What sensor sends vehicle speed info to the ECU?

WaltNYC · May 28, 2016

Looking at my OBDII live data I can see that my truck thinks it is going over 120 mph on occaision. The speed reading on the OBD reader is all over the map, spiking from 70 to 120 mph. This forces the ECU to retard timing until the vehicle slows. I'm guessing I have a bad sensor.

What sensor sends vehicle speed to ECU.

I am pretty sure the VSS only runs the gauge on the dash.

AfiRover · May 28, 2016

what you got ?

discostew · May 28, 2016

You probably have a Disco 1 right? Your probably right about whats happening kind of. I think the computor shuts down a whole bank of cyliners to slow you down. or at least 2 from each bank. The speed transducer is on the rear of the transfer case, on the rear output or tailshaft.
If it was a disco 2 it would be throwing all your three amigos because that one uses an average of all the wheel speed signals.

WaltNYC · May 28, 2016

Disco 1, yes. (1997). Speed transducer you refer to on the T-case is the VSS, I think.

Not throwing any codes.

discostew · May 28, 2016

WaltNYC said:
Disco 1, yes. (1997). Speed transducer you refer to on the T-case is the VSS, I think.

Not throwing any codes.

Because the signal is breaking up it never leaves the window that the ECM sees as plausible. The pcm believes that you are going that fast and is trying to slow you down. My hats off to you for figuring that one out. That would fuck with some of the best of them.

WaltNYC · May 28, 2016

So if I am understanding you correctly, you believe the VSS is the data source for the vehicle speed at the ECU, correct? I was under the impression the VSS only fed the gauge on the dash, not the ECU.

here is a video of the live data.. A bit shakey but you can see the vehicle speed in excess of 100 mph. I assure you we were rolling no more than 75mph.

click below to see video. The gauge on the bottom is the vehicle speed reading. The top is just temperature.
https://youtu.be/hajB-TZTBIk

discostew · May 28, 2016

Your right that it goes to a buffer in the instrument cluster. Then it gets passed on to the multi function unit, the cruise ecu, and the ECM. It was usually the speed transducer. I don't know why it never really seemed to effect the speedo itself. But people used to tell me that it happened at 55 mph ( like I was a cop and gave a shit) I would always say thats weird cause it almost always happens at like 70 mph. Then they would agree that that's how fast they were going. So yours has all the trademark symptoms. I get kind of leary as these trucks get older and I figure shits starting to fail thats outside of the norm. But it kind of fits with all the hallmark signs of a speed transducer. Most probable cause is what I tell my guys to say in a case like this.

discostew · May 28, 2016

It does go to the instrument cluster. Then it gets passed on by a buffer there to three places outside of the cluster and the speedo head inside the cluster. The three places it gets sent to outside the cluster are the cruise ecu, the multi function unit and the ECM. Maybe the service interval counter as well. Just going from memory here. For some reason it never effected the speedo head, maybe because that signal is filtered. I kind of get concerned sometimes that we start seeing strange failures that are way outside the norm because these trucks are getting so old. But you have all the symptoms of a speed transducer. Especially since you say it starts at 70 mph or so. It's the most probable cause of what you have going on.

1920SF · May 28, 2016

Good news is its an easy fix, and if you still have the original VSS on your 97 it almost certainly is the culprit.

I had mine go out halfway through Ohio (an absurdly large state east/west) on a trip from VA to WI and the symptoms you describe were identical to mine as both through the feel of the truck and the SG II read out. I unplugged the sensor and carried on with no speedo until I could get the replacement part.
r-
Ray

jhmover · May 29, 2016

Tech section link below shows how to do it.

https://discoweb.org/vss/index.htm

WaltNYC · May 29, 2016

I replaced the original about two years ago. At that time the symptom was a speedometer that read zero while the truck was in motion. The "bogging down" wasn't a problem at that time.

Thanks for the help.

Levi · May 29, 2016

Mine did both, I had no speedometer when it was really cold, around 0, but would then start working once I was driving for awhile and would also randomly bog down between 70-80 mph. I opened the old one up after I took it out (it basically fell apart) and the top washer was as thin as a piece of paper.

WaltNYC · May 31, 2016

first follow up... codes P0300 (random multiple misfire) and P1316 (Excess Emissions Misfire Fault) did eventually appear. I assign this to the ECU shutting down cylinders to protect against an erroneously high road speed reading. VSS on order.

Also, I found this thread which seems to support the failing VSS diagnosis.

https://discoweb.org/forums/showthread.php?t=11295

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What sensor sends vehicle speed info to the ECU?

WaltNYC

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