Mercruiser 120 Help with wiring the ignition coil (and distributor, possible upgrade to electronic ignition)

[mjricha83]

Hello! i have a 1978 mercruiser 120 2.5l 4cyl. ever since i've gotten this boat its run great but after about an hour it shuts down on the water and wont restart. only happens in the water. Im 99% sure I have it solved, I think its the ignition coil. the previous owner had changed it, and A) its the wrong one (its a thunderbolt and I don't have thunderbolt ignition) and B) it Gets EXTREMELY hot under load. like burn your fingers hot. and C) there's no ignition ballast resistor anywhere. it does still have points ignition.

im having two issues. one, he has added a new wire to the positive terminal of the ignition coil. it goes under the dash and to the key. there are also two wires that look original that go to the positive (cant trace them) and two that go to the negative (ground and distributor).

my question is, how SHOULD it be wired? the two original wires that go to the postive terminal are supposed to come from (or go to ) where exactly?

I'm looking at upgrading to the pentronix igntion, and I want to make sure everything is wired correctly.

second issue is the distrubtor. I called pentronix to make sure i ordered the right one but they said they cant 100% be sure that i have the correct distributor. i have looked around online and i see theres supposed to be a stamp with the number on it but I dont see it anywhere. all i see is s sticker that reads "j354" on the outside and a stamp that says delco remy on the inside. according to AI that means i need the pertronix 1143 for 4 cyl delco distirbutors. So I just wanted to see if I could find someone who might have a little more insight or that has possibly put them in before.

any info would be greatly appreciated. thanks!

 

[Droid]

The 2 wires come from:

A) The ignition switch thru the big black plug then converts to a resistor wire within the engine harness a few inches from its big black plug. It looks like a normal wire & connects to the coil. Voltage probably will be about 9 volts.

B) The second wire comes from a small stud on the starter solenoid. I just can't remember but think it's labeled "R". Regardless, it is the opposite stud from the yellow/red wire. It will only be hot (12volts) during engine cranking, creating a hotter spark.

In my opinion, DO NOT waste your money on a electronic ign system.
Yes I'm sure you have the correct distributor installed. Just replace the points/condensed, set dwell & timing. AND install a new coil that requires an external resistor. Delco D511, I've used Standard UC-12 on a few occasions.
If coil is hot, not warm, as in too hot to hold, it's junk or someone has bypassed the ballast wire. If the engine harness from the black plug to the coil has not been sliced open for someone to perform surgery, I'd assume it is then good. This ballast wire really can only fail open & then it would start & die when key is released.

 

[mjricha83]

The 2 wires come from:

A) The ignition switch thru the big black plug then converts to a resistor wire within the engine harness a few inches from its big black plug. It looks like a normal wire & connects to the coil. Voltage probably will be about 9 volts.

B) The second wire comes from a small stud on the starter solenoid. I just can't remember but think it's labeled "R". Regardless, it is the opposite stud from the yellow/red wire. It will only be hot (12volts) during engine cranking, creating a hotter spark.

In my opinion, DO NOT waste your money on a electronic ign system.
Yes I'm sure you have the correct distributor installed. Just replace the points/condensed, set dwell & timing. AND install a new coil that requires an external resistor. Delco D511, I've used Standard UC-12 on a few occasions.
If coil is hot, not warm, as in too hot to hold, it's junk or someone has bypassed the ballast wire. If the engine harness from the black plug to the coil has not been sliced open for someone to perform surgery, I'd assume it is then good. This ballast wire really can only fail open & then it would start & die when key is released.

Thats part of the issue. Im getting 12v all the time. so now that makes sense. the previous owner added a new wire which runs directly from the ignition switch to the ignition coil positive. that must just be a regular wire not a resistor wire. the distributor cap, rotor, condenser, wires and plugs are all new, and gapped and set appropriately. im just trying to peg down this overheating coil issue.

so start the motor let it run and check the voltage on coil positive to make sure its not less than 12? ill do it again to be safe but im pretty sure it was 12 while running not just cranking.

so if i dont have a resistor wire, can i just get the coil that needs a resistor and a seperate ballast resistor instead? or should i try to find a resistor wire that i can add in line to his current wire that goes between the ignition switch and the pos terminal on the coil?

