Iv'e got a 98 B 2500 with a 4 cyl engine 2 wheel drive. The problem the truck is giving me is - the speedometer works intermittently. It seems to work until the truck has been running for awhile like 15 miles or so then it drops off to zero. Then sometimes if I hit a bump in the road it will work for a short period. It seems to be heat related like a bad solder trace on a pc board in the ecm. Has anybody encounterd a problem like this at all.? I hate shaking wires all day.[X(]

 

Any warning lights on like the check engine or ABS? Do you have 4 wheel antilock brakes or only rear antilock? Vehicle speed is taken from speed sensor mounted on the top of the rear differential on rear anitlock and from the ABS control module on 4 wheel antilock. If no warning lights on then the signal is ok but is either being lost at the speedometer head itself or is not being output by the GEM module.

 

Thanks for the info Babyhuey,

I have rear anti-lock only, and it's a two wheel drive. Also, no fault lites either. So I should check either the connection at the top of the differential or at the speedometer connection? It gives me a place to start, I will let you know what I find.

Thanks again, Vortilon out...

 

Another question,

What is the best way to access the back of the instrament panel to check the speedometer connection; also, is it direct reading from the pickup on the differential or through a control module? I removed some screws from the panel around the instrament panel and tried pulling the panel away, well I didn't want to break anything, so I stopped and decided to ask for some more tips on how to go about this without creating more problems and expense.

I have a different problem as well. My blower motor only works on HIGH, nothing on MED or LOW. What is the most likely culprit on this issue? Relay or switch? and where is it located?

Thanks in advance, Vortilon

 

Would be a lot more suspect of the speedometer and printed circuit. The GEM/CTM module gets the vehicle speed signal along with the rear antilock module. GEM/CTM then outputs signal to both the instument cluster and the PCM from same wire but branched. If speed sensor was out then RABS light and CEL should be on. If GEM/CTM were not outputing a good singal then at least the CEL would be on due to not getting signal. Think you might was to concentrate efforts at instument cluster circuit board and speedometer itself.

 

BabyHuey,

Where is the printed circuit board located? Do I have to tear apart the dash to access it?

Thanks V

 

BH,

Hey I got the cluster as well as the PC board out - not too bad. Anyway, I inspected for obvious problems - none found. Reinstalled, now the speedometer doesn't even work part of the time. It appears as though what I did, put it over the edge completely. That narrows down the troubleshooting to the instrament cluster anyway.

 

Yes it certainly does. Wish I knew of a way you could test one or the other without trying a known good part but that is pretty much the only way at this point without any obvious visual damage.
Parts a bit on the spendy side. Speedo head is almost $400. Odd thing about the large printed circuit board on the back of the cluster though. There is a production date split for that part. Trucks built after 8-17-98 get a very cheap printed board at only $35.20 whereas on trucks built before that date the board is a bit over $354.00!!! Just be aware of the build date split if you are shopping for some good used part. Speedo heads are all the same except for difference between US and Canada as we use mph and they use kph.