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

no, an internal resistor coil with resistance wire would have smaller (than normal) voltage drops due to the increased resistance...

When cranking, the indicated voltage goes into the expected range...

So, changing the source of the the B+ changes the measured voltage at the coil...
 
OK I am confused.
wouldnt the two 2 Ohm loads now in series added up, equal 4 Ohms?

Wouldnt the higher resistance reduce the applied measured voltage?

So with key on only, with 2 ohm wire resister only one would see 7-9 VDC at external resistor coil + Correct?

So with a 4 ohm load using the wire and internal resistor coil, wouldnt it now be less? Like 4-6 VDC at coil +
 
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i take 'internal resistor coil' to be one with extra resistance inside, where there is no access to measure it.

so the measurement is made at the same point - the + stud on the coil.

the extra resistance would combine with the resistance wire; this reduces the current flowing, not the voltage. so the voltage drop across the resistance wire would be less. so the measured voltage at the + stud should be higher with the internal resistor coil, everything else being the same.
 
makomark said:
Tthe 4.6 VDC reading suggests, to me, the key switch is burnt/corroded on the IGN lead...
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well he did add the red wire to the same post as the purple lead at the key ignition,, and that does get full 12v at the coil. ( i didnt test it connected to the coil though)
 
kghost said:
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
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yes it could be because you're essentially getting two resistors (one on the wire and one in the coil). but yeah will test again soon. new coil should be here monday.
 
hello all, new external resistor coil has been installed.

PO red wire removed

key on engine off only resistor wire installed on coil pos terminal- 12.4v

both wires connected (purpleyellow/resistor) running:11.5-12v

battery charging at 13.8v while running
 
Droid said:
Let us know how it works
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I'm gonna take the boat out within the next couple days. I'm gonna take a temp gun and keep an eye on the coil while I'm out because I always end up stranded and have to troll motor my way back in.

You think that 11-12v is good though? According to my reading it should be lower between 9-10 right?
 
kghost said:
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.
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AFIAK, the resistor should only affect the voltage when the key is in the Run position, when cranking, the resistor is being bypassed and is only there to save the points while in the Run position through the voltage drop. Maybe the ignition switch or starter is mis-wired?

My 120 is from 1984 and the distributor & alternator are original, to my knowledge. The previous owner certainly didn't do any maintenance or service, so I'm assuming that, although I have to say that I'm happy with the way it runs and that I didn't need to deal with a rat's nest of 'expert-level' wiring changes.

Let me know if you want me to shoot photos and verify wire colors & locations in my boat.
 
mjricha83 said:
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
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Do you have the distributor diagram at the bottom?

Why are you calling the purple wire 'resistance wire'? That's the ignition wire and should be at battery voltage. The crank wire

Look at the starter fore wire colors- you should have the battery cable and a red wire on the battery post and one a smaller wire on the solenoid, to make it crank. That may be yellow/red.

Let me know if you need me to look at my boat- it doesn't have a one wire alternator, but should have the same starter wiring.
 

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jimn said:
Do you have the distributor diagram at the bottom?

Why are you calling the purple wire 'resistance wire'? That's the ignition wire and should be at battery voltage. The crank wire

Look at the starter fore wire colors- you should have the battery cable and a red wire on the battery post and one a smaller wire on the solenoid, to make it crank. That may be yellow/red.

Let me know if you need me to look at my boat- it doesn't have a one wire alternator, but should have the same starter wiring.
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The purple wire which connects to the coil (point ignition only) the last 3 or so feet between harness and coil + is a different wire type and is called the RESISTOR WIRE used on all point ignitions by Mercruiser. Which is the BALAST RESISTOR. It is folded over on itself a few times and wrapped in the OEM tape used on engine harness.
 
jimn said:
Do you have the distributor diagram at the bottom?

Why are you calling the purple wire 'resistance wire'? That's the ignition wire and should be at battery voltage. The crank wire

Look at the starter fore wire colors- you should have the battery cable and a red wire on the battery post and one a smaller wire on the solenoid, to make it crank. That may be yellow/red.

Let me know if you need me to look at my boat- it doesn't have a one wire alternator, but should have the same starter wiring.
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I went thru ALL THE WIRING over the phone as he sent photoa of all the critical connections.
 
The other thing that may be of benefit is to check the dwell angle on the points...and get that correct BEFORE you verify the timing...and while you have the timing light out, make sure the timing advances with RPM...
 
mjricha said:
How wide is it supposed to be?

I set it at 0.020 which is what was in the instructions with the new points and condenser
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see image

1978 inline 4 ignition specs.png
 
That is a good start but dwell is a more accurate setting.

Dwell is the number of degrees of DISTRIBUTOR shaft rotation the points are closed. In other words, the time the current if flowing thru the coil. Too long and coil will get hotter, too short & perhaps the coil won't have time enough to fully charge & a weak spark results. So a big gap decreases closed time or a lower dwell angle. As the points are used, over time there is wear to the contact point (burning/arcing) & to the rubbing block. Typically over time the wear causes the dwell to increase ( gap decreases).

So the net result is points slowly wear, the gap decreases so the points open later which causes spark to retard. This is why I said way back it's best to set the dwell to the low end of the specs.

