Code 24, when going down hill.

[Pmcb]

My 87 Sierra 5.7 TBI, is throwing a code 24 when i go down hill. Just got the timing reset, engine is running good. I live in a very hilly area. Not always but maybe 50% of the time i am on a long down hill run it throws a code 24. My speedometer is about 5 mph off, probably from larger tires. What could cause this?

 

[RustyPile]

According to google, this:

GM OBD Trouble Code 24

Vehicle Speed Sensor Error (Open Circuit or No Activity) The Vehicle Speed Sensor signals to the ECM the current speed at

repairpal.com

 

[gmbellew]

loss of VSS signal indicates a wire may be broken or mostly broken and going down hill moves it just right so it loses connection.

I'd be looking at the harness to the VSS running alongside the transmission and/or transfer case as the most likely spot for an issue like that.

just fyi - I had a rub through on VSS harness in that area once, but it popped the ECM1 fuse and that shut the truck off. replaced the fuse and fixed the rub through and all good. it never set a check engine light.

I would also adjust your DRAC to match your tire size. the TCC lockup uses VSS and it can affect drivability if you are too far off. the computer may use VSS for some other stuff also, but I don't know how for sure.

 

[bucket]

'87 doesn't have a DRAC module. The VSS reads off the back of the speedometer.

To set a code 24, certain parameters need to be met, involving engine rpm, load, egr position, etc. I don't recall exactly what they are though. If you were to ride the brake and apply a little throttle while going downhill, the code would likely not set. I say this because I had a Camaro with a broken speedo cable and therefore an inop VSS. It was a manual transmission. Coasting at higher speeds (probably 30mph+) it would set the code 24, unless I left it in gear and let the engine help slow down the car, then it would not set code 24.

My guess is a faulty VSS or a problem on the back of the cluster.