No Power to Speedometer?

[Ruff Idol]

1992 C1500 with the 700R4 here. So I've had an intermittently working speedometer, most days it's just totally dead no matter what speed you're going, and if I'm lucky enough to hit a serious pothole it comes back briefly. Started by replacing the VSS and its pigtail (the old wiring was soaked in ATF from a bad o-ring so I thought it was the issue) and made no difference. Wound up putting the original VSS back in with a new o-ring as the aftermarket one I got was noisy. Still an intermittent speedo after all that.

At this point like an idiot I remembered I have an electronics background and am more than capable of a little troubleshooting rather than throwing money at more parts. I pulled out the glove box and supplied 12VDC+ and - via jumpers to terminals C8 and C9 on the VSSB/DRAC unit (in case anyone should know, the DRAC is behind the glove box on 92-95 but built into the inst. cluster on earlier models). With this jumper setup terribly jerry-rigged up, I went on a test drive and the speedo came back perfectly. While driving, I disconnected the makeshift ground I had connected to terminal C8, and the speedo was still going! So this tells me it is not a bad ground somewhere like I would think and is a 12V+ issue.

The next thing I did for peace of mind was take apart the 12V+ connections on the firewall junction block one by one and clean them real good with a wire brush. Reinstalled each nut TIGHT and ensured each lug of every connector was making good contact. Welp, upon start of the truck and test drive all my electronics worked except the speedo again like usual. I don't understand. To my knowledge there is no dedicated fuse for the speedo or the VSSB, just 1 that serves the whole cluster. Where does the power actually come in to terminal C9 of the VSSB? I would assume it directly leaves the glove box through the firewall, connecting to that junction block via a fusible linked cable like many of the other directly powered electronics on the truck. If that was true though, wouldn't my cleaning of the connections have fixed the issue? I'm PRETTY confident I got all those wire lugs clean, shiny, and bolted down tight.

Not sure if this helps, but the speedo rarely ever operates in cold weather, and will come back pretty reliably upon hitting a decent pothole on a hot summer day. This drives me nuts because it makes me immediately think of a bad ground, but my road test with the jumpers to C8 and C9 likely rules that out.

Also not sure if this helps, but in early 2022 I had somebody swap in a used 350. Truck originally had a 305. This made me think of the engine ground connected to one of the thermostat housing bolts, so I cleaned those up to no avail. At this point I'm pretty sure all the necessary grounds are fine but I could be wrong.

 

[Komet]

If you've got all your signals reliably going into the DRAC, the only path left from there is a single wire that goes straight to the speedometer. So either that wire is fragged, or the cluster itself is having an intermittent connection. Does it EVER go intermittent with the jerry-rigged setup? Perhaps the solution is as easy as making that a little more permanent.

 

[Ruff Idol]

If you've got all your signals reliably going into the DRAC, the only path left from there is a single wire that goes straight to the speedometer. So either that wire is fragged, or the cluster itself is having an intermittent connection. Does it EVER go intermittent with the jerry-rigged setup? Perhaps the solution is as easy as making that a little more permanent.

I only went for a short drive so I can't say atm. But it was working, and accurate, for that entire drive and I was going over some considerable bumps!

 

[JozefRoberts]

I have an issue on my ‘94 K2500 with the speedo not working and the parking brake light always being on, used a test light and found out there’s no power going to my fuse labeled “brake” so I used a fuse tap on the ignition fuse and ran it to the “brake” fuse and now everything works fine. Still tracking down the real issue, but it works for now. Might be worth looking into for your issue?

 

[caminonut]

1992 C1500 with the 700R4 here. So I've had an intermittently working speedometer, most days it's just totally dead no matter what speed you're going, and if I'm lucky enough to hit a serious pothole it comes back briefly. Started by replacing the VSS and its pigtail (the old wiring was soaked in ATF from a bad o-ring so I thought it was the issue) and made no difference. Wound up putting the original VSS back in with a new o-ring as the aftermarket one I got was noisy. Still an intermittent speedo after all that.

At this point like an idiot I remembered I have an electronics background and am more than capable of a little troubleshooting rather than throwing money at more parts. I pulled out the glove box and supplied 12VDC+ and - via jumpers to terminals C8 and C9 on the VSSB/DRAC unit (in case anyone should know, the DRAC is behind the glove box on 92-95 but built into the inst. cluster on earlier models). With this jumper setup terribly jerry-rigged up, I went on a test drive and the speedo came back perfectly. While driving, I disconnected the makeshift ground I had connected to terminal C8, and the speedo was still going! So this tells me it is not a bad ground somewhere like I would think and is a 12V+ issue.

The next thing I did for peace of mind was take apart the 12V+ connections on the firewall junction block one by one and clean them real good with a wire brush. Reinstalled each nut TIGHT and ensured each lug of every connector was making good contact. Welp, upon start of the truck and test drive all my electronics worked except the speedo again like usual. I don't understand. To my knowledge there is no dedicated fuse for the speedo or the VSSB, just 1 that serves the whole cluster. Where does the power actually come in to terminal C9 of the VSSB? I would assume it directly leaves the glove box through the firewall, connecting to that junction block via a fusible linked cable like many of the other directly powered electronics on the truck. If that was true though, wouldn't my cleaning of the connections have fixed the issue? I'm PRETTY confident I got all those wire lugs clean, shiny, and bolted down tight.

