700r4 VSS and speedometer

[RShrimp]

1990 C1500 and my speedometer works intermittently or jumps. Seems to work more often when the truck is fully warmed up. I get a CEL code 24 when it doesn't work. That tells me that the issue is with the VSS or the connection from the VSS to the ECM.
I replaced the VSS (Better know in the autoparts industry as a ABS sensor), as it was leaking there anyway. I tapped it in with a stick to make sure it was fully seated. At least the leak is fixed. The speedo works more often now but still not 100%. Do the tume rings go bad? I really dont see how? Pretty sure I am working in the right area as the symptom did change. It used to work 20% and now it works 60% of the time but still mainly after warm up. Any ideas on how to proceed?

 

[b454rat]

The VSS sensor doesn’t touch the tone ring, so that can’t wear out. I’d try the connection between the dash cluster n wiring pigtail.

 

[someotherguy]

Haven't seen the VSS referred to as an ABS sensor! But I guess terminology gets swapped around depending on where you look.

The "jumpy" speedometer (and the leak you mentioned) would have had me looking at the VSS as well. Is your code 24 gone now, too?

The 1988-1991 cluster design can be a little sketchy once they get old. They can develop cold solder joints internally and just get unreliable. However, I'd start by popping the cluster out and spray the connector socket with DeOxit contact cleaner, and carefully disassemble the cluster and pop the gauges out of their sockets - each gauge has studs on the back that simply press into sockets on the circuit board. I'd spray those with DeOxit, too.

You can access the gauges to remove them by first removing the lens/bezel assembly from the front of the cluster -

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Example of the sockets (this one is with the speedometer removed) -

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If you're not used to working on electronics, read up on grounding yourself before digging in. The circuits inside that cluster can be damaged by electrostatic discharge (ESD)

Richard

 

[RShrimp]

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[NickTransmissions]

I replaced the VSS (Better know in the auto parts industry as a ABS sensor), as it was leaking there anyway.

LOL at Carquest, they need to change their website...As I'm sure you know, the VSS and ABS are not the same thing...The vehicle speed sensor is in the transmission while the ABS sensor is elsewhere, usually at one or more wheels. As mentioned above, the tone ring / gear never contacts the sensor unless the output shaft is bent or something else catastrophic has occurred.

You can actually test your VSS by using a multimeter, set to 'Volts' while spinning the output shaft (have vehicle in neutral, rear wheels on jackstands, front wheels chocked or also on stands) and see the change in voltage generated by the sensor as you spin the drive shaft...If there's no change as you vary the spin rate or no voltage present at all, the sensor is bad...If voltage appears to be responding normally, the problems is elsewhere, as mentioned above.

 

[1998_K1500_Sub]

I replaced the VSS ... The speedo works more often now but still not 100% ... It used to work 20% and now it works 60% of the time but still mainly after warm up. Any ideas on how to proceed?

Can you check the air gap b/t the VSS and the tone ring? I'm not sure what the spec might be, but just as a guess .020-0.050 or in that range. I'm sure there's a spec around somewhere for this or similar sensors (crank position sensor, ABS sensors,...).

 

[RShrimp]

VSS speedo update. After replacing the VSS The speedo works like this.
Doesnt work at all when cold CEL light comes on and sets a code 24 )No VSS Signal)
After 5-10 miles it will start to jump a bit and CEL goes out,
After 10-16 miles it works perfectly and continues to work perfectly until the next cold start.
I also notice that the TC Lockup does not work until the speedo starts working.

I seems like maybe a clearance issue between the tone ring and the sensor but I dont see any way to check it. I triple checked to make sure the VSS was fully seated. Tapping it with a wood drift just to make sure.
The issue has to be between the Vss and the ECM as it does set a code which it would not do if the issue was between the ECM and the dash.
Any insight would be great please.
I am tempted to just try another sensor as changing it did change the symptoms and todays parts are crap!

 

[RShrimp]

I decided to just throw another sensor at it today (Advance Auto Warranty) It did not help and the issue actually got worse. I really think it is the connection at the sensor. (works when hot, works for a couple seconds after you hit a bump.) I was able to find a new VSS connector Carquest PTA1003 in case anyone needs one in the future. I was also able to download the Trouble Tree from the Emissions Manual Posted on this site. This evening I will be going through the proper diag procedure (which I should have done in the first place)

 

[RShrimp]

Just to report back. Problem solved. Did the diag procedure from the manual and found no signal at ECM. Used the new plug to test the sensor (volt meter and spin the drive shaft) and that worked fine. Cut off the old plug and tested voltage there with key on. 50ish millivolts on one wire and the other is ground which is correct. Wired in new plug and took it for a ride. Immediate Joy!

 

[Muddy]

Just to report back. Problem solved. Did the diag procedure from the manual and found no signal at ECM. Used the new plug to test the sensor (volt meter and spin the drive shaft) and that worked fine. Cut off the old plug and tested voltage there with key on. 50ish millivolts on one wire and the other is ground which is correct. Wired in new plug and took it for a ride. Immediate Joy!

Do you think the same issue could occur on a 93 K 1500 a lot of the symptoms are the same