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Electronic Speedometer ?'s

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rusty79beast

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I just purchased another Blazer. This time it's a 1991 and I'm new to the whole electronic speedometer setup. Well first off the speedometer doesn't work, so my question would be what steps would I take in diagnosing the non-working speedometer? Is there a sensor some where? If so how do I check it? I also read in a past column you can somehow check to see if the computer is getting the correct speed signal? How is that done and what does that tell you?

I couldn't resist the price on this "new" blazer. $1000 dollars with the only major just being the rocker panels and a little over the rear fenders. The A/C even still works. It has 148000 miles on it, but the previous owner says the speedometer just stopped working while he was selling it so who knows! Well I hope someone can help me out!

Thanks guys :bow:
 
My speedometer was intermittant and then just quit working on my '90 K5 which, I think, is the first of the electronic speedometers. I replaced the plug on the transmission but that didn't work. It ended up being the printed circuit. I ordered one from LMN but the one they sent me was not for an electronic speedo. I ended up getting pushed for time so I just took it to the dealership and let them fix that problem plus a few other things. Works perfectly now. Good luck, and in my opinion the 90-91's are the best year models. :D
 
If you've got cruise and it works, you've just eliminated the VSS on the tranny, and the wiring leading up to the DRAC.

A scanner hooked up to the ECM will let you see if the VSS is working right, in the case of no/non-working cruise control. You can see the vehicle MPH in the scanner display. If it reads reasonable (tire size/gear changes will throw it off of course) then the VSS is good.

The DRAC is what interprets the VSS signal and distributes it to ABS, cruise, ECM and Speedo, and although highly unlikely, since each output is a separate wire, it could have issues even if other components work as they should.
 
The cruise control doesn't work, so do I need a scanner to check to see if the vss is working correctly or is there a simpler way to check the vss? If I was going to buy a scanner, how much could I get one for?
 
I don't believe there is a simpler way to test VSS. Of course if you have a labscope lying around, you could test it with that. :rolleyes:

Autozone apparently has scanners that you can use freely, I'd check with them if you have any close by. You just need to drive with it hooked up and on "monitor" and it will indicate MPH. Without VSS signal to the ECM, problems when you let off the gas and are moving may crop up, so that might point you one way or another as well if that is or is not occurring.

I have an OLD autoxray XP240, but it works fine. I bought mine for about $100 off ebay, it came with cables for Ford and Dodge as well as GM.
 
Make sure the sensor is plugged in and doesn't have a ton of oil all over it. It is located at the rear of the tcase on the slipyoke shaft housing.
 
I have been reading up on the VSS and I have read that it also controls the air/fuel ratio etc. So does this mean if it runs fine/ doesn't stall that it is OK?? I am new to the electronic speedometer, so I want to learn more before I go out and buy a new VSS and find out that it isn't the problem. I guess I could always go to the junk yard and find a used one for cheap. One last question would be that my brother has a 90 silverado but it is a manual transmission, so could I swap that in to see if it worked? Also what years of pickups could I look for a VSS in a junkyard that would work with my 91 K5?

Thanks guys :bow:
 
Actually missing/bad VSS will typically show up on deceleration...idle speed is tied into VSS for that, so it may stall if the VSS isn't working. Other than that, you probably would never know it wasn't working, short of the speedometer, cruise, etc.

If you find another donor, post up. I pulled one from an S10 t-case, it's not the same. Pulled the VSS connector from a late 80's or early 90's Astro with 700R4, and the connector was the same as my '91 205 VSS. Couldn't get to the sensor to pull it though, didn't even think about it. :doah:

ACdelco shows two different sensors for that time period, but I can't tell what the difference is. Both seem to have been used in C/K/R/V trucks for quite some time.
 
rusty79beast said:
I I am new to the electronic speedometer, so I want to learn more before I go out and buy a new VSS and find out that it isn't the problem. I guess I could always go to the junk yard and find a used one for cheap.
Click to expand...
Do yourself a BIG favor and buy a factory shop manual set for the new rig. It has great troubleshooting flow charts for all of the various sensors. You can quickly spend a LOT of cash throwing parts at phantom problems on EFI rigs. A good scanner will also help a bunch. The scanner and manuals will pay for themselves in parts that you don't have to buy. :D
 
tiger9297 said:
My speedometer was intermittant and then just quit working on my '90 K5 which, I think, is the first of the electronic speedometers. I replaced the plug on the transmission but that didn't work. It ended up being the printed circuit. I ordered one from LMN but the one they sent me was not for an electronic speedo. I ended up getting pushed for time so I just took it to the dealership and let them fix that problem plus a few other things. Works perfectly now. Good luck, and in my opinion the 90-91's are the best year models. :D
Click to expand...
I know this is an old thread.... But I just picked up a '90 K5.. Speedometer works intermittently. I checked all the wires going down the tunnel and installed new conduit over them to insulate them.. Still same issue... Do you remember what the dealership did to fix? I would love for mine to work reliably. I noticed that mine tends to work the best when I first start the Blazer up.. After a few miles... it is a crapshoot.. I have had it work for 20 miles... Other times it works for 3-4 miles. When it works, it seems flawless... Then it starts to bug out and just stop. I need mine to make my Cruise Control work (other than the fact that I am OCD about everything on my vehicle to work as designed) Thanks for any input
 
rusty79beast said:
I just purchased another Blazer. This time it's a 1991 and I'm new to the whole electronic speedometer setup. Well first off the speedometer doesn't work, so my question would be what steps would I take in diagnosing the non-working speedometer? Is there a sensor some where? If so how do I check it? I also read in a past column you can somehow check to see if the computer is getting the correct speed signal? How is that done and what does that tell you?

I couldn't resist the price on this "new" blazer. $1000 dollars with the only major just being the rocker panels and a little over the rear fenders. The A/C even still works. It has 148000 miles on it, but the previous owner says the speedometer just stopped working while he was selling it so who knows! Well I hope someone can help me out!

Thanks guys :bow:
Click to expand...
Did you ever make any progress on yours? I would love to know what you did if you fixed it...
 
Odd. On a smartphone I cant see your whole post. Quoting I can.

If cruise doesnt work, the speedometer isnt the problem, or not the only problem.

I assume cruise doesnt work only when the speedo doesn't? If so, the problem is most likely between the VSS on the t-case and the DRAC that it feeds under the dash. Visibly the VSS is about all you can get your eyes on easily, pretty unlikely it is going to be intermittent, I'd expect bad wiring or connector somewhere in that system.

Link in my sig will let you download the service and wiring manuals.
 
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