 

[makomark]

The coil voltage will likely be at least 12VDC wired as you described.

Yes, you can get a regular ballast resistor and use in with the modified setup as long as you get the correct coil. If you go this route, connect the wire from the starter's solenoid (yes, the R terminal) to the coil , either directly or oil the coil's side of the ballast resistor.

 

[Droid]

I'd start by unreinventing the P.O.s do it yourself wires. I assume the factory wire still comes out of the factory harness near the ignition switch. Presumably it's purple but I'm not sure. If you find it, disconnect the DIY wire from ignition switch & reattach the factory wire.

If you can find & do the above then continue:

Question: I reread your original post. Are there the 2 factory wires AND the P.Os DIY wire on the coil positive post? If so, disconnect all 3 wires then proceed below. IF NOT, jump down 1 step.

If yes above:
*You have disconnected the 2 factory wires from the coil positive in step above. Turn key on & verify the approximately 9 volts in the factory resistor wire.
*Now check for 12 volts in the 2nd wire (coming from solenoid) when turning key to crank. This 12 volts should stop when key is released from crank. The approx 9 volts will still be present in 1st wire so long as key is in ON or CRANK position.
***Just to clarify, you are totally ignoring P.O.s wire except to disconnect both ends.***

*IF only 2 wires are currently at coil positive (one being P.O.s DIY wire & I assume the other comes from solenoid) then disconnect both & ignore DIY wire. Find factory resistor wire hopefully still coming out of & tied back from factory harness near the coil. Then proceed to check voltage as outlined above.

If yes to both voltage checks above:
Buy a new coil as stated in my above 1st post.

I would set point dwell and not just gap, it is more accurate. Off the top of my head I'm thinking 31-34 degrees dwell. I try to set towards the low end as rubbing block wear causes dwell to slowly increase. As dwell increase with wear, timing retards. Be sure to put a small dab of grease on the distributor cam lobe, this reduces rubbing block wear. Then set timing. Dwell always before timing.
And remove the rotor, put a few drops of motor oil on the wick in the center of the shaft. This oil lubricated the centrifugal advance mechanism.

 

[Droid]

You say the negative post has 2 wires, one ground & 1 to distributor.

Can't be, if 1 went to ground it wouldn't run. Black goes to distributor & gray goes back to tachometer.

 

[mjricha83]

You say the negative post has 2 wires, one ground & 1 to distributor.

Can't be, if 1 went to ground it wouldn't run. Black goes to distributor & gray goes back to tachometer.

i just checked you are correct it is black (dist) and grey (tach)my apologies

there are three wires going to the positive terminal
Purple, purple/yellow and red

red is the wire he added that goes to the ignition switch
purple is hooked to the distributor positive and is UNHOOKED from the alternator.
purple/yellow goes to starter solenoid

im going to do what you mentioned in the post above. im going to try to find the solid purple wire behind the ignition switch. im going to guess there will be no voltage because its unhooked from the alternator. if 0V jump broken purple wire to alternator and test again for 9v

 

[Droid]

That 9 volts is an approximation.

I'm used to the older single wire alternator vs the one I see in that other post.
If that wire diagram in other post reflects your boat (as in multiple wires to alternator & electric choke stove) then you may not have 12 volts until it starts. I say may not because I can't see what happens at the alternator.

Diagram is missing the key switch. So I'm not sure if you're looking for purple or purple/red under the dash. But purple/red sounds right - you need to look really close to see the red tracer.

Does the alternator charge? Do you see 13.8 to 14.2 volts after it's been running awhile? Does the choke heat get hot & open?

I'm pretty certain if you wire it properly and resistor wire is not damaged (most likely is good) it will run. You can use an ohm meter for static testing of wires vs having battery hooked up. We know the key switch is good.

 

[kghost]

i just checked you are correct it is black (dist) and grey (tach)my apologies

there are three wires going to the positive terminal
Purple, purple/yellow and red

red is the wire he added that goes to the ignition switch
purple is hooked to the distributor positive and is UNHOOKED from the alternator.
purple/yellow goes to starter solenoid

im going to do what you mentioned in the post above. im going to try to find the solid purple wire behind the ignition switch. im going to guess there will be no voltage because its unhooked from the alternator. if 0V jump broken purple wire to alternator and test again for 9v

If you look at the schematic that was shared for your POINT ignition, You will see the PURPLE wire has multiple attachment points.
(From Ign switch to engine harness and there is a multi wire connection all taped up in the engine wire harness which then goes to the following, [Choke if electric, Alt, and is attached to the resistance wire going to the Coil +]..........)

