Tachometer stays at 0 RPM or jumps to max while engine is running

[Ranger_Trivette]

I’m running out of ideas. This situation just doesn’t make sense to me.

This winter I restored a 1985 Glastron V194 with an OMC 3.8L V6.

The boat runs fine. Maybe it should be a bit faster, but before doing any serious troubleshooting I really need to know what RPM the engine is actually turning.

During the refit I replaced all of the gauges. Everything works except the tachometer.

The tach powers up and the backlight works, but it does not show the correct RPM.

It is set to “3”, since it is a gasoline V6.

The three wires on the back are connected to the original gauge wiring:
positive, ground, and the grey signal wire.

I then bought a second tachometer.
Again, three wires, again set to “3”.

This time I connected the tach directly to the battery for power and ground, and connected the signal wire directly to the negative terminal of the ignition coil.

Still nothing.

At this point I really don’t know what else to check.

The engine runs, so the ignition coil is obviously switching and producing spark. My understanding is that the tach should read the pulses from the negative side of the coil.

What am I missing?

 

[makomark]

assuming the tachs are good, I would suspect some aspect of the interconnecting wires ....

the tachometer model number(s) for the replacements would be good to have for those of us not present...

 

[Ranger_Trivette]

assuming the tachs are good, I would suspect some aspect of the interconnecting wires ....

the tachometer model number(s) for the replacements would be good to have for those of us not present...

one from amazon, one from aliexpress.
i'm attaching a screenshot.

I also suspected the wiring, so for the second tach I completely bypassed the boat wiring.
I powered it directly from the battery (+ and ground), and ran the signal wire directly to the negative terminal of the ignition coil.
Still no reading.

 

[Ranger_Trivette]

I went back to the boat today.

I reinstalled the ignition coil, and the engine starts and runs perfectly.

This time I connected the original tachometer, which I had actually never tested before:

• Positive directly to the battery
• Negative directly to the battery
• Grey signal wire directly to the negative terminal of the ignition coil

Still no RPM reading.

One additional thing I noticed: when I press the tach signal wire against the negative terminal of the ignition coil with my finger, I can feel a very slight electrical tingling through my finger. There is no visible spark or arc, just a faint sensation of current.

Maybe that is completely normal for the negative side of the coil, but I thought it might be worth mentioning.

So at this point I have tried three different tachometers, including the original one, and none of them will read RPM from the coil negative. The engine itself runs perfectly.

 

[Droid]

Ok, I can easily assume your Chinesium tachs are junk BUT you say the original tach is inop as well. Highly unlikely all 3 are junk.

Let's start with the basics:
Tach needs 12V pos (most likely purple at the dash panel), 12V neg & a signal wire from the coil (typically gray). It sounds as if you bypassed the boat's gray wire for testing purposes.

You are correct, the tach signal comes from the coil negative. Are you sure the coil is not wired into the engine backwards? Normally ignition positive from the key goes to coil POSITIVE terminal (along with the ballast bypass wire from the starter solenoid). Wire to distributor points goes to coil NEGATIVE terminal.
Tach gray also goes to coil negative also & maybe the shift assist module but IDK for sure.
It is my understanding that a coil can be wired backwards & produce spark. If I recall from 50 years ago, spark will jump from spark plug center electrode to the ground if proper & the opposite direction if reversed.

I'm assuming this is a Cobra & not a stringer drive but I doubt it matters.
I'm also assuming your motor had from the factory a points style conventional ignition.

If someone has installed an aftermarket electronic ignition, all bets are off. Good luck to you.

Resident OMC Cobra Guru Louc should be by shortly to help.

 

[Ranger_Trivette]

Ok, I can easily assume your Chinesium tachs are junk BUT you say the original tach is inop as well. Highly unlikely all 3 are junk.

Let's start with the basics:
Tach needs 12V pos (most likely purple at the dash panel), 12V neg & a signal wire from the coil (typically gray). It sounds as if you bypassed the boat's gray wire for testing purposes.

You are correct, the tach signal comes from the coil negative. Are you sure the coil is not wired into the engine backwards? Normally ignition positive from the key goes to coil POSITIVE terminal (along with the ballast bypass wire from the starter solenoid). Wire to distributor points goes to coil NEGATIVE terminal.
Tach gray also goes to coil negative also & maybe the shift assist module but IDK for sure.
It is my understanding that a coil can be wired backwards & produce spark. If I recall from 50 years ago, spark will jump from spark plug center electrode to the ground if proper & the opposite direction if reversed.