Not sure if this helps, but the speedo rarely ever operates in cold weather, and will come back pretty reliably upon hitting a decent pothole on a hot summer day. This drives me nuts because it makes me immediately think of a bad ground, but my road test with the jumpers to C8 and C9 likely rules that out.

Also not sure if this helps, but in early 2022 I had somebody swap in a used 350. Truck originally had a 305. This made me think of the engine ground connected to one of the thermostat housing bolts, so I cleaned those up to no avail. At this point I'm pretty sure all the necessary grounds are fine but I could be wrong.

If you find out it is the Circuit Board in the Cluster causing the problem with your Speedo not working all the time, here is a link for a 1992 on e-bay: https://www.ebay.com/itm/315035625866

 

[Road Trip]

I have an issue on my ‘94 K2500 with the speedo not working and the parking brake light always being on, used a test light and found out there’s no power going to my fuse labeled “brake” so I used a fuse tap on the ignition fuse and ran it to the “brake” fuse and now everything works fine. Still tracking down the real issue, but it works for now. Might be worth looking into for your issue?

Greetings JozefRoberts,

First of all, welcome to the GMT400 forum. You may be new to where all things GMT400 are
discussed, but at the same time your troubleshooting method tells me that this isn't your
first rodeo. :0)

****

Background info for anyone researching an inop speedo on the ('94-previous? '95-previous?) GMT400s:

When the electric speedo in a GMT400 vehicle doesn't work, the first troubleshooting question to
figure out is which of the 4 following categories does the failure fall into:

* Bad positive power
* Bad ground
* Power is good, but varying signal representing vehicle speed is missing.
(VSS not generating > or DRAC not processing > or both components OK but interconnect wiring circuit open/shorted.)
* Power & signal are good, but speedo assy itself has failed.

By temporarily subbing in +12v from another circuit, you have neatly narrowed the scope of your problem
to a single category. Nice troubleshooting on display vs just loading the parts cannon and letting 'er rip,
and possibly introducing more unknowns into the mix while doing so.

****

Being new to this forum, you may not as yet discovered that there's a repository of nearly all of
the C/K Factory Service Manuals available for free download. They are no doubt the next level above
the aftermarket repair manuals. Sure, they aren't perfect, but they are an unfair advantage for anyone
trying to keep their 30-year old GMT400 on the road. (LINK)

Anyway, I'm going to open the '94 C/K Driveability, Emissions, & Electrical Diagnosis service manual and
see where you can take your troubleshooting results and turn it into a permanent fix.

Step #1 - Understanding the malfunctioning upstream circuit layout feeding the BRAKE fuse:

You must be registered for see images attach

Lots of relevant troubleshooting info can be gleaned from this single page. Here's one possible way
through the maze:

* Starting at the fuse & working upstream, instead of just assuming that the ignition switch is the
root cause, let's see if we can eliminate that first. The first check is to see if the subsystem fed by
the 4WD FUSE is still working? IF so, then the (Circuit) 300 ORN (Orange) wire is still successfully feeding power
all the way to the fuse block, and you should find the open right there in the immediate vicinity of the BRAKE FUSE itself.
(NOTE: Physical fuse block layout to be shared shortly.)

If not (or the 4WD system isn't working and can't be used to help troubleshooting) then my very next
check would be to see if the subsystem being fed by the HTR A/C fuse is still working? (ie: Fan, etc)

If the heater fan is still operating but the BRAKE FUSE is still unpowered, then possibly the problem is
located in the vicinity of the S203 splice?

Given the above, ONLY if the BRAKE, 4WD, and HTR A/C systems are all inoperative should the troubleshooting
progress into the ignition switch. Given your original problem description, I'm going to assume that the only
issue you are currently experiencing is the speedo dead + brake warning light on, so I won't go any further
into the ignition switch until you report back with your observations and can confirm/deny this.

****

OK, now that we've waded around in the circuit theory a bit, let's now get a view of the physical fuse block.

Customer side first:

You must be registered for see images attach

The above is pretty self-explanatory. Just remember that when you see the reverse side of this same fuse block in the
following page that you have to remember that it's a mirror image. (I'll use the orange color to help with the orientation.)

You must be registered for see images attach

If you compare the physical wiring layout to the 1st schematic you can see everything matches up. The
top 3 fuses in the second column are all fed by the same bus. The BRAKE fuse does feed power to the VSS buffer
(DRAC) in addition to the Antilock Module. (explaining the Brake Warning Light)

****

To recap, thanks to your temporary jumpering of power to the BRAKE FUSE, we are focused on the power
upstream of the fuse. Could be a simple break right at the fuse itself, or an issue at Splice 203, or even the
feed from the ignition switch. But thanks to understanding the circuit layout, your repair focus will be directed
by how many subsystems are missing power. If everything lines up just right, you will find/fix this problem
on the first try...thereby earning the coveted Troublesniper status.

That's more than enuf for now. Let us know what you uncover. Bonus points for a sharp photo of
the actual failure, as well as your fix. This way we can help others in the GMT400 community trying
to sort out a similar failure.

Happy Hunting! Cheers --