Based on your description, if the PURPLE wire on coil + is coming from the wire taped harness it may still be the resistance wire. Disconnect it and measure voltage on that purple wire with the key switch in the ON position, it should measure less than battery voltage- Approx 8-10 Volts DC. If it does it is still the resistance wire. If not, then find out where it goes.

A typical ignition switch is 3 wires, Input is RED - Battery voltage, Purple - Ignition On, Yellow with Red stripe-Crank

The small two wires to Alt are Red with purple stripe and Purple, One is Excite Purple and the other is Sense purple with red stripe.

There would be NO reason to disconnect the wire at the Alt as the multi wire connection in the harness would still allow voltage on the resistance wire to coil +.

If not mentioned earlier.

Two oil filled canister type coils. They look identical and not all are marked for what they are.

1. External resister coil. Needs a ballast resister to reduce running voltage below 12VDC. Mercruiser uses a resister wire within harness. Used on Point ignitions

2. Internal resister coil. Only requires ignition circuit voltage (running battery voltage). These are typically used on Electronic ignitions. (Thunderbolt)


If you use a external resister coil and supply it with running battery voltage it will over heat. This is a common mistake done on Thunderbolt ignitions when the boat owner uses the incorrect coil when replaced.

If you replace the coil in a point ignition with an internal resister coil and use the resister wire in harness the coil will have a very week spark due to very low voltage (two resisters, one external limiting voltage to coil combined with one internal limiting voltage to the actual internal coil windings)

If what you describe is true, The previous owner thought he could get away with using a internal resister coil but did not understand the wiring.

If I were you,

1. Confirm resister wire is still in harness - If not, an external ballast resister will be needed. (both internal (wire) and external ballast are ~ 2.0) OHMS
2. Buy a EXTERNAL resister coil. (Can be purchased at any Automotive store) But you must specify EXTERNAL RESISTER.
3. Replace point, rotor and condenser as previously suggested- adjust dwell and timing.

4. REMOVE the added wire from previous owner. The only reason to use a bypass wire would be if there was a break in the purple wire between key switch and engine wire harness causing the loss of IGNITION ON voltage from key switch.

 

[Droid]

I was pondering why did P.O. install a new wire?

Possible reasons:
*Resistor wire bad resulting in a start then die when key released? Highly unlikely as these wires rarely fail.
*Hard starting or no start because no current from solenoid during cranking? Wire would be a bandaid fix that gets engine to start & causes coil to overheat. Correct fix for this issue is new solenoid or repair faulty purple/yellow wire from solenoid to coil.
*Shift interruptor switch interrupting when it shouldn't & he was shotgunning ideas. This would not have fixed the shift interruptor issue but he just left it in? Result of leaving bandaid wire installed would be overheated coil. All while shift interrupting circuit was still malfunctioning.

IDK but Ghost & I are on the same track. His diagram is better than the 1 provided yesterday.
His step 4 above is correct but let me emphasize this is not a permanent repair. Maybe P.O. fixed 1 problem, created the hot coil issue & just sold boat as is?

 

[kghost]

I was pondering why did P.O. install a new wire?

Possible reasons:
*Resistor wire bad resulting in a start then die when key released? Highly unlikely as these wires rarely fail.
*Hard starting or no start because no current from solenoid during cranking? Wire would be a bandaid fix that gets engine to start & causes coil to overheat. Correct fix for this issue is new solenoid or repair faulty purple/yellow wire from solenoid to coil.
*Shift interruptor switch interrupting when it shouldn't & he was shotgunning ideas. This would not have fixed the shift interruptor issue but he just left it in? Result of leaving bandaid wire installed would be overheated coil. All while shift interrupting circuit was still malfunctioning.

IDK but Ghost & I are on the same track. His diagram is better than the 1 provided yesterday.

With regards to no start due to solenoid,

Typically the indicator of that is, no start in cranking position but when key is released to run position engine fires off.
This is as you said, bad starter solenoid contacts. Either rebuild or replace solenoid.

