85 vette digital dash issue. speedo not working

Ranman

Ranman

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2 You're 10
Aug 21, 2013
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ancaster
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1985
Someone please talk me off the ledge!! I bought this car a couple months ago and for the life of me I cannot get the speedo to work!!
I have had the dash out a couple times. Replaced all the bulbs, checked all the connections. I originally thought it was the vehicle speed sensor on the transmission so after a month of looking for a part that GM discontinued making I finally found one. I had it installed today and,,,same issue,,, the speedo still isn't reading. it is stuck on 0. In the meantime I am using my GPS to read my speed but now its personal. I need to find out how to resolve this.
On a side note, my odometer is stuck and no longer moves. Also on the trip setting it does not move? somehow they must be connected. I don't really trust the gas guage as it never goes below half(even after putting in a new sending unit a couple weeks ago.
oh ya, my check engine light comes on after being on the highway for a few minutes but I don't feel any difference in the performance. took it to a GM dealer today and they said, sorry, they don't have a scanner t read the codes on a car that old.

any ideas??
 
I sure wish I could help you with the dash problems. But I just don't know what to tell you. I do know that the early C4 star wars dash has been a source of many a head ache. There is a number of places that rebuild these things. Often there is trouble with the power supply and solder joints on the circuit board. Just a thought.

As for checking to see what code is being set. Have a look at this page. Much easier than me typing it here for you.

C4 Corvette Diagnostic Code Recovery Techniques

I have an old Snap-on MT2500 (red brick) Diagnostics Scanner. Not much good for anything today, but works great for our older C4's. These can be had at a reasonable price today, but they must have complete up-to-date software with them or they are useless. Software is no longer available.

From the sounds of it. You cruise along, engine light comes on, but car is running just fine ? I will almost bet that is setting a code 32 (egr). There is a temperature sensor on the egr that is often the problem.
 
thanks gang! im going to try the paper clip thing today to determine the code. I just replaced my speed sensor and the speedo and odo still don't work.
stay tuned.
 
Good going guys. Some of these issues can be a real headache on older vehicles.


G/L Ranman on solving the problem. Keep at it and let us know how you make out.

Colin.
 
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Ok first of all, thank you Brian! your paper clip trick worked! thanks for sharing. The car is reading a code 24- Speed sensor issue, and a code 44- lean exhaust issue,,,, now I am working on a resolution,,,,,,
 
update,,i disconnected the battery which reset the computer. did the paper clip test again and now I am only getting code 12 *all clear* now I wait until tomorrow and see if I get the codes back after it warms up. stay tuned.....
 
Code 24 (speed sensor) I believe is going to be your problem. I know you said that it was replaced just recently; however the after market Chinese crap that gets sent around is often faulty right from the get go.
Good luck with the situation, hope you get to the bottom of things soon.
 
**update** I tested the speed sensor today. (took it out of the tranny and manually spun it with a drill) and low and behold the speedo displayed a reading!
so in my opinion, the digital dash works correctly, the speedo works correctly, the odometer works correctly and the speed sensor works correctly and everything is wired correctly and properly grounded.
I believe I have ruled everything out except the only thing left that I can think of is that the gear(teeth) inside the transmission is my issue?? Anyone have any ideas?
 
Ranman said:
**update** I tested the speed sensor today. (took it out of the tranny and manually spun it with a drill) and low and behold the speedo displayed a reading!
so in my opinion, the digital dash works correctly, the speedo works correctly, the odometer works correctly and the speed sensor works correctly and everything is wired correctly and properly grounded.
I believe I have ruled everything out except the only thing left that I can think of is that the gear(teeth) inside the transmission is my issue?? Anyone have any ideas?
Click to expand...

I think you may be on to something there. Good news that the dash is OK.
 
Well, it is official, I have a speedo again! It is what we thought it was. A $2.00 clip that held the gear on that drove the speed sensor gear fell off. 6 hours later and I am back in business! now all 4 bulbs on the dash are out?? I am going back under tomorrow night to check the ground connection as I just replaced the bulbs a couple weeks ago. so I do have all gauges now working. Just cant see them very well :):):)
 
Glad to hear that you have everything working again. Now to just get the lights going and your back in business.
You say you just replaced the four backlight bulbs. If my understanding is correct, they are the 882 bulbs. (g4base 12v 5w). Did you replace with these bulbs and did you replace the sockets as well? Also, where did you purchase these? Or did you change out to something else?
Just the other day I lost the right side of my dash. I see the LCD's are still working, so it is just the back lighting I'm sure.
Thanks
 
Hi Brian, I have done both. I have replaced just bulbs from the front of the dash. And I replaced bulb and socket from the rear. I haven't had to buy them yet as the previous owner left me a few. I have attached a pic of what I used.

photobulb.JPG
 
I:t is official! I am back in business!! I soldered all 12 pins on the back of the board and it works awesome!!
No more issues. All gauges and lights are working perfectly. Now time for some cruising! :D :D :D :D

photodash.JPG
 
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