I have a 1993 Silverado Extended Cab C3500HD Dually with 6.5 Diesel engine. The tachometer does not work. The manual says nothing about it.

I would like to know how to diagnose the issue. If you've had experience with this model, please let me know.

Thank you.

 

Did it suddenly quit or did it seem to be acting up prior to quitting? Any other gauges not working or cluster issues?

The tach gets its signal from the white wire on the alternator. This terminal gets the AC voltage before it’s rectified to DC, so the easiest check to start with is to check the AC voltage at the alternator with the engine running. I don’t know what the voltage should be, but If you have a signal, the wiring or instrument cluster is likely to blame.

You could also pull the alternator and have it checked by an auto electric shop if you suspect it to be the problem - they would have access to specific technical info. This would be a shop that specializes in rebuilding alternators, starters, and other electrical components.

 

Thank you very much.

This old truck was used by a couple to pull a 5th wheel trailer. One day, they had an alternator fire so they were towed to a campground. The fire had burnt the connector at the alternator. They bought a new unit and added a new connector pigtail.

When they sold the RV and truck, another person had it for awhile before me. There is no knowledge that it worked after the fire. The cruise control and the radio also do not work. The radio turns on but noting works and no sound. Everything else on the dash works fine. I suspect that these issue all stemmed from that fire, but cannot say for any certainty. Perhaps some sort of short.

That new replacment alternator stopped putting out enough and had a bad regulator. It was just replaced and there are two new batteries in this diesel. All has been retested and working great.

Got a used cruise control module since the diagnostics tree said it was the issue. It wasn't. Still doesn't work. Back to the manual.

However, the tach appears to be a simple device but not mentioned in any of the manual DVD. I do not see a white wire at the alternator, but will look again.

So your contention is that the tach is simply run by the signal from the alternator. Is that correct?

Thanks again.

 

To clarify what I meant:

The tach signal is received from the alternator, however, the gauge itself is operated using a stepper motor and must have power and ground to work.

Everything in the cluster shares a common power and ground feed, so given that all the other gauges are working fine, it’s certainly possible the fire shorted out the tach stepper motor without damaging the rest of the cluster.

If the pigtail for the alternator was replaced, you’ll probably have to open up the wiring harness to find the splices and look at the original wiring colors to see which terminals they line up with. I can only tell you what the wiring diagram shows me - they are posted below.

The first is the alternator end in the engine bay, the second is the tach end in the cluster (the circuit is hard to trace so I marked it), and the 3rd is the cruise module and switch. If you know your way around a meter, these circuits should be easy to trace.

About the cruise - try pulling the brake pedal back toward you and see if the cruise starts working. It’s very common for the brake switches to wear out and not fully return to a released state - it’s enough to turn off the lights but not clear the signal to the cruise or PCM that the brake pedal isn’t being pressed.

 

Thank you for the good info. Very useful.

 

You’re welcome. Were you finally able to resolve the tach and cruise problems?

 

I have been very busy on other things, but have noticed a couple factors. One is that when I washed off my engine at the manual car wash, the tach started working. That would suggest an open circuit in the engine compartment area.

I am in the process of rewiring the major cables, moving one battery and adding a heavy bus bar for expansion. When that is finished, I will wet down the various cables to find that open circuit.

Thanks again.

 

You’re welcome. Thanks for remembering to keep us updated.

EDIT:

One other thing to check is the pass through connections at the firewall. It’s not uncommon for them to loosen up over time, and it would be an ideal place for water to penetrate. I’d expect other electrical issues if that were the case, but I’ve seen stranger things happen where water was the cause.

 

I do not necessarily think that water is the cause. Rather that a loose connection exists or a fusible link is broken or anything where an open circuit is closed when water completes the connection.

 

Okay, here's another update. The instrument cluster was sent off for diagnosis and repair. The odometer had stopped working and so the shop replaced the motor for it. (It is interesting to note that the tripometer still was working.)

The whole thing was completely checked out, including making sure the tach worked. When reinstalled in the dash all is good except that the tach still does not register.

All I know is that if I take it to the manual car wash and wash down the engine compartment, the tach will work for a short time.

So I just decided to try the water trick with a spray bottle. I sprayed one section at a time. No matter where I sprayed, it did not start working. I really tried to saturate each section of wiring, especially toward the rear of the compartment. No joy.

This is bizarre.


View of firewall wiring.
[img alt="
Alternator wiring."]https://cimg0.ibsrv.net/gimg/chevroletforum.com-vbulletin/2000x1504/img_1361_4fdddd096131524c734b8051416ad79c9cf8da03. jpg[/img]
Alternator wiring.

The new wiring is working great, so the issue is somewhere in the old stuff.