I'm assuming this is a Cobra & not a stringer drive but I doubt it matters.
I'm also assuming your motor had from the factory a points style conventional ignition.

If someone has installed an aftermarket electronic ignition, all bets are off. Good luck to you.

Resident OMC Cobra Guru Louc should be by shortly to help.

On Monday I’m going to test the Chinese tachs with a signal generator, so we can definitively rule out the possibility that they are actually all faulty.


In the first configuration, I connected the boat’s gray wire to the new tach. When I turn the ignition on, the tach lights up and the needle sweeps up and back down, but it doesn’t detect any RPM signal.


For all the subsequent tests, I mounted the tach temporarily at the stern and connected its signal wire directly to the ignition coil, to completely rule out any problem with the boat’s wiring.


I’ll now check whether the ignition coil might be wired backwards.


It’s an OMC Stringer 800 with SelecTrim.


The ignition system is the original conventional type, with a distributor and breaker points.

 

[Ranger_Trivette]

So, I tested the tachometers.


The original one is broken, but the new ones work properly.


After consulting the electrician at my company, we think the problem could be caused by the condenser in the distributor. I’ll replace it, but first I’ll try adding a 1–10 kΩ resistor to the tach signal wire to see if cleaning up the signal helps.


Unfortunately, the alternator doesn’t have a W/tachometer output, so I can’t connect the tachometer there.


It has three wires: a large orange one, which is the alternator output, and two smaller wires, one red and one purple.


Any idea what the red and purple wires are for?

 

[Droid]

You are correct, orange is alternator output. The 2 wire connector (red & red/purple) go to the internal regulator.

Regarding the gray tach lead:
The OMC manual shows the gray lead going from the coil neg to the shift assist module but fails to show how it also gets to the main engine plug. Yet it shows the dash end of the harness going to the tach.
Back to the shift assist: Is it goofy/out of adjustment, faulty? Would that stop the tach signal? IDK. But for testing purposes only I would disconnect the ESA module & see it it restores tach function.

Regarding a tach signal from the alternator: On an outboard, since the stator is essentially the alternator & it sits under the flywheel there is no gear reduction. One revolution is one revolution.
On a belt driven alternator, there is a gear reduction thru the pulleys & belts. A worn belt tightened back up would ride lower in the pulleys than a fresh belt. A loose belt might slip at high alternator output. Each would affect tach accuracy. So, I've never seen that idea used.

And yes, #3 is the correct tach setting. Although 2 & 4 should read a signal but be inaccurate.

 

[louc]

Interesting problem
I am wondering about the cannon connector between the engine harness and the dash/instrument harness, I wonder if that grey wire has oxidized terminals inside the cannon connector? In my 1988 wiring schematic, on the negative terminal of the coil there are 2 grey wires, one goes to a connector that leads to the ESA module the other goes right to the cannon plug. Does yours have only one grey wire on the negative terminal of the coil?
On my boat (88 Four Winns 4.3 Cobra) the dash gauges are all original and still work (Medallion gauges) I bought a set of Faria gauges about 15 years ago and never installed them! The bezels were a different size in the dash which complicated the install a bit so I have been waiting for one of the originals to quit working but not so fast lol.
On the alternator, the 2 smaller wires on mine are Purple (Voltage sense wire) and Red/Purple (alternator excite). here's an odd tidbit. Don't know if it applies to yours as well but is common on some OMCs (I learned this from ARCO Marine). So in the sense wire circuit, there is a splice that apparently causes high resistance. This will cause the sense wire to read low voltage to the alternator's regulator, which then increases charging. At the same time the dash gauge reads more or less normal (13-14 v) but back at the batteries, it was 15.5 V! When I bought a new alternator from ARCO the instructions in the box described this problem. What they advised was to disconnect and tape up that purple wire at the alternator and run a jumper wire from that terminal (S) to the big B+ terminal on the alternator. With this change, charging voltage at the batteries was now normal. In fact, later OMCs and Volvos (Joint venture) are wired the same way. I thought of taking that splice apart and fixing it, but again, lazy and that fix worked fine now for like 10 years. Don't know if you had the same issue. Might want to check actual charging voltage at the batteries instead of "believing" the dash gauge!

