Need help diagnosing speedometer/odometer not working 1993 GMC Topkick

[zieg85]

On to the next problem.  Again, spent time looking for info on this medium duty truck to no avail.  How does one determine if the VSS or the speedometer is the problem?  I have no check engine light present.  It is a gas engine 5 speed manual transmission, no cruise control.

[VileZambonie]

You would look at your scan tool datastream and observe VSS. You need to purchase the factory service manual for this vehicle in order to understand how to properly troubleshoot and diag. Is it your truck?

[zieg85]

You would look at your scan tool datastream and observe VSS. You need to purchase the factory service manual for this vehicle in order to understand how to properly troubleshoot and diag. Is it your truck?

Not my truck, thanks Vile.

[VileZambonie]

Good, then your customer can pay for it. Once you have that, you will have endless power to fix his truck. Unfortunately most of us auto guys don't have access to YMM specific data beyond 3500 series trucks. I already pay over $6k per year for my information & programming systems and let me tell you, I have to work hard to recoup my costs. Favors are fine as long as you aren't footing the bill. eBay is a great source for the manuals too. If you need help deciphering anything, I'm sure we can help, but I don't know of anyone personally on here that can access the heavy truck systems and provide limitless data. You need "description and operation", diagrams, TSB's, component and cell references as well as unit repair info if you are going to tackle serious repairs.

[zieg85]

Good, then your customer can pay for it. Once you have that, you will have endless power to fix his truck. Unfortunately most of us auto guys don't have access to YMM specific data beyond 3500 series trucks. I already pay over $6k per year for my information & programming systems and let me tell you, I have to work hard to recoup my costs. Favors are fine as long as you aren't footing the bill. eBay is a great source for the manuals too. If you need help deciphering anything, I'm sure we can help, but I don't know of anyone personally on here that can access the heavy truck systems and provide limitless data. You need "description and operation", diagrams, TSB's, component and cell references as well as unit repair info if you are going to tackle serious repairs.

I gave it a shot, very limited info is found on the internet for the medium duty trucks.  Not a customer but a very good friend who has helped me out more times than not.  Without run time data I can't tell anything for sure but I have to assume that since the service engine light is not on and everything runs like it should I am leaning towards the speedo cluster needs attention.  He has his diesel mechanic that works on his other stuff, hopefully he can scan it for him.

[bd]

FWIW - Vile expressed the truth of the matter absolutely perfectly.  I've been away from the Topkicks long enough that only vague memories remain.  Although the medium truck systems from that period were fairly simple, it remains a fact that w/o the appropriate data stream readouts and references, you are gambling with your diagnoses.  If your friend intends to keep the truck in service, it would behoove him to attain the factory manuals, of which there were three or four.  Nonetheless, from recollection, the majority of speedometer inop issues were split between DRAC faults and cluster failures.  If you have an o-scope or AC voltmeter, you can validate the VSS signal for a go-no-go check of the VSS.  You can also verify harness connections to the cluster, B+ and B-.  Beyond that, it is guesswork.

[zieg85]

I get it, no problem.  I have all the other issues sorted out safety wise with the brakes and lights.  Everything now works except the speedometer.  GPS's on phones in the meantime.  On some groups regarding the medium duty bad VSS caused the check engine light to stay on in several cases which leads me to think it is the cluster but I am not going to make that call.

I appreciate all the knowledge I've gained just by following these threads.  I chime in when I can but still have a lot to learn.  I started out down the path to being a mechanic but before I went off to Wyoming Tech I thought it best to keep it as a hobby and pursue a different career path.  Turning wrenches 10-12 hours a day, I sure didn't want to continue after work.  My mechanical ability got me into Sherwin-Williams Laboratory R&D to keep up the maintenance of the various spray equipment which lent me a 33 year career and squarebody hobby on the side.