 

[mjricha83]

If you look at the schematic that was shared for your POINT ignition, You will see the PURPLE wire has multiple attachment points.
(From Ign switch to engine harness and there is a multi wire connection all taped up in the engine wire harness which then goes to the following, [Choke if electric, Alt, and is attached to the resistance wire going to the Coil +]..........)

Based on your description, if the PURPLE wire on coil + is coming from the wire taped harness it may still be the resistance wire. Disconnect it and measure voltage on that purple wire with the key switch in the ON position, it should measure less than battery voltage- Approx 8-10 Volts DC. If it does it is still the resistance wire. If not, then find out where it goes.

A typical ignition switch is 3 wires, Input is RED - Battery voltage, Purple - Ignition On, Yellow with Red stripe-Crank

The small two wires to Alt are Red with purple stripe and Purple, One is Excite Purple and the other is Sense purple with red stripe.

There would be NO reason to disconnect the wire at the Alt as the multi wire connection in the harness would still allow voltage on the resistance wire to coil +.

If not mentioned earlier.

Two oil filled canister type coils. They look identical and not all are marked for what they are.

1. External resister coil. Needs a ballast resister to reduce running voltage below 12VDC. Mercruiser uses a resister wire within harness. Used on Point ignitions

2. Internal resister coil. Only requires ignition circuit voltage (running battery voltage). These are typically used on Electronic ignitions. (Thunderbolt)


If you use a external resister coil and supply it with running battery voltage it will over heat. This is a common mistake done on Thunderbolt ignitions when the boat owner uses the incorrect coil when replaced.

If you replace the coil in a point ignition with an internal resister coil and use the resister wire in harness the coil will have a very week spark due to very low voltage (two resisters, one external limiting voltage to coil combined with one internal limiting voltage to the actual internal coil windings)

If what you describe is true, The previous owner thought he could get away with using a internal resister coil but did not understand the wiring.

If I were you,

1. Confirm resister wire is still in harness - If not, an external ballast resister will be needed. (both internal (wire) and external ballast are ~ 2.0) OHMS
2. Buy a EXTERNAL resister coil. (Can be purchased at any Automotive store) But you must specify EXTERNAL RESISTER.
3. Replace point, rotor and condenser as previously suggested- adjust dwell and timing.

4. REMOVE the added wire from previous owner. The only reason to use a bypass wire would be if there was a break in the purple wire between key switch and engine wire harness causing the loss of IGNITION ON voltage from key switch.




View attachment 36962

I have replaced the rotor, point, condenser, cap, and have timed and adjusted the point. wires and plugs also new. When i was first working on this a few months ago i noticed the cap had a crack in it, so went ahead and replaced all those things as they were very old.

as far as the wires go, it looks like everything is wired as in your picture with a few exceptions.
1) there is no yellow/red wire coming from the starter solenoid at all,
2) there is no red purple wire at the alternator
3) there is a circuit breaker button next to the coil, and it has two red wires going to it, which are connected at the button, but disconnected at the alternator.

4) only wires present at alternator are black (connected), orange (connected), purple (disconnected) large red to circuit breaker (disconnected).

i did get a shop manual from where you sent this picture from so I have that now. and I can check and see what post on the ignition switch the red wire he installed goes to. but for now will follow the steps you said and check the wires I can connect.

I had already previously ordered a external resistor coil because I knew i needed one to replace the thunderbolt before I made this post its just not here yet.

 

[mjricha83]

I was pondering why did P.O. install a new wire?

Possible reasons:
*Resistor wire bad resulting in a start then die when key released? Highly unlikely as these wires rarely fail.
*Hard starting or no start because no current from solenoid during cranking? Wire would be a bandaid fix that gets engine to start & causes coil to overheat. Correct fix for this issue is new solenoid or repair faulty purple/yellow wire from solenoid to coil.
*Shift interruptor switch interrupting when it shouldn't & he was shotgunning ideas. This would not have fixed the shift interruptor issue but he just left it in? Result of leaving bandaid wire installed would be overheated coil. All while shift interrupting circuit was still malfunctioning.

IDK but Ghost & I are on the same track. His diagram is better than the 1 provided yesterday.
His step 4 above is correct but let me emphasize this is not a permanent repair. Maybe P.O. fixed 1 problem, created the hot coil issue & just sold boat as is?

i can check the voltage at the solenoid during cranking as well just to be sure this isnt the reason why he added the red wire

 

[Dieter]

I was pondering why did P.O. install a new wire?