It is possible to test condensers, you need a meter that measures microfarads, there is a spec for it in the OMC shop manual. For the V6 the spec is from .25 to .28 microfarads. I bet no one else has ever measured this lol. But I did out of curiosity. I hear comments that the quality of points and condenser sets isn't as good as it was, so maybe measuring is a good idea!
I think you probably have the same Prestolite centrifugal advance & points distributor that I have.

 

[makomark]

I dn't think think a 10K resistor is going to help, especially when its connections are described. And I don't believe the condensor is having an adverse impact on the tach.

See if you can get an oscilloscope from the same source you got the signal generator from...and then see what appears on the "-" terminal of the coil as then check the tach's signal input lead.

Many times, the input circuitry used by the tach maker is far from optimal ,especially for older ignition systems...there are a few ways to "clean up" the tach signal so they are recognized. But until you know the signal provided (waveform details), its hard to make progress...

 

[Ranger_Trivette]

Big UPDATE

I did some tests.

• The original tachometer is broken. Giving it an impulse doesn't make it show anything.
• The first tachometer I bought works. The needle moves if I repeatedly tap the signal wire against 12V. By accident, I set it to 1.0 instead of 3.0. Connected to the negative terminal of the coil, it reads about 2100–2200 RPM. However, if I set it to 3.0, it stops reading completely and drops to zero.
• The second tachometer I bought goes full scale, both when set to 1 and 3. I tried increasing the reduction factor a lot. At around 20–25, it suddenly drops to zero. I couldn't find any setting where it shows an intermediate value: it's either zero or full scale.

I’m attaching a photo of the tachometer showing 2100 RPM.
Great success! Finally, a step forward!

Stupid question: is it possible that one cylinder isn't firing, and that this is causing an irregular tach signal? The engine should still run on 5 cylinders.
The boat seems to run correctly, but it doesn't reach the speed it should. That's why it's important for me to get the tachometer working: I need to understand whether the engine itself isn't reaching the correct RPM, or whether the problem is somewhere else.

You are correct, orange is alternator output. The 2 wire connector (red & red/purple) go to the internal regulator.

For now, I’ll leave the alternator replacement idea aside.
Having a W terminal output would be useful, but then I’d still have to deal with the ratio between engine RPM and alternator RPM, because of the pulley ratio.
For now, I’d rather postpone those tests.

Regarding the gray tach lead:
The OMC manual shows the gray lead going from the coil neg to the shift assist module but fails to show how it also gets to the main engine plug. Yet it shows the dash end of the harness going to the tach.
Back to the shift assist: Is it goofy/out of adjustment, faulty? Would that stop the tach signal? IDK. But for testing purposes only I would disconnect the ESA module & see it it restores tach function.

Regarding a tach signal from the alternator: On an outboard, since the stator is essentially the alternator & it sits under the flywheel there is no gear reduction. One revolution is one revolution.
On a belt driven alternator, there is a gear reduction thru the pulleys & belts. A worn belt tightened back up would ride lower in the pulleys than a fresh belt. A loose belt might slip at high alternator output. Each would affect tach accuracy. So, I've never seen that idea used.

The ESA is working correctly.
The wiring diagram doesn’t show exactly how the gray wire reaches the dashboard, but I assume it’s simply a wire that goes from the negative terminal of the coil to the connector of the main 10–12 wire harness, and from there it just continues forward to the dash.

And yes, #3 is the correct tach setting. Although 2 & 4 should read a signal but be inaccurate.

I’m not sure why, but as I wrote above, based on the tests I did today, I only get a readable signal when it’s set to 1.0.

Interesting problem
I am wondering about the cannon connector between the engine harness and the dash/instrument harness, I wonder if that grey wire has oxidized terminals inside the cannon connector? In my 1988 wiring schematic, on the negative terminal of the coil there are 2 grey wires, one goes to a connector that leads to the ESA module the other goes right to the cannon plug. Does yours have only one grey wire on the negative terminal of the coil?
On my boat (88 Four Winns 4.3 Cobra) the dash gauges are all original and still work (Medallion gauges) I bought a set of Faria gauges about 15 years ago and never installed them! The bezels were a different size in the dash which complicated the install a bit so I have been waiting for one of the originals to quit working but not so fast lol.