Another thing to consider is up to now we have talked about dwell. Since dwell is distributor shaft rotation & the distributor shaft turns at 1/2 crankshaft rotation, 1 degree of dwell equals 2 degrees of crankshaft rotation. So 1 degree of dwell change (due to point wear) over time equals 2 degrees of retarded engine timing.

Regardless to all of this, point gap most frequently is .016 to .020 across most engines. Using dwell is more accurate because of variables like one person's opinion or skill of how to use a feeler gauge vs the next guy. And more importantly WHY POINTS ARE SET BEFORE THE FINAL ENGINE TIMING IS SET regardless if using a feeler gauge or dwell meter.
 
I see why people switch to electronic ignition now.

Ok well I don't have a dwell meter but I can set the points to .022 as listed above and check the timing. I can't take the boat out for a couple days anyway because I just found out one of my trailer bearings is completely toast and there is grease all blown out under the trailer so I've ordered that and waiting on delivery
 
Dwell meters are more accurate than feeler gauges for adjusting the points...but you gotta do what you can with what you have...
 
When using a feeler gauge, I find it best to have good lighting, a delicate hand & be sure to keep it straight (as in 90 degrees to points). Holding it at any angle will cause points to be pushed open & give a false reading. Bracing your arm across the valve cover or your other arm helps keep you straighter. Being at a full arms length just invites discrepancies.
 
That's a good helpful picture.

And like I had said earlier, a few drops of engine oil in the center of the distributor shaft. (You can see the wick clearly in that picture but it's not pointed to). This lubricates the advance mechanism below & out of sight.
Put the rotor on & give it a gentle twist each way. It should spring gently back to its starting point.
 
And if you're pulling 1 wheel hub apart, do both because the other typically fails soon after.
Then the lights will drive you crazy until you throw them away & install a new LED kit with a complete new harness.

I assume you're young & it's your 1st boat and 1st time with old school points/condenser.
This has been a fun thread!
 
Droid said:
And if you're pulling 1 wheel hub apart, do both because the other typically fails soon after.
Then the lights will drive you crazy until you throw them away & install a new LED kit with a complete new harness.

I assume you're young & it's your 1st boat and 1st time with old school points/condenser.
This has been a fun thread!
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not super young (43) but yes first boat. have worked on cars my whole life and assumed that getting a boat with an inboard would be easier for me to work on than an outboard. don't think i've ever worked on something without electronic ignition though. (most cars/trucks Ive rebuilt were 80s-90s) but have ground up rebuilt/refurbished several cars in the past. also grew up doing boating activities (skiiing etc as opposed to fishing) and got a cheap boat to fix up so when my kids are a little older I can take them tubing and/or knee boarding, wake board etc. we live within 2-3 ,,miles of about 10 public boat able large lakes so its a good spot. however having my old boat die outside of someones 5 mil dollar house/ and or boat house is rather embarrassing haha


but yeah im doing both sets of bearings.
 
43 is young to me, I could be your father.

You have a very simple nearly indestructible engine. I agree easy to work on. But behind the flywheel things are more difficult than a simple outboard. That being said, old I/Os are what I own. After 47 years of having old MerCruisers & OMC electric shifts in my shed I'm all too familiar.
 
mjricha said:
I see why people switch to electronic ignition now.

Ok well I don't have a dwell meter but I can set the points to .022 as listed above and check the timing. I can't take the boat out for a couple days anyway because I just found out one of my trailer bearings is completely toast and there is grease all blown out under the trailer so I've ordered that and waiting on delivery
Click to expand...

Dwell meters don't need to be expensive but a low priced one is good to have if you own anything that might need it. I bought a multimeter with Tach & Dwell for about $20 in the '90s and have compared it to other, more expensive meters- the Tach is only counting pulses, so that's easy to design and Dwell is only measuring time, also easy to design into a meter.
 
jimn said:
Dwell meters don't need to be expensive but a low priced one is good to have if you own anything that might need it. I bought a multimeter with Tach & Dwell for about $20 in the '90s and have compared it to other, more expensive meters- the Tach is only counting pulses, so that's easy to design and Dwell is only measuring time, also easy to design into a meter.
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same here my tach died but the dwell still works. Same with the timing light I bought back in 1990. My dad was cleaning his garage out and gave me his tach and dwell he bought in the 60's still works.

to the OP - find an older guy who is a shade tree mechanic. bet he would lend you a dwell meter ha hasn't used in years
 
I purchased a multimeter that has dwell for 4/6/8 cyl built in. I'm 1000% sure this boat will be the only thing I ever use dwell for. Besides if I can't get this ignition issue fixed where i keep overheating coils I'll be switching to electronic ignition anyway. But regardless, I'll use a multimeter whether I need dwell or not so I went ahead and got one. I know a couple shade tree mechanics but none with a dwell meter (I asked).

So once the meter gets here, before I take the boat out, I'll check dwell, check timing and adjust appropriately. That 11 volts running on the coil with the appropriate coil and wiring still gives me a bad feeling. Everywhere I read points to, that's too high. I don't necessarily like getting stuck in the middle of the lake with only a trolling motor to get back to the dock haha. But getting everything back to complete stock is important for further problem solving so that's where I'm at currently. Hopefully getting trailer bearings today. (And hope they fit, they were very worn and hard to read).

Edit: not getting a multimeter with dwell on it. Checked reviews and they were all bad for inconsistent dwell/rpm readings. Buying a snap-on dwell meter used on eBay instead
 
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