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Speedometer frustration

Rhinodrvr

Rhinodrvr

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Friends,

My 91 K5 has developed a weird problem. I have recently installed a new GM crate motor in my truck and we are now back up and running.

One of the things I wanted to fix was the odometer and trip odometer. Both had developed offset numbers and I could not reset the trip odometer. So I found a used gauge cluster in great condition, sent the speedo off to be refreshed and updated the dash lights. I got the speedo back last week and spent my holidays getting the cluster back together and back in my Blazer. Everything went in and when I fired it up all of the engine guages came to life, fuel gauge worked and all....VERY EXCITED!! I engaged the 4x4 to verify that the "idiot" dash light came on (it did) and then it was time for a test drive.

I went down the road about 1.7 miles and suddenly the speedo pegged over last 85MPH and is laying on the trip odo reset rod. VERY FRUSTRATED!!

I am send the cluster back to the guy that did the resto work....very nice guy and I am not mad at him. I am concerned that there is a switch or component that I am missing that could be related to this problem. I put the original cluster back in and all the gauges work except for the original speedo.

I know that in 1991, the speedo was electric with no speedo cable like the older C/K10 series trucks...any one else have these problems or know of a possible solutions

Cheers,
Chip
 
Start from the t-case and follow the twisted wire up to the DRAC unit. Be sure they aren't melted or damages.

Have you tested the cruise control? How does the engine run? If cruise works and engine ok, then I would look at the speedo. Otherwise, it may be the DRAC.

Did the speedo ever work properly before the new engine?
 
Yes, the speedo did work before we did the engine swap. The cruise did work yesterday when I engaged it.

The first speedo does not move at all and the second pegged over to the trip odo reset arm.
 
So, if the cruise worked yesterday, that would lead me to think the twisted wires are OK and the DRAC is OK. You could go to the junkyard and get another electric speedo to see if it's indeed the speedo not working correctly.

But with what you've said, I'd feel pretty confident that the gauge needs to go back to the guy who did the work. He must of screwed something up. Cluster may be OK....just the gauge itself.

Is this just a simple, like for like engine swap? No computer mods or anything?
 
Just a simple instrument cluster swap. I wanted to keep the original cluster intact and put in a new one that was reset to 0 miles for the new motor and correct a few problems with the original speedo.

Both speedos (original and the one I had restored) are not functioning, which is what led me to ask if it could be something else.
 
Depending on what you have for diagnostic equipment, hooking some sort of data viewing tool (XP240, winALDL, Tunerpro RT, etc) up to the ECM will also indicate if the VSS/DRAC stuff is working correctly. Does not mean the Speedo is getting the proper signal, but if cruise and ECM are both getting proper speed readings, doubtful the DRAC wire to speedometer is the culprit.

Only additional test I might perform is to make sure (edit)cruise engages ONLY around 25MPH and up, however you can determine your MPH (GPS?). Cruise should not allow you to set it below 25MPH, but if the DRAC is somewhat screwy (unlikely, but maybe a bad solder joint on the "jumpers" for gear ratio) it fools the cruise module into thinking you are going faster than 25, when you aren't, and will let you set it.

Had this problem when I wired my DRAC in, the online DRAC pinout I used had a couple pins mixed up, so at 15 the cruise module thought I was going 30, and I could set the cruise.

Not likely, because your failure mode is different with two speedometers, but IMO these things seem to be pretty robust, and having two fail pretty much back to back would make me pause. The fact that one was just worked on makes it easier to swallow that two are improperly functioning, but I'd still want to verify the instrument panel connection, DRAC function, etc.
 
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Could the transmission transducer be the problem? I finally got the speedo cluster back but they said it had been blown by a voltage spike? Thoughts?
 
I'm the first to say that I'm no electrical expert. But my understand of how the tone ring/VSS setup works is that there is a small current developed, and that is what the VSS reads. I don't see how that could provide too much voltage.

You aren't seeing above 14.5 or so at the alternator are you?

There are only three wires to the speedometer, power, ground and the DRAC feed. The power is 12V, so no idea how that could be excessive unless your alternator is putting out too much in terms of voltage.

I really don't know how the input to the speedometer "works", its always referenced as 2k or 4k PPM (can't recall which the speedometer gets) so I'm guessing you can't just measure voltage on that wire.

Can't see how it would be a problem in your case, but did you crawl under the truck and follow the VSS wiring to ensure it didn't meet the exhaust, or get pinced in the bellhousing when the engine was swapped?
 
I will check tomorrow. I just got the cluster back right at the end of the year and just now getting back into this.

What is the VSS and the DRAC?
 
VSS is the sensor on the t-case output that "reads" how fast a metal ring on the output shaft spins. That sends its signal to the DRAC (Digital Ratio Adapter Controller). Two wires from the VSS to the DRAC.

The DRAC interprets the VSS input and sends an appropriate signal to the various components that need it...ABS, Cruise Control, ECM, and Speedometer.
 
OK...I finally found it!! In my defense, the truck has been in storage for almost 5 years and just got it running (new motor, trans rebuild, etc.).

I pulled the DRAC out and checked the leads and everything seems to be correct. What is the best next step? I have replaced the VSS module with an AC Delco one, I've check fuses, I've had one cluster rebuilt, the original is back in the truck now and everything works except the speedo. Is there anyone that can rebuild these?

I did notice that when I dove the Blazer that the E-Brake light was on and the Odometer had quit working. Plugged the DRAC back up and the light was out and the odo was back working...best I can figure is at least something in the DRAC is working but not the speedo.

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If the speedometer is suspect, I'd just buy one from a 1990-91 K5 or Suburban. Crazy expensive online for some reason, I pay about $25 each for them. Better than what the seller will get for scrap. :)
 
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