Possible reasons:
*Resistor wire bad resulting in a start then die when key released? Highly unlikely as these wires rarely fail.
*Hard starting or no start because no current from solenoid during cranking? Wire would be a bandaid fix that gets engine to start & causes coil to overheat. Correct fix for this issue is new solenoid or repair faulty purple/yellow wire from solenoid to coil.
*Shift interruptor switch interrupting when it shouldn't & he was shotgunning ideas. This would not have fixed the shift interruptor issue but he just left it in? Result of leaving bandaid wire installed would be overheated coil. All while shift interrupting circuit was still malfunctioning.

IDK but Ghost & I are on the same track. His diagram is better than the 1 provided yesterday.
His step 4 above is correct but let me emphasize this is not a permanent repair. Maybe P.O. fixed 1 problem, created the hot coil issue & just sold boat as is?

if he ran another wire did the PO do a Points to electronic conversion like pertronix ? This would require 12 v at the coil vs the reduced 9 -10 v

 

[kghost]

"""only wires present at alternator are black (connected), orange (connected), purple (disconnected) large red to circuit breaker (disconnected)."""

Black is ground
Orange is 10 ga wire and is Alt output and is typically connected on Main + stud on Starter.

Red and Purple should be 14 ga and would have bullet connectors. See image-Note due to age of engine the actual Alt may look slightly different.

Without those two smaller wires connected I doubt the Alt is charging unless he installed a ONE WIRE alt.

If he did there would be NO purple or red 14 ga wires hanging off the Alt. You do not say where the disconnected wires are????? In harness or coming from Alt????

""i can check the voltage at the solenoid during cranking as well just to be sure this isnt the reason why he added the red wire""

He shouldnt have connected the added red wire from the key switch for cranking voltage to the coil. If he did it will be on the same key switch terminal as the Yellow with red stripe wire on key switch.

Also to check this you best get a TEST LIGHT. Better for testing if battery voltage is present by the brightness of the light vs a Volt meter which can show voltage but wont show if there is sufficient current.

Also if he wanted, the OP could have tappped of the slave solenoid large output terminal for the cranking 12 volts to coil + vs key switch.

There is absolutely no way to understand why someone would alter the factory wiring especially the way he did.

 

[mjricha83]

if he ran another wire did the PO do a Points to electronic conversion like pertronix ? This would require 12 v at the coil vs the reduced 9 -10 v

no it was all standard points/condenser inside the distributor. I replaced them a few months ago with new ones. and dwell/gap as well

 

[mjricha83]

"""only wires present at alternator are black (connected), orange (connected), purple (disconnected) large red to circuit breaker (disconnected)."""

Black is ground
Orange is 10 ga wire and is Alt output and is typically connected on Main + stud on Starter.

Red and Purple should be 14 ga and would have bullet connectors. See image-Note due to age of engine the actual Alt may look slightly different.

Without those two smaller wires connected I doubt the Alt is charging unless he installed a ONE WIRE alt.

If he did there would be NO purple or red 14 ga wires hanging off the Alt. You do not say where the disconnected wires are????? In harness or coming from Alt????

View attachment 36970





""i can check the voltage at the solenoid during cranking as well just to be sure this isnt the reason why he added the red wire""

He shouldnt have connected the added red wire from the key switch for cranking voltage to the coil. If he did it will be on the same key switch terminal as the Yellow with red stripe wire on key switch.

Also to check this you best get a TEST LIGHT. Better for testing if battery voltage is present by the brightness of the light vs a Volt meter which can show voltage but wont show if there is sufficient current.

Also if he wanted, the OP could have tappped of the slave solenoid large output terminal for the cranking 12 volts to coil + vs key switch.

There is absolutely no way to understand why someone would alter the factory wiring especially the way he did.

there is no red/purple wire coming out of the harness at the alternator

there is a purple wire with a bullet connector coming out of the harness at the alternator but the end that would connect to the alternator is missing so its not connected
the red wire is coming out of the harness and is cut before it connects to the alternator

 

[kghost]

send me your phone number and when to call (if you are in the USA) I am available all day

 

[Droid]

Ghost,
I'm sure you & I could unreinvent the P.O.s prior work quickly if we saw it in person.

