Speedo moves with RPM

Double-A

Double-A

Quitter
I have a feeling I'm going to answer my own question here, but I wanted to see if anyone else had this happen.

In park and revving the engine, the speedo climbs to ~10mph, then falls back when allowed to fall back to idle. I'm assuming I have a bad ground somewhere (great) but wasn't sure if it could be an ECM issue instead. Keep in mind that the truck isn't 100% healthy after it's spontaneous combustion event, and my OEM boost gauge is constantly pegged also - perhaps related? All other OEM gauges seem fine, and the alternator (although a few years old) has ~5 miles on it. The truck is almost running well, but still has a burp when getting on the gas.

Like I mentioned, just curious if anyone went through this before, and if there was a best place to start tracking this down. I do have a spare cluster to swap if need be.
 
Slyclone

Slyclone

Well-known member
Re: Speedo moves with RPM

I tried to PM you, you are pretty close to where I am.

I am not 100% sure but some where along the lines I recall tach bouncing issues and all that happy stuff with something linked in the distributor. Like I said I'm not 100% sure some one should be able to jump in on the fun and get you in the right direction.

Or you could have a bad gear in cluster or something is not set right. The tach issue could be electrical where the cluster issue with mph could very well be mechanical. I think one end is gear driven in the cluster. I never figured out where the other end went .
 
DaveP

DaveP

Active member
Re: Speedo moves with RPM

Double-A said:
Like I mentioned, just curious if anyone went through this before, and if there was a best place to start tracking this down. I do have a spare cluster to swap if need be.
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I haven't had this particular problem, so I'm just going to make some educated guesses about your issue. None of the gauges have anything to do with the ECM. The boost gauge is really just a volt meter that reads the MAP signal on the grey wire. So don't get sidetracked thinking the ECM may be causing this. It isn't.

Are the speedo or tach, or any of the gauges affected by turning on, or dimming the panel lights, or the turn signals? If so, this would indicate a ground problem. If not, I would lean more toward a cluster or motherboard problem.

The ground for the cluster eventually ends up at the big black wire at the front of the intake manifold. It may also have a poor connection at the bulkhead connector. If turning on other stuff in the cluster doesn't affect the problem it isn't a ground problem.

Let me know if you need more.
Dave
 
randy merritt

randy merritt

Donating Member
Re: Speedo moves with RPM

DaveP said:
I haven't had this particular problem, so I'm just going to make some educated guesses about your issue. None of the gauges have anything to do with the ECM. The boost gauge is really just a volt meter that reads the MAP signal on the grey wire. So don't get sidetracked thinking the ECM may be causing this. It isn't.

Are the speedo or tach, or any of the gauges affected by turning on, or dimming the panel lights, or the turn signals? If so, this would indicate a ground problem. If not, I would lean more toward a cluster or motherboard problem.

The ground for the cluster eventually ends up at the big black wire at the front of the intake manifold. It may also have a poor connection at the bulkhead connector. If turning on other stuff in the cluster doesn't affect the problem it isn't a ground problem.

Let me know if you need more.
Dave
Click to expand...

Quote from Star Trek IV. Dave is the Sy/Ty Spock, he knows his stuff:

"Kirk: Mr. Spock, have you accounted for the variable mass of whales and water in your time re-entry program?

Spock: Mr. Scott cannot give me exact figures, Admiral, so... I will make a guess.

Kirk: A guess? You, Spock? That's extraordinary.

Spock: [to Dr. McCoy] I don't think he understands.

McCoy: No, Spock. He means that he feels safer about your guesses than most other people's facts."
 
Don W.

Don W.

Stab it and steer it
Re: Speedo moves with RPM

randy merritt said:
Does the DRAC control the tach as well as the speedo?
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Nope, looking at the wiring diag, not as far as I can tell. Subject to correction... Hi Dave.

(I've met Dave, he's cool.):tup:

Edit: The grounds are common to everything on the panel but they are also "connected" through the box the drives the check-gauges light. I don't know where that is located but, and it's a rather long shot. if it can be disconnected try that. Proceed at your own risk.
 
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Jimmy

Jimmy

Banned
Re: Speedo moves with RPM

When my speedo was jumping all over the place,I checked the DRAC connection and it wasn't fully seated.Plugged it up and problem disappeard.
 
randy merritt

randy merritt

Donating Member
Re: Speedo moves with RPM

Jimmy said:
When my speedo was jumping all over the place,I checked the DRAC connection and it wasn't fully seated.Plugged it up and problem disappeard.
Click to expand...
Speedo I understood, just not the DRAC to tach connection.

EDIT: sorry, I am an idiot. This conversation was never about the tach.
 
Double-A

Double-A

Quitter
Re: Speedo moves with RPM

Still have it, 9 years after the fire and still not roadworthy. Nobody looking for a deal and small headache I guess.

Stumbled across some things, wanted to post an update since who knows when I'll get to work on it next. (Maybe next year...)

Pulled ECM/DRAC, everything looks normal. Swapped cluster with an extra I have laying around, no difference. Then I got to wondering...

I had to replace the engine harness due to the fire, and I had to splice the green & purple wires (which I believe go to the speed sensor) due to the used replacement harness having them smashed. Perhaps I did a sucky job of splicing them. (It was tight, and many years ago, anything is possible I guess.) Guess I can rip everything apart yet again and look at/solidify the splices. Not sure how easily the wires come back through around the trans and tunnel, but maybe I'll go that route. (I spliced them with the wires still attached and tight to the firewall.) The only other option I can think of is bad DRAC - when dad gets his truck back from the shop (long story) I can borrow his DRAC, although I've read it's rare that they fail so I'm not optimistic about that fixing it. More likely my screw-up.

The boost gauge didn't work (move) on cluster #2 as well, so I'm going to test the map sensor when I get a chance. I don't have a code thrown, but it's pig-rich, stumbles on throttle, and doesn't register boost to the oem gauge. (Aftermarket not installed yet, but I should have one around to double-check.) It should be a quick test, but it's 1:30am and I gotta sleep some - project for another day.

Just moving it around the driveway not running right, I can tell she's gonna fly. (I've never driven her healthy.)

Thanks everyone for the posts!
Aaron
 
T

TurboSBC

New member
Re: Speedo moves with RPM

IIRC, most, if not all of my S10s and Blazers over the years did this. Even my 2nd Gens have done it :dunno: I always assumed it was supposed to happen.

Has anyone tried it to see if it's not a single, isolated incident?
 
Double-A

Double-A

Quitter
Re: Speedo moves with RPM

I've never noticed it, but since I bought mine not running, it may have always done that and I just never knew. It only started to move above 4k rpm, so I guess as long as it runs "no harm, no foul".

I'd be curious to find out more from others. Not considering it a dead issue yet.
 
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