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1990 K5 Speedo malfunction

Justin Fleming

Justin Fleming

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Guys, just starting to look into this, but the speedo is not working on my 1990.(every once in a while it will bounce up to speed) This is the electronic version with out a true speedo cable. I am also getting the 24 error code Vehicle Speed Sensor Error. Anyone have any suggestions for trouble shooting this?

Is the VSS the sensor that is plugged into the xfer case? Looks like the wires going to it are twisted/braided....
 
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My suspicion would be the VSS itself on the t-case, but if you've got cruise, see if it works.

Its possible the power or ground for the DRAC is bad, you'd have to check whatever fuse feeds it.

The speedometer and ECM are fed off different circuits after the DRAC, so it's unlikely anything short of the VSS, wiring to the DRAC, or the DRAC itself are the problem if it's not power or ground to the DRAC. ECM is telling you it's not getting a signal from the DRAC, speedometer is most likely telling you the same thing, cruise would be just a third indicator.

Ive had a VSS go bad but it was because the output shaft was loose enough to hit the VSS and grind the small nub off. Probably not real likely if a slip yoke t-case.
 
Where is the DRAC located and what is the widget that I am looking for? Should I have power when key is on or power at all times?

It seems they are pretty proud of the V$$ as it is a bit pricey.... so I would like to prove it being bad before just throwing money at it.
 
also if I pull that sensor out, should there be much fluid at this location/level to worry about?
 
DRAC is a white plastic box up under the dash. I actually don't know where they are mounted on these trucks. I want to say drivers side or mid dash somewhere.

Only power with key on. GM may have used the same VSS on a bunch of rigs, so junkyard may be a source. I never complain about spares.

If I figured out AC Delcos catalog, looks like it was used on a bunch of different vehicles up until 2006. S10s seem to be the best source.

Shouldn't be much fluid up there. IIRC they use a gasket or o-ring to seal, might need to be careful removing if just checking.
 
I pulled the sensor out, no physical damage to the unit.... all fuese check good, I have not located the VSS yet.
 
The VSS is what's in the t-case. DRAC is what you mean I assume.

Reseating connectors doesn't hurt either. Have you tried after pulling the sensor?
 
Is there a ohms test or something on the sensor that can be done? Ironically I did have a u joint go and broke the tail shaft house last summer. I wonder is something happened to the sensor when this all happened. The truck hardly gets driven so it hard to tell. But no physical damage on the sensor. There was a little metal shaving/fuss mud on there as it seems the sensor is magnetic
 
You'd probably have to google and see, and make sure it's the same type VSS as there were a few different ones on other vehicles. Its function is tied into the reluctor wheel turning past it, I suspect it may not be testable without some specific equipment.

I'd be careful randomly poking and prodding things with a voltmeter, IIRC, the VSS output is AC, not DC like everything else on the truck.

Too bad the VSS is so pricey, this is one of those that I'd generally throw a part at otherwise. Known good parts are handy at times. Far faster than actual diagnosis in this case.
 
running to the store to throw money at it and hope its the fix. I will tell you in about 1 hr
 
Crossed fingers. Really, outside the DRAC and VSS, the wiring can be the only issue, and that would be though to inspect.

VSS is the most likely culprit however IMO.
 
Drac module is directly below the radio clipped to the inside of the sheet metal part of the dash. There is a big wire bail holding it in. Pop the bail and the drac should pull right out from underneath. It’s white plastic and a little larger than a deck of playing cards.
 
to be continued, all three local store had no stock and the snow storm is rolling in so she has to go back together for now....
 
My two cents. Getting a code in the ecm and erratic speedometer means whatever you have going on is common to the two.

if it was just the speedo and no code or vice versa I’d be looking at the circuit between the drac and the offending device.

However with both acting up it could be in the drac, wiring from the drac to the vss or the vss itself. A drac could fail but it’s pretty rare. My money is on the wiring and or sensor. The wiring is the twisted pair you saw at the sensor. Check for any pinch points or areas where the harness rubbed that might be causing the circuit to have an intermittent open. Check the connection as well.
 
does the twisted pair go directly from the sensor to the DRAC, If so I could check continuity on both wires and short to ground? Is there a ohms test or something for the sensor? Is there any testing or checking I can do on the DRAC module
 
Well I had not given up, started chasing the wires down on Sunday and of course I found someone hack job. I am certain if and when I repair this we should be back in action. The question I have is should I try and twist the wires up like the original wires were twisted? It looks like someone just took a piece of a speaker wire and put it in place. My guess is it was hitting the exhaust at one time and that is what melted the spiral wrap.

vss issue 1.jpg

vss issue 2.jpg
 
just when you thought over all the years you fixed all the hacks from people over the past 32 years...
 
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