1994 K3500 DRAC/VSSB trouble shooting.

[yellowK20]

So I got a screaming deal on a ex army K3500 SRW with a nice utility bed .

Kid was selling it because the Truck went in to limp mode , no speedo. I figured it was a VSS , or maybe the DRAC/ VSSB .

Got it home , went for the swaptronics with known good parts...........nope still in limp mode.
At this point I realize the brake light is also on . So I start actually trouble shooting, instead of just throwing parts at it .

I find that I’m only getting .833 volts to the DRAC/VSSB. I back probe the connector with 12-volts and bam , speedo works , no more limp mode and the brake light goes out .

In the limited prints I can find online , the ABS module and DRAC/VSSB are powered by the same circuit. So that’s my next step .

I did see through some google-fu that the transfer case range switch could possibly be bad , but looking through the prints I have doesn’t show those circuits being connected (doesn’t mean they aren’t)

If anyone have a quick tip or idea on this? I’m only a couple hours into the project so far not getting desperate, just looking for some wisdom .

 

[dyeager535]

What's the voltage at the fuse? I have a hard time reading the wiring diagram, but looks like there is a brake fuse that feeds the DRAC?

 

[yellowK20]

12-volts , whatever is bad is after the fuse block but before the the VSSB and ABS module because both are not receiving switched 12-volts.


Edit: I didn’t get a chance to mess with it last night, I’m going to try to find some better prints today.

 

[Russell]

Sounds like a bad termination or broken wire on the 12V supply wire. Maybe someone installed one of these shoddy splicers onto the wire and it broke in half as a result?

 

[yellowK20]

Sounds like a bad termination or broken wire on the 12V supply wire. Maybe someone installed one of these shoddy splicers onto the wire and it broke in half as a result?

View attachment 338285

I’m thinking it’s a broke wire or a bad connection leaving the fuse block I have voltage with key on (.833 volts) and it drops to zero volts with key off so the connection isn’t totally bad .

The spliced wire thing is not out of the question, but so far all the wiring on this truck is unmolested.

There is a CB radio that’s plugged into the cigarette lighter , direct replacement LED headlights, and LED taillights,and the entire truck was bedlinered . Other then that the truck is stock.

 

[TreeFarm]

Check the output pin of the fuse in the fuse box. On my GMT400 the spring inside the pin loosened up and cased it to not contact. Mine happened to be the fuse that fed the brake lights. Also, grounds on these trucks are weird. I would check those too.

 

[yellowK20]

Grounds are good , I checked them the other day , even the one on the back of the block lol.

I’m definitely going to have to check the fuse block pins !

I also eliminated ground issues when I back probed the connector at the VSSB with 12-volts and all problems went away .

The ghetto fix would be to just run a new 12-volt switched wire and call it good. But the truck is nice enough to fix right.

 

[dyeager535]

Wow, are the fuse block terminals for the fuses all set up that way? I was imaging the square body design with the brass terminal strips that are a real bear to deal with if something goes wrong with them.

You've also just swapped another fuse into that position for the heck of it? I've seen some real shoddy fuses coming out of china (shocker, right?) that failed to blow at even double what they were "rated" at, it wouldn't surprise me that a mismade fuse could potentially cause this problem too.

 

[yellowK20]

Wow, are the fuse block terminals for the fuses all set up that way? I was imaging the square body design with the brass terminal strips that are a real bear to deal with if something goes wrong with them.

You've also just swapped another fuse into that position for the heck of it? I've seen some real shoddy fuses coming out of china (shocker, right?) that failed to blow at even double what they were "rated" at, it wouldn't surprise me that a mismade fuse could potentially cause this problem too.

I didn’t try a new fuse but I have 12 volts to ground on both sides of it when installed .

I’m going to pull the fuse tonight and apply 12-volts to the load side just to be sure.

 

[Russell]

Does the fuse have an LED indicator to show it is blown?

 

[yellowK20]

Does the fuse have an LED indicator to show it is blown?

No, it’s the regular blade style mini fuse .


I have .833 volts on that circuit with my fluke 87 , with key on . Key off is zero .

Sorry for no updates, work has been crazy last couple days . Been getting home late and going to bed.

 

[TreeFarm]

I didn’t try a new fuse but I have 12 volts to ground on both sides of it when installed .

I’m going to pull the fuse tonight and apply 12-volts to the load side just to be sure.

As long as the fuse is good, and the pin on the supply side of the fuse is good. You will get 12 on both sides of the fuse if testing from the top test points. If the load side pin is bad, that 12v my not go out into the harness. I kind of doubt this is what it is but its an easy check. Just replace the fuse with a piece of wire that you know makes good contact.

GM used a lot of splices in these harness. I found one that was like 4 or 5 wires soldered to one with heat shrink over it. Also GM used two grounds at least in my 98. There is reference ground and chassis ground, just something that may help. I know it caused me issues with the interior wiring.

IDK if the diesel is different or how the years changed but on my 98 some internal cab wiring went through the firewall on the passenger side. There was a bulkhead seal with a connector just inside the firewall below the HVAC box. It may be worth it to get under there and check for power at that connector. Or maybe that's where you're already checking for power at. The DRAC is on that side right? Or maybe just take off and reset the main connector by the fuse box.

its easy enough to pull the full engine harness/fuse box to test if needed. It is completely separate from chassis wiring and has connectors at all points

 

[79rustyk10]

Did you try turning it off, waiting for a few minutes, then turning it back on? Lol, kidding.

 

[yellowK20]

I brought my power probe home from work , time to get serious lol