It is located on the drivers side near the rear of a 5 spd
manual transmission on a 99 silverado. Looks like a
hazard for getting snagged by road debris or a tree branch
and ripping the wires out of the nearby electrical connector.

There was about 4 inches of electrical tape dangling, so
I suppose the loop was originally taped up.

Any one know what the loop is for?

 

I have no clue, but it doesn't look OEM. Try cutting the electrical tape off and see what it connects to. There's only two wires going into that connector, so perhaps it's some sort of wire tie to hold the wires back, or it was used to hold the wires if someone was working on it at some point.

 

Thanks, good idea. i unwrapped some of it and took more snaps.

My guess is that the bare wire is some sort of OEM electrical
shield for the sensor leads. One end of the loop becomes insulated
like a conventional electrical wire and feeds back into the wire harness.
The other end seems to remain bare, and feeds into the blue/silver foil
wrapped pair of leads for the sensor, which are purple-white and
yellow-black. The sensor leads seem to be twisted together along their
length. Twisting like that, i've heard, is a method to ensure equal noise
in the two leads, and equal noise makes the true electrical signal easier
to detect. but this is total speculation.

any other ideas what this might be about? how to best re-wrap it? which
sensor?

 

Just wrap it up like you found it with new electrical tape. If the wires are all slimy and gunked up, spray some brake clean on them and wipe them off. Let dry and then re-wrap the wires in electrical tape like it was before.

Yes, twisting the wires does help to keep both the wires at the same amount of electrical noise, but, afaik, it also helps to reduce the overall noise.

 

The sensor pictured above on the rear driver side of the 5spd manual transmission is in fact the Vehicle Speed Sensor. Well, it is the VSS in so far as driving the truck with it unplugged triggered the ABS code C0236 "Rear Speed Signal Missing".

I taped it back up, then did some testing with the scan tool Actron CP9580.

With the engine running, unplugging the sensor (or jiggling the senor wires to test for a loose connection) did not trigger any codes. With the sensor unplugged, letting the truck idle for 6 minutes and then several engine restarts all failed to trigger any codes. I finally was able to trigger a code by taking the truck for a short drive, which triggered the C0236 and the ABS + Brake lights on the dash. After turning off the engine, then restarting, the code was still stored as History, but only the ABS light came on. Odd. I Turned the engine off then used the scan tool to clear codes (success). After restarting the engine three times, the ABS light was lit up, but scan tool reported no codes. Odd. Finally, I replugged the VSS connector and restarted the engine. the ABS light still lit up despite no codes. Odd. After driving a couple hundred yards, the ABS light went off.

There is a second sensor on the transmission, located towards the front. Not sure what it is. It is the reddish one seen in this snapshot:

 

Well, my guess is that the little red connector is part of the clutch safety switch, backup light switch, perhaps a transmission oil level light switch? Could be any of the related.

By the way, unplugging things to see of they trigger engine/abs lights wont necessarily trigger the light immediately. For example, my check engine light only comes on (it's for the EGR) when my truck is in overdrive (an automatic transmission thing) while going 45mph+, when in reality, the EGR is malfunctioning all the time.

 

If your scanner has live data you could watch the sensors while the engine is running to get a better understanding of what its doing. The VSS is used by the computer to measure speed, in autos help with shift points, and also works with the Anti lock Brake system. If your truck was automatic trans, unpluggin it would make it shift weird. And after replacing or plugging a sensor, it takes a few key cycles sometimes for the SES light to go off, if its not manually cleared.

 

live data - yep my new CP9580 displays live data, pretty spiffy. but its another thing to interpret it. i'm pretty disappointed in the user manual. its kind of worthless at this point other than for reading codes. i'm missing something, at the very least specs for the sensors, and their behaviors in different driving situations.

anyone know of a good manual for scan tool data?

 

i would suggest all data or mitchel on demand also check with the dealership if they can print you off the specs or what its sposed to be at a haynes or chilton may have the info but i dont think it does