We assume boat is a 78. We assume engine is the original, a 78 or perhaps a left over 77. We don't have the serial number but it appears it should NOT be a single wire alternator going from the appropriate serial number range. 49xxxxx to 51xxxxx. It is possible the engine is not original & wire harness is not comparable with the alternator installed or visa-versa.

But we have no knowledge of why a new wire was run or why they are disconnected from the alternator. Or for that matter why the coil was replaced & by what appears to be the wrong one.

I'm going to step back & just follow along now.

In a shop environment; Research, head scratching, making/running new proper color wires would rapidly make this boat an uneconomical job.

 

[kghost]

I have been on the phone with him

1. Alt is a ONE WIRE alt.
2. Engine wire harness seems to be correct and all wires are present.
3. He voltage tested resistance wire alone with key on and it showed battery voltage. I would think it should less more like 10-11 vdc (Any input would be appreciated).

Additional info to follow.

 

[Droid]

With essentially zero current draw from the meter IDK how much voltage drop to expect for sure.
But we know the wire isn't broken/open.
For gee whizz I wonder if the wire is hooked to coil positive as it should be in real life & coil negative jumpered to ground (or points closed) what the voltage would read at the positive post? Then do similar test using a solid wire from battery to coil positive (with resistor wire disconnected) & read voltage. I'd expect maybe 2 or 3 volts difference. But what do I know.

 

[kghost]

He is going to remove the OP red wire to coil + and Ign switch.
He ordered a External resistor coil and will install when he gets it.

The Purple Ign on wire to coil + was combined with the Crank wire in one ring terminal which he said looked original based on the professional crimping so it appears that is OEM configuration. I also asked him to reconnect the Purple wire alone to Coil + and turn key on and remeasure the voltage as I was thinking without the load of the coil the resistance wire may show full battery voltage.

After he receives the internal resistor coil and installs it with only the OEM wires he will start engine and check voltage at the coil + to confirm resistance wire is good (I believe it should be around 9-11 VDC. (Note: he did say resistance wire is a different type of wire. Which I know to be true. It is not at all like standard wire, it is a single insulated wire about 3 feet long and bent over on itself within the wire harness as I have peeled the harness back on many to check the connections in the harness in the past). If when checked after all back together and the voltage at coil + is still battery/charging voltage He understands that a stand alone ballast resister will be needed and the resistance wire cut out of harness and a normal 14 ga stranded purple wire to coil + put in its place.

 

[makomark]

There WONT be any voltage drop in a good piece of resistance wire when there is no current flowing thru it....

 

[kghost]

There WONT be any voltage drop in a good piece of resistance wire when there is no current flowing thru it....

 

[Dieter]

1. Alt is a ONE WIRE alt.

Years back I ended up going to a single wire alternator on my grandfathers 1966 straight 6. Just hooked up the red wire and tapped off the other two, disconnected the external voltage regulator. Worked for years

 

[Droid]

I have one of those old electronic external regulators, used & condition unknown. Both my MerCruisers are early 70s single wire design.

 

[mjricha83]

test results:
1) purple resistance wire disconnected at at coil key on =12.6v
2) purple resistance wire connected on coil key on= 4.6v ( not 14.6, actually 4.6 )
2) purple/yellow disconnected at coil during crank =10.6v to 11.2v

red P.O wire traces to the same post as the purple wire on the back of ignition switch, 12.6v key on

i did all the tests three times to make sure. these were the voltages and didnt vary much.

But i will hold off for now until I get my external resistor coil in the mail, then install it with stock wires and re-test

send me your phone number and when to call (if you are in the USA) I am available all day

 

[kghost]

The 4.6 volts seems low but you have a internal resister coil now.

Most likely that will go up to 8 volts or higher which will be correct with the new external resister coil

 

[makomark]

Tthe 4.6 VDC reading suggests, to me, the key switch is burnt/corroded on the IGN lead...

 

[kghost]

Tthe 4.6 VDC reading suggests, to me, the key switch is burnt/corroded on the IGN lead...

You dont think it is because of the resister wire and the Internal resister coil?

When measuring only the resister wire not attached to the coil, It measured battery voltage