Speed sensor operation

Posted by: Bill Bamberg

Speed sensor operation - 01/20/03 02:59 PM

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I have a 1987 roadster with a 4+3 manual transmission.

My speed sensor had erratic behavior until, looking for a loose connection, the connector close to the transmission came apart.

When it's working, it works perfectly, and all the functions it controls work and display properly.

I took it to a mechanic along with a replacement speed sensor. He repaired the connector, but the original sensor tested the same as the new one, so he told me to get a refund. The speed sensor still didn't run the instrument panel. He didn't have time to do more, so I took the car. To my surprise, after 10-12 miles, the whole system turned on and worked perfectly.

Recently, I had the car in for a starter, and asked him to fix the speed sensor once and for all. By chance he had just disassembled a blown up 4+3, and he suspected my problem was with the plastic gear between the transmission and speed sensor. Sure enough, there was a slight flat worn on part of the gear. He replaced the gear and repaired some damage to the harness caused by touching the exhaust system. He also welded the exhaust system back to it's proper shape and rehosed the cruise control.

When I picked up the car, the speed sensor ran the IP and all related functions perfectly, except the cruise control. Again, to my surprise, after 10-12 miles of driving, the speed sensor went dead. What he had done exactly reversed the problem.

I've been driving the car like this and have made some further observations:

1. When I leave the car parked for more than about half an hour, the speed sensor works perfectly on start up, but again shuts off after a few miles. The maximum range is about 12 miles, but this drops if the car hasn't been stopped for very long. (transmission temperature related?)

2. If I'm at the 8-12 mile range, driving up a steep hill, or hit a bump, invariably turns the sensor off. The sensor can negotiate the same hill or bump earlier in its on cycle. I tested this on the same hill about 2 miles into operation.

3. I have the factory manuals and don't see anything between the sensor and the IP except a couple of connectors. My electrical sense hints at a bare wire shorting out from a thermally induced condition. If somehow the mechanic reversed two wires when he repaired the harness, it might explain why the operation cycle exactly reversed.

Does anyone have any ideas?

Bill Bamberg
Posted by: Magster

Re: Speed sensor operation - 01/22/03 11:41 AM

Hey Bill this is a C5 forum !!

I would suggest getting a different mechanic.
Posted by: NYCHASM

Re: Speed sensor operation - 01/23/03 11:36 AM

Quote:
Originally posted by Magster:
Hey Bill this is a C5 forum !!

I would suggest getting a different mechanic.
Hey Magster, you are correct. This is a C5 forum. However, there IS a C4 Section!!

I would suggest that someone capable of doing so would move this thread to the proper location, with the hopes that this member could get some answers to his questions.

Charlie
Posted by: BruceF

Re: Speed sensor operation - 01/23/03 01:27 PM

Moving to C4 smile
Posted by: 86Z51

Re: Speed sensor operation - 02/14/03 06:02 PM

Sorry it took me so long to post this, but I was away for a few weeks and just saw it last night.

What I am thinking is that there is maybe some mechanical resistance that has caused the plastic gear to strip again. Have you checked it again since it was replaced?

I'm looking at a transmission (4+3) overhaul, myself. Do you have any opinions on who does good work in that department?

I was kind of surprised by the comments your posting drew: I thought a Corvette was a Corvette. I still consider my 86 Coupe to be a superior autocross car to most of the C5's until the Z06 came along.