Intermittent Elec Speedo 91 Az.

[globaljames]

This has me stumped. When its cold or after it rains my speedo occasionally wont work for a while. It may be coaxed into functioning after a bit by restarting the truck or when its good and ready to. It is sometimes ( not always ) followed up by a service eng light or the brake light flashing a two long 3 short code that maybe two seperate codes.
I have replaced the sensor on the tranny checked the wire it connects to, The computer connections look good and the speed od sees voltage.
I live in Phx so we dont really have much cold or wet weather to speak of But I wonder if a intermittent signal has an impact on fuel etc. Its a 91 1500 350 /700/ 241. I shudder to think what kind of labor I will be hit with if I ask my Mech to search and destroy... Thank you for any help James

 

[dyeager535]

The EASIEST way to track this down better is to get a scanner on the truck, and see if the ECM is getting a MPH reading while the speedometer is acting up.

If neither the ECM nor speedo is getting a good signal, then it's drac, wiring to drac from VSS, or VSS itself.

If the ECM is getting a good reading, it's a safe bet that the speedometer itself is the problem, either wiring to it, or internally. The DRAC output COULD be bad I guess, but it should save you from further replacing parts that aren't broken.

What is a code 23, assuming you have read that right. If it's a VSS code, then the issue is not going to be solely speedometer.

 

[tiger9297]

My speedo on my '90 K5 was doing the same thing. Sometimes worked fine other times not at all. The other problem associated with mine included my oil pressure gauge not working. Turned out to be the printed circuit and a few other small odds and ends. Parts to fix mine were a little over $100. Labor on the other hand was around $300. Took them a while to run it down. Actually, it was a very good deal b/c they had my truck for a while but if you figure $300 @ $60/hr. is only about 5 hours labor. I KNOW they spent more time than that on it. Even though it was alot of money I walked away VERY happy b/c I know it could have been much worse. You can replace the printed circuit yourself just make sure when you order it that they send you the one with the plug for the speedo. Good luck.

 

[globaljames]

This is the reason I love this site. I did not have a clear understanding of the path involved. Everytime it turn the key the speed o raises up a bit so I figure it is getting some signal. The whole cold wet thing is throwing me off but now I have all kinds of avenues to explore.

I hope in time I will be able to give some relative feedback to someone elses question Thanks again James

 

[dyeager535]

I don't have digital speedometer in my truck (yet) but I would assume a tach and electronic speedo operate on the same principle, which would mean the initial needle movement is the ignition 12V source working correctly, and the ground being ok. EIther could very well be intermittent though!

The actual "signal" wire to the speedometer that uses the DRAC pulses to display the speedo is the one that does the actual work, if you will. It very well could be printed circuit, wiring, etc., but if you get a scanner on it, you should be able to eliminate about 50% of the potential problems as possible causes.

If the VSS reading from the DRAC to the ECM is corrupt/nonexistent, the most noticeable symptom is letting off the gas and having the vehicle stumble/stall.

 

[mouse]

if it were a soldered connection starting to break, or some sort of weak connection, it usually starts as an intermittent problem. Of course moist air would cause the circuit to have different resistence and could cause it to act wierd.