I have a 1985 and the speedometer quit working, it stays at 0 and the bar graph does not move either. I have gotton an intermittent code for the TPS. Does the TPS have anything to do with it??? I do have the FSM but can't seem to find anything on speedometer repair. Thanks for your help

I'm not gonna steer you in the wrong direction, so I'll have to double check my FSM to give you an definitive answer. I would first check that the connection to the vehicle speed sensor (VSS) is connected on the rear driver's side of the transmission, and/or doesn't have any breaks in the wiring. That would be my first inclination. I would also make sure my grounds are good. It could also possibly be a problem with the CCM, but I doubt it.

As far as the TPS, probably not related, but that's another thing you should test and repair.

Matthew

PS - how long has this been going on?

I tested the speed sensor and it was out of range according to the FSM so I replaced it and the connector looked fine, however it did not fix the speedo. It hasn't worked for about a month now. thanks again

Sorry, I haven't checked the FSM yet, but that looks like your culprit. I honestly didn't know you could check the VSS with diagnostics. I would double check the connector to make sure it's on all the way and check the wiring for any breaks. Also, although it's hard to install it upside down and still clamp it down properly, make sure it's installed the right way up (not upside down).

What did you replace the VSS with? OEM/aftermarket, used/new? There are also several different sensors, make sure you have the right one. Make sure it's installed all the way and the gears all mesh. (When replaced did you put the gear back in?)

Here's a link with some info on my VSS when I did my rear end swap:

http://www.corvette-guru.com/modules/ ... =ASC&type=&mode=0&start=0

Good luck,

Matthew

If your throwing a TPS code Replace the TPS it's easy the hard part is setting it to .54v. I have an 85 also.

On a side note WECLOME ABOARD.

Do the mpg readings work?

Does the car have the original rear end ratio? I'm wondering if someone changed the speedo gears or sensor to the wrong ones and the gears chewed themselves up.

The speedometer does effect some of the other sensors on the car, but I don't think the TPS is related.

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Do NOT simply replace the TPS out of hand because of the code. It is too easy to test to just throw away without knowing. The code could have been caused by something as simple and cheap as a poor electrical connection.

A code gets set when the ECM receives an out of parameter reading. That is a reading that doesn't match the readings of the other sensors, something out side of the expected range. The possible causes of codes are multiple. Trouble shooting is necessary to determine the cause of each code. So often I see posted, "The code (computer) said I had a bad XYZ sensor. There are no codes that call for the replacement of anything.

RACE ON!!!

CFI is right. You should test it first. Mine went bad and didn't throw a code.

The biggest problem with early C4's is wiring. The insulation shrinks, dries out and causes bear wire to appear in all the wrong places.

It also chaffes from wear, heat and chemicals.

I would, in addition, check the wiring.

A friend has a C4 with a 4+3 and his OD unit stopped working - the entire OD harness was toast. Worn away by heat, grease and such.

Yes that entire harness is prone to burning on the exhaust if its not secured properly.

Gm speedo electrical is tricky.

Normal applications have a 4-out buffer. Speedo in, one wire out to the ECM, another to the dash.

So the two wires (purple and yellow) from the VSS go into the buffer. Brown wire is 4k PPM output to the ECM. Green with black stripe goes to the cluster. Black/white is ground, and pink/black is power.

However, on your '85 the cluster IS the buffer, and sends a 4K ppm signal to the ECM. The good news is, the "speedo" and "buffer" are independent circuits.

My starting point would be to plug a scanner into the ALDL port and see if you see a valid MPH while driving. This would prove the VSS good. If you don't, it still could be the VSS signal, the buffer, or wiring.

I have scanned copies of the manual, as well as notes if you need help trouble shooting.

C4 electrical is a pain. I abandoned my dash swap on the '87 I used to have because I just didn't want to be chasing down gremlins for the rest of my life.


-- Joe

Thanks for all the info, I have a 4 day weekend coming up and will be working on this. I will plug my scanner in and check the VSS and chase all the wiring down.

The TPS is at .54v currently however when I push it to WOT the voltage is over 7v and that seems high, sometimes the car will cut off when I am at WOT. I will keep working the adjustment and see if I cant get it closer to spec.

Thanks again

Quote:

Hrmm.. If it was my car I'd probably consider repinning the ECM connectors to run a later ECM, like a 165 or a '730.

The 1985 ECM ('870 right?) is a turd. Next time you see a '89 being parted out, offer $40 for the ECM. It will delete the cold start injector, and you'll have a slightly more aggressive advance curve to boot.

-- Joe