Speedometer reads way too fast

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bigcountry78

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I searched, but I didn't find the answer. I'm not sure this is in the right spot, but here goes. I just bought a 97 Cheyenne, 4.3 v6 and 4l60e transmission. The speedometer reads 0 when sitting still, but quickly pegs out at 100. It reads about 60 when the truck is going 30. By the time I'm at 50, the needle is pegged at 100. And it stays there until I slow down, then it reads 0 when I'm stopped again. And it reads consistently, no bouncing or fluttering, and the movement is proportional to the movement of the truck. It's just way too fast. Any ideas? Truck is completely stock, 265 75 16 tires. All other guages seem to be working normally. Thanks.
 

95sierra2500

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check all of your grounds first, theres one under the dash, could be that the vssb or drac box is also shot
 

east302

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The vehicle speed sensor (VSS) sends a signal to the computer (VCM) that processes the signal and provides an input to the speedometer and uses it for cruise control.

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Does your cruise control work? If not, I'd suspect an issue with the VSS.

If the VCM doesn't detect a signal from the VSS in 5 seconds at, generally, just above idle, it will throw trouble code P0500.

All of that is to say that it may just be a problem with your instrument cluster. You could grab a salvage yard cluster and see if it resolves. Odometers are easy to swap between clusters.


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bigcountry78

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So a quick test for the VSS would be the cruise control? If it works then it's good, if the cruise doesn't work, then it's bad? I haven't had the chance to try the cruise. I'm not getting a SEL either.
 

east302

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I would think that if the cruise control works, then that rules out any issue with the VSS.


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bigcountry78

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Thanks, that's where I'll start tomorrow. Need to top off the transmission fluid before I drive it anymore, found out it was a bit low and I definitely don't want hurt it.
 

bigcountry78

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Well the cruise control works, and unhooking the battery for 30 minutes did nothing. Looks like I need a new cluster. I will check the grounds, but with my luck it probably won't be that simple.
 

mike8449

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I searched, but I didn't find the answer. I'm not sure this is in the right spot, but here goes. I just bought a 97 Cheyenne, 4.3 v6 and 4l60e transmission. The speedometer reads 0 when sitting still, but quickly pegs out at 100. It reads about 60 when the truck is going 30. By the time I'm at 50, the needle is pegged at 100. And it stays there until I slow down, then it reads 0 when I'm stopped again. And it reads consistently, no bouncing or fluttering, and the movement is proportional to the movement of the truck. It's just way too fast. Any ideas? Truck is completely stock, 265 75 16 tires. All other guages seem to be working normally. Thanks.

Not sure of the differences between your 97 and my 94 C1500, but I had somewhat similar problems with my speedo. Ended up being the circuit board in the cluster that controls the tach and speedo. Couldn't get a replacement circuit board, so a junkyard cluster solved my problem.

I posted a thread a few weeks ago about my problem. Not sure if there's any more info for you there, but here's the link if you want to take a look.

http://www.gmt400.com/threads/highly-erratic-speedometer.38832/

Good luck!
 
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Blackwater

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If only your Speedo is effected and not your other gages then it is not a grounding issue since they all ground together on the chassis behind the engine. There is a Field bulletin stating that the wiring harness eventually gets damaged over time. GM recommends replacing the wires from the sensor to the PCM. Note: You will need to twist the wires to reduce interference from other systems. This technique is used if shielding is not used. It is also noted to check the reluctor wheel by removing the sensor from the transmission (4x2) or Transfer case (4x4) and using a flat tipped screwdriver and try to rotate the reluctor wheel. if you can move it then the reluctor wheel has come loose and is spinning on the shaft. This is a rare occurrence but has happened. After you have checked those then the next logical place to look is the DRAC/VSSB. My understanding DRACs where used from 1991 to 1995 in gas and all Diesel models. The VSSB was used from 1994 to 1995. Not sure on 1997 models. If you have a DRAC/VSSB module the open it up. be cautious about ESD. Electro Static Discharge (ESD) will damage the circuitry. I am very familiar with this in the job I do with the military/s that use the system I am trained to maintain. you can get a grounding strap from Radio Shack if they still have them or amazon for $6. Ground the wrist that will be touching the breadboard and ground the other to a good common ground source, preferably a chassis ground. When you opened the module you will see a set of jumpers to calibrate the speedo. DRACs have 7 pins while VSSB have 14 pins. Using a magnifying glass, inspect the solder joins for small cracks (aka cold solder joints). If not, use a ESD safe solder hook to lightly tug on the jumper to check if it has com loose from the breadboard. if so I suggest you find another one where as the connection point has come loose from the breadboard. If you have to replace the DRAC/VSSB you may have to calibrate it if it came off another truck with different tire size and/or differential gear ratios. With liquid electrical flux, 60/40 .8 mm to 1.0 mm dia. solder. Preferably non-rosin core and not leadless. leadless tends to not flow very well and all professionals don't use leadless for quality inspections. They tend to have numerous issues with cold joints. Rosin core if in a pinch but can be found. If you need more jumpers you can make some from 14 gage solid wire. Tin the jumper by applying flux the heat from a clean, fluxed and fresh soldered tip of the iron and as the flux burns off add solder between the iron and the work. To easily calculate which jumper to use if you need to change your calibration in the event of a tire size change I have included a web site that has a easy calculator and a chart of the jumper configurations needed for your application. I hope this helps you solve your issue. Note: When calibrating a VSSB the calibration chart is for the seven pins from the right. The pin order is 7654321 1234567. What jumpers you don't use on the right pins you use on the inverse (left 7 pins). Example: you use 1,3,5,6 and 7, on the inverse you use 2 and 4. http://tbichips.com/?page_id=504
 
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