I will let you know. I will probably start working that problem in about 2 weeks.

 

well I finally got back to the problem --- initially my diagnosis was the transmission speed sensor -- which I replaced (autozone $37). This didn't fix the probem. When I first turn on the switch the speedometer will read 0, once I start the car randomn numbers just start appearing in the speedo. With only the switch on I'm getting 12VDC from the battery but once I start the car I' getting 12VDC from the alternator and I think the ECM is getting more involved in the process. My leaning for now is exactly what the gentleman posted in this thread earlier -- the ECM. When I bought the car I noticed the ECM was not firmly mounted in its bracket (no screws), it was just laying in the bracket so for some reason it has been messed with before. Unfortunately I cannot speak to the previous owner to get to the bottom of that issue. I first intend to make absolutely certain I have the right part number ECM --- then if I do -- get it rebuilt. I will keep you guys posted..

 

the ecm is not usually the problem. mostly ground issues. the correct ecm part#is 16159278. the ecm is anchored with a post at front center that slips into bracket and 1 bolt at center on each side. nothing in the back where wire harness connects. the chip inside ecm is removable so you can use the proper one for standard or automatic trans. the ecm runs $300 plus on ebay, used. take it to a store that will send it to be repaired. this is the ONLY ecm that fits a 92 corvette , it can be found in chevy impala and z28 ,pontiac trans am with lt-1 engine. on the speedo , my problem was the wiring to the speed sensor on trans was corroded. i removed as much old wire as i could and replaced it. i removed pins from connector at speed sensor and replace wire all the way. its working. keep us informed please. chuck

 

thanks Chuck,
I will check tomorrow to make certain it is the correct P/N. I visually checked the wiring when I installed the new speed sensor and it all (including the sensor connector) looked in great shape. Be fore I ship the ECM off for rebuild I'm going to do some continuity checks and ground cleaning. I will keep updates on the search posted.

 

the corrosion is not visable, try stripping the insulation off near the connector, you will probably find the copper braids are black. this changes the amps. of signal. also there are several grounds on a bolt above the starter and several grounds on a bolt on back side of the left cyl. head. both are hard to reach even with car on a lift. , chuck

 

I flipped the ECM over to check the part number and found some interesting infomation. The first sticker said "Use your original Prom". The second sticker said: "Made in Mexico" and had the following numbers --- ECU --- EM9278-----0 91769 47525 3----577 1908 09929 850. I noticed the last 4 numbers of the EM9278 number do correspond to the last 4 numbers of what you listed as the correct ECM. Any ideas? Meantime I'm checking grounds, wiring and connectors.

 

yes its probably correct ecm . i have seen remanufactured ecms with similar part numbers usually ending with last four numbers of gm part number. my guess is previous owner tried to fix same issues your working on. the only problem ive ever had with an ecm is i ignored a check engine light too long (as did the previos owner) and burned out a quad internally.

 

I too suspect what you suspect --- the previous owner worked this same issue. Unfortunately I can no longer get in touch with him. My gut feeling is that it isn't the ECM. This weekend I plan to get into the wiring -- grounds -- connectors, etc. I will post again on Sunday with my results.

 

do you have a factory service manual? very helpful and confusing. there are more grounds than you can believe. good luck

 

If the 92 is an LT5 then P/N 16159278 would be wrong.

If its a base model 92/93 car then P/N 16159278 should work.
If the car is a ZR-1 the P/N would be 16163993

 

researched the shop manual thoroughly: the speed sensor on the transmission sends a signal to the ecm --- this same signal is then used for SCV (sound compensated volume), cruise control and through the CCM to the speedometer. All 3 work correctly except for the random numbers that appear on the speedometer prior to the car starting to move. While the car is in motion the speedometer works perfectly. Once I come to a stop with the motor running the random numbers start again -- I've looked - there is absolutely no pattern to the numbers. I've ran diagnostics on the ccm and ecm with no codes noted. I replaced the speed sensor on the transmission - no change. With the ignition switch on, the speed reads 0. As soon as I start the engine the numbers start appearing in the speedo. This weekend I unplugged the connector at the speed sensor -- ignition on---reads 0, start engine -- still reads 0 but of course the speedo doesn't work. Pulled the battery and cleaned all terminals --- removed the grounds from the side of engine block and cleaned thoroughly (according to the shop manual, these are the grounds for the ECM). Put it all back together and no change, problem still there. Everything on this car works perfect except the speedo problem and the LTWPS advisory light, But even this goees out once the car starts rolling a couple of hundred yards -- I'm pretty sure this is caused by the LTWPS module behind the instrument panel that is suppose to retain the memory of the tire pressure when the car is shut off. But this speedo thing has me stumped. Now I'm leaning (not too heavily) towards the CCM even though all of its other functions work perfectly as the signal goes from the ecm to the ccm then to the speedo --- this is an unusual one that currently has me stumped ---- ANY IDEAS?: Let me add this one other note: when the speedo numbers are jumping around so are the instantaneous MPG numbers right beside the big numbers. But they also start reading correctly once the car is moving -- could this be an instrument panel issue? *****:

