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Background: I don't know much about electricity, and my truck is in the process of getting restored. My truck originally had the full gauge package, which includes the tachometer.
Problem: The tachometer on my truck is just - DEAD. For the last few years, the tachometer has only worked when it's warm, or not at all.
Troubleshooting: I tried two different instrument cluster assemblies in much better shape than mine, and the tach is dead on both of them. The rest I am going to relay to you what was told to me by the mechanic at the restoration shop:
The "TACH TEST" wire on the coil and the tach plug off the Duraspark ignition harness were both tested and are fine. The problem was traced to a module located under the dashboard. He told me it was defective because power was going into this box, but it wasn't coming back out. I researched this module and what he is referring to is the cruise control amplifier. That sort of makes sense, because my cruise control hasn't worked in years.
But what does the function of the tachometer have to do with the cruise control amplifier?
Results: I picked up a used one from the junkyard and tried it, but the tach is still dead. Then, I purchased a NOS module from Ebay and tried it - but the tach is still dead.
Now, I find it hard to believe BOTH instrument clusters have a dead tach, and BOTH cruise control amplifiers are defective.
IIRC the tach leads have absolutely nothing to do with the cruise control module, that shop is most likely snowing you (but I don't have CC on my truck so can't verify that).
The wires are black with green dots or green hash or some such, and they're sort-of add-on wires.
It is LOTS easier to do this wtih the dash cover removed, but I would suggest you physically trace the wires, there are only two of them and they're both in one connector that houses only those wires, that connector is near the DS2 box (or where such a box would be) on the driver's side wheel cover under the hood.
Crap, I had all kinds of good pictures of this stuff but my picture-hosting company went to a for-pay service and none of my pictures work anymore.
Lariat, do the other gauges work with the clusters that you tried? I don't know alot
about these clusters, but some of the other members possibly can help.
Chris is right - it has nothing to do with the cruise control. Here's a link to the EVTM: Start, Ignition, & Carb Circuits - Gary's Garagemahal. Page 19, the second page, shows the tach and its wiring. And the two wires at the bottom go to E and F on Page 18 and right to the DS-II module. E is the ground that makes it read correctly for an 8 cylinder, so it isn't a problem at the moment since even if it was bad you'd have a reading. So, concentrate on F. Do a resistance test from it at the tach or the connector going to the tach and again at the ignition module.
The speed control amplifier has nothing to do with how the tachometer works. Some speed controls did get a tach signal to kick out if the engine ever over reved(ie clutch pressed in manual...) but I don't think these year Ford's had that.
The tach signal comes from the neg side of the coil and is pretty much a direct run to the tachometer's input. Check the wire's resistance from the coil neg side to the tach's input, should be less than 5ohms. I don't have the color code with me but I recall its a green wire for tach signal.
If you can still run the truck engine in it current condition I would do some voltage checks also.
Put the positive lead of the volt meter on the "TACH TEST" (green wire - coil negative)
The voltage you see at idle and other different RPM's should be the same at the coil neg AND the tach post on the tach itself.
While you are at the back of the tach, put the meter leads on the Neg and positive post of the tach. You need power, ground and a tach signal to make the needle move, and in your case and extra ground on the "8" post for the tach to read correctly for a V8 engine.
Personally I'd take your gauge cluster out and examine the PCB and the inside of the tach. I suspected mine would have some glue causing a short across circuts on the main gauge PCB or on the circut board of the tach, clean as a whistle. Turns out MY tach wasn't working because someone bought a factory tach, didn't have the matching PCB and they never wired it in!
Is your tach actually hand wired in, or are all the proper connections on the main gauge flex circut board? Thats where I would start
But his used to work then stopped. He also tried two other gauge clusters and got no tach response from any of them. Pretty much means his problem is not the cluster.
I think I tried 4 factory tachs that at best worked intermittently. I finally had Rocketman Classic Cougar replace the guts with new and now it works great. Had the ammeter changed to a voltmeter at the same time. Couldn't be happier.
I didn't see where it was verified the other tacs worked before your testing.
On mine that did not work, pulling the cluster reveled the cab bracing actually wore a cut in the PCB right at the hump for the tac.
Soldered in a small jumper and all if fine now....
The original tachometer cluster I had (200.000 miles!) wouldn't work at all when the weather was real cold, but would work better (the needle would move but it would "stick" for a while) as it warmed up and then it would work perfectly fine in hotter weather. All the other gauges work fine.
I put in a very low-mileage instrument cluster in, but I have the same results.
I tried a third instrument cluster with low miles, and again, the same results.
My restorer/mechanic did some voltage checks and determined that the "tach test" wire on the coil is good, and the tach wires attached to the Duraspark ignition harness were good.
He traced the wires and once again the tach wires led to the cruise control amplifier box, and from there to the cluster harness. We replaced the cruise control amplifier box TWICE, with the same results as above. I know you guys told me that the tach has nothing to do with cruise control, but it was explained to me that in order for the cruise control to function, it must be able to read what the engine is doing?
He doesn't know what else to do to fix it, and I don't understand electricity enough to help him.
Does anyone have a wire diagram for vehicles with cruise control, or any other ideas?
I already suggested you/he try and physically trace the wires again as they are not related to the cruise control stuff at all... here's proof, scroll down to Page 67 for a diagram of the Speed Control unit (the Servo is the component under the hood; the other box is mounted on the steering column inside the cab):