Everything you said looks correct to me.
I have two gray wires: one goes to the ESA module, and the other goes to the cannon plug.

On the alternator, the 2 smaller wires on mine are Purple (Voltage sense wire) and Red/Purple (alternator excite). here's an odd tidbit. Don't know if it applies to yours as well but is common on some OMCs (I learned this from ARCO Marine). So in the sense wire circuit, there is a splice that apparently causes high resistance. This will cause the sense wire to read low voltage to the alternator's regulator, which then increases charging. At the same time the dash gauge reads more or less normal (13-14 v) but back at the batteries, it was 15.5 V! When I bought a new alternator from ARCO the instructions in the box described this problem. What they advised was to disconnect and tape up that purple wire at the alternator and run a jumper wire from that terminal (S) to the big B+ terminal on the alternator. With this change, charging voltage at the batteries was now normal. In fact, later OMCs and Volvos (Joint venture) are wired the same way. I thought of taking that splice apart and fixing it, but again, lazy and that fix worked fine now for like 10 years. Don't know if you had the same issue. Might want to check actual charging voltage at the batteries instead of "believing" the dash gauge!

I basically rewired the electrical system from the cannon plug forward.
I’m attaching two photos. I now have two voltage displays that read directly from the two batteries. They’re mounted just below the steering wheel in the photo, so they’re not very easy to see.
And yes, while charging, they actually show 15.5 V.
I’ll definitely keep this modification in mind, but I’d prefer to postpone it until this winter, since it’s still warm here and I’d like to keep using the boat for another month or so

It is possible to test condensers, you need a meter that measures microfarads, there is a spec for it in the OMC shop manual. For the V6 the spec is from .25 to .28 microfarads. I bet no one else has ever measured this lol. But I did out of curiosity. I hear comments that the quality of points and condenser sets isn't as good as it was, so maybe measuring is a good idea!
I think you probably have the same Prestolite centrifugal advance & points distributor that I have.

The boat is from 1985, but it’s actually almost like new. It only has about 50 hours of use.
The distributor looks to be in excellent condition. I’m not an expert, of course, but it looks almost new.
I can’t say for sure whether it’s the original distributor or whether it was replaced at some point for some reason.

I dn't think think a 10K resistor is going to help, especially when its connections are described. And I don't believe the condensor is having an adverse impact on the tach.

See if you can get an oscilloscope from the same source you got the signal generator from...and then see what appears on the "-" terminal of the coil as then check the tach's signal input lead.

Many times, the input circuitry used by the tach maker is far from optimal ,especially for older ignition systems...there are a few ways to "clean up" the tach signal so they are recognized. But until you know the signal provided (waveform details), its hard to make progress...

The resistor had no effect at all, just as you suggested.
The tachometer is the only thing connected between the distributor and the coil, which is why I started wondering whether it could somehow be related to the problem.
Today I finally managed to get a readable signal, but if I divide it by three, the reading disappears completely.
Tomorrow I’m going there with a multimeter that can measure frequency. That’s about the most I can do for now.
Let’s see what frequency I get.

 

[Droid]

Back to the subject of pulse setting switch on the tach:

In the early days (mid 70s & earlier), you bought a tach for the particular engine size you were working on.
Later the "one size fits all" tach came to be. Per OMC your tach needs to be on #3. I DO NOT KNOW IF #3 IS CORRECT ACROSS THE THE INDUSTRY OR ON OMC BRANDED INSTRUMENTS.

I have a friend that does some boat restorations & he calls me to do some I/O installs & wiring. Customers buy the cheap Amazon instrument kits. I installed 1 & will not do another. I can't read Chineese & I won't deal with trying to splice 16 gauge boat wires to the 20 or 22 gauge little pig tails provided. You wont get a durable connection & all the bouncing around in a boat will just cause the cluster of poor butt splices to fail quickly.

You've got a lot of time in what should be a simple issue & you sound knowledgeable. Just buy a quality instrument. Actually, I don't understand why you decided to upgrade the whole dash.

 

[makomark]

I doubt the multimeter will provide the necessary fidelity of waveform details.

Looking at the taCH ON THE 'NEW' photo, I don't see a brand name on its label...which is a red flag here, too ( good telltale for what Droid calls Chineesium).

The 'old' tach likely didn't work as it was an outboard tach...most of them work off the alternator/stator output...