I am in the process of purchasing a 1993 gmc 1 ton dually with a 6.5l td, and will have it this weekend. The only issue with the truck that I have found is the speedometer. It seems to not operate until it reaches about 65mph, then works just fine until you shut down the truck, then go through the same thing the next time it starts. The owner said his mechanic claimed the sensor could be dirty. Is there a servo moter behind the dash that would effect this, and what would be a good way to troubleshoot this before buying several parts? I have been told the servo can be an issue, and that the speed sensor on the tranny is not really prone to issues. The sensor would be an easier replacement I would think. Any insite would be great. Thanks.

By the way, I know this is a premature post as I will not have the truck for a few days. I just want to get started as soon as possible. Again, thanks. -

the chevys dont use a mechanical cable that goes to the dash at least ive never seen one. my 95 didnt. normally they just use a speed sensor at the back of the transmission (2wd) or the back of the transfer case(4wd) that sends out a pulse signal and the speedometer reads the pulse signal and transfers it into a mph signal. so odds are its probably a sensor or electrical problem -

tltruckparts's Sig:

1995 Chevy K2500, 1993 Cummins 12 valve Turbo Diesel IC, Auto Trans 46rh (518) w/ OD,
NP205 Transfercase, SAS w/ GM Dana 60 front, Corporate 14 1 ton rear with disc brake conversion on 35's. 1/8" timing bump, upgraded pump with 3200 spring, SDX 5x14 injectors, airdog II 165, Autometer Ultra-Lite EGT, Boost, and Trans Temp gauges, M&H M3 Fuel Pin

I finally got the truck from the seller today. I pulled out the speed sensor from the tranny and cleaned it, but that did not help. I also took out the instrument cluster and blew it out with air, and also without help. I am getting a new speed sensor tomorrow and will put it in as soon as I can. I am having a very hard time finding the instrument cluster for my truck. I can't seem to find them on the internet. That will be my next step in the speedometer repair. Any other options or suggestions would be great, and of course, maybe a heads up where to find a used cluster.....

I also noticed that the speedometer didn't work much this time when it reached a higher speed, and hence, neither does the odometer. It would move only slightly when the speedometer would kick in, but then stop immediately when the speedometer stopped. I called the seller, and he said he hasn't driven it much in the last few months, and the problem only manifested this summer. So the addage of mileage without recording it is minimal.

Thanks for your help and any suggestions. -

finding those clusters for a gas truck is easy, but finding them for a diesel is impossible almost. ive looked high and low to find one for mine. the speedometer signal is a fair simple process. here is a list that should cover the whole sytem.

1) there is a vehicle speed sensor signal buffer directly above the heater fan behind the dash that could have gone bad. there is a dark green wire that comes from port c15 of the buffer module directly to the speedo. check that wire for continuity. THIS IS THE WIRE THAT ACTUALLY SEND THE SIGNAL TO THE SPEEDOMETER
2) there is a light green wire/black stripe that goes from the vehicle speed sensor to the c7 port of the signal buffer module. check those wires for continuity as well to eliminate any bad wires
3) there is a purple/white stripe wire that goes from the vss to the signal buffer port c12 as well check that for continuity.

if all your wiring is ok. either the vss is bad, the signal buffer module is bad, or the speedometer unit itself is bad.

here is a picture for how to diagnose the speed sensor itself



after writing this whole topic i just realized it is for a 95 not a 93. i remember 95 is the first year of the new dash so i dont know if its the same or if its different. but follow the image as for testing your modules at least -

Thanks for the info. I will have the new vss tonight, and hopefully put it in tomorrow. As for the cluster, would a salvage yard be agood place to start? And I am fairly new to all this vehicle stuff, so I will have to look into the checks that you wrote for me. That will be more of a saturday thing as I know it takes time to get to those parts. But thanks a bunch for the start.

By the way, where are those pictures from that you posted?

Jim -

sometimes you will get lucky at a junkyard. didnt make many early 90's 6.5 so your kinda going to be hard pressed to find little parts like that -

Woody35's Sig:

1997 Chevy K1500 4X4 EC/SB L56 6.5 Liter Turbodiesel
9.5 Inch 14 Bolt Axle ,3.42 Limited Slip Posi, 4L80E Tranny, HD Tranny Cooler,
265/75R/16 UniRoyal Liberators A/T
2005 VW Passat TDI 2.0 Liter Daily Driver
1938 Allis Chalmers Model B
MODS: Straight Pipe, and Metal Step
Dynoed at 12.5 HORSEPOWER

ive looked up and down for those parts theyre extremely hard to find for the diesels. as for the picture i posted, i got it off of alldata.com i have a subscription on there. and just to let you know the picture that is on there now, IS NOT the picture i posted i have no idea what happened with the link but thats not it. but if you got any questions feel free to ask i know these trucks bumper to bumper i build them for a living

edit:
everytime i reload that image it comes up with a different picture. must be a way to keep people from posting there stuff -

Thanks to you all for your help. It turns out the VSS was the culprit. I changed it out and now all is well. I look forward to help on some future but soon postings. -

glad you figured it out! -