 

maybe try taking fuse for radio out and check speedo and disconnect cruise and check speedo?? anything on that circuit with a weak ground will feedback. im thinking, just like a bad ground on your taillights you can get a running light and a reverse light when your expecting running lights. YOU GOTTA HAVE PATIENCE WITH THESE CARS! i worked on mine all day today replacing the optispark. when its right , i like driving it.

 

your definately right about the patience. I will try your ideas and have also thought of another possibility. It may have been coincidence but today I was sitting in the car while it was running, watching the speedo jump around -- numbers as low as 3 and as high as the 80s. I started bumping around with my fist on the drivers side of the console, around the area that covers the CCM. The numbers started going from 3 as the low but never any higher than the 20s. I shut the car off, retarted it and tried the same thing---exact same results. I plan on going into the CCM area and check connections and any grounds in the immediate area tomorrow if time permits. Thanks again for the thoughts!
BTW: how difficult is the opti-spark work? It looks rather close around there. That ground on the engine block was a bear.

 

well , don't put water on it like i did and you'll be alright. had the alternator off to rebuild it and thought i'd clean the area under it while it was off , bad move! but for me it might have been a good thing because the bearing was on it's way out. who knows what would have happened. took some time , not as bad as some make it out to be. drain the antifreeze, take belt off, remove water pump(6 bolts), remove damper pulley(3 bolts) just tap it off from under car using a peice of 3/4" x 3/4" x 12" wood and small hammer. remove optispark. be sure to label spark plug wires. i added vent system to opti as outlined at www.corvettefever.com/howto/16758. check it out.

 

I am still saying your ECM is causing all your speedo problems.

 

I will definately remember "no water". It sounds like another "patience" job. Sounds like this would be a good time to replace the front main crankshaft seal if you were having any leaks there.

 

That was my very first thoughts, but the fact that all 3 functions of the speed sensor go through the ECM and work correctly and the fact that the ECM has already been replaced once ( I suspect trouble-shooting this same problem) has steered me away from the ECM. Is there any particular reason you suspect the ECM?

 

well did you resolve the speedo issue?

 

Not yet. Here is the latest. I got down to the CCM which is buried in the bowels of the instrument panel and found everything looking brand new. I cleaned the pins and both connectors that go into the CCM and reinstalled. To take out the CCM required removing a number of other components including the am/fm/cd control head. The CD had been acting up so I decided to have it restored by DR Dons in Texas (he is working on it now). I was also so deep into the dash that I went ahead and removed the LTPWS module and have a replacement on the way (hope that fixes that light). Lastly I spoke with a gentleman (can't remember his name) that was recommended to me by an ECM rebuilder. This guy is a vette expert from way back (he helped write the shop manuals) and he is convinced (after a 20 minute conversation which he charges you for) that the problem is in my ECM. So, I sent the ECM off for rebuild as well. The parts should start coming back in this week and hopefully I can get it back together this weekend with all the problems solved. I will keep you posted.

 

OK, here is the latest. I got back my AM/FM/CD freshly rebuilt from Dr Don's in Dallas. I also had him add an MP3 jack. Got it back and already have it installed. I also bought a used LTPWS module from a corvette parts guy in North Carolina ---- already installed it. I also had K and B Specialties in Alphretta Georgia rebuild my ECM --- already installed it. Here are my results:
1. CD player works like a champ -- the ejection problem is cured.

2. I no longer get the LTPWS service light -- the system works like it is suppose to work.

3. Even with the newly rebuilt ECM, the speedo continues to give random numbers once the engine is started until the car starts moving and then it reads correctly until I once again come to a stop.

I took the drivers side kick panel off and cleaned the grounds and got no change.

There are grounds on the passenger side under the kick panel and I will clean them tomorrow but I don't think that is the problem.

At this point the speedo problem has me whipped.