Bouncy, inaccurate speedometer at speed

[Texasdoc]

Tremec TKO 600. I think I'm using the electronic sensor, not the mechanical one but will have to verify it the next time I'm under the car. VDO speedometer gauge. I'm having a small problem with it.

At most speeds, it is accurate (adjustable). When I am coming to a stop (around <5 mph), the needle will jump up to about 30 mph for a second, then go to zero.

I've been living with this for years because it was mostly accurate. The other day, I may or may not have been traveling at triple digit speed (um, on the track, verified by gps) but the speedometer read 10 mph slow. At this speed, the needle was varying up and down by about 4 mph while the car was at a constant speed. When I slowed down to 70 it was accurate again.

I don't know if my problem is in the guage itself (doubtful), somewhere in the cable (also doubtful), or in the sender (possible).

Any thoughts on how to test which part of the system is bad?

[patrickt]

Here are some tips on dealing with a mechanical speedometer and cable. Lubricating the Smiths Speedometer Cable To Stop Needle Bounce I have no personal experience with TKO electric speed sensors, but I would suspect that they either fail entirely or work normally. If you're handy with electronics, you could likely bench test the cable and gauge by mimicking the output of a normal electric sensor without having to remove it. If I had to bet, I'd bet on a sticky gauge mechanism, or mechanical cable if you have one.

[Texasdoc]

Definitely not a mechanical cable. Either straight from the electronic sensor to the gauge, or the mechanical output to a driven gear sensor.

[Gaz64]

I would say to read 30mph for 1 second at <5mph intermittently is a sensor or wiring issue.

[PHMCobra]

Similar problem with my Backdradft Cobra (1360)--shortly after I purchased the car. The speedometer would jump erratically at times just as you have described. I cleaned the electric coupling from the tranny to the speedo panel--located from the tranny to the passenger side of the tranny under the car. Perhaps a poor connection from installation-- just whatever. I have had problems since! Drive SAFELY! Phil

[PHMCobra]

Missed and important word in my last Post. I HAVE HAD NO PROBLEMS SINCE CLEANING THE TRANNY WIRE TO THE SPEEDO WIRE. Good luck in your fix!

[Texasdoc]

I'm thinking it is probably the sensor wire. I'm going to pull the old wire and install an entire new run. Should this sensor wire be shielded any grounded on the sensor end?

For future searches: What I've learned is the Tremec's built-in sensor is an inductive sensor (not hall-effect). The plug for the electronic speed sensor is a 2 pin Delphi connector. This is located on the right side of the TKO.

Some people also suggest adding a diode on the "sensor wire". There is some discussion of electronic noise picked up by the gauge thru the sensor wire.

Edit: looks like if running an inductive sensor, you need to use shielded wires. If using a Hall-effect sensor, you use the diode.

[Gaz64]

Yes, probably run shielded sensor wire, with the sensor end of the outer shield grounded to the transmission case.

Gary

[Ron61]

I had the VDO gauges in my Cobra and had the same problem. I finally traced it down to the ground and sending cable had worn a hole in the insulation and at time they would touch. I took out all three cables and ran new better ones and never had a problem after that. I also had one VDO speedometer head that went bad and caused that kind of problem but it was one that had been modified and they replaced it. I liked their gauges, especially as they were accurate and it was easy to change the speedometer if I changed gears or tire sizes. In the 15 years that I had the car I never had any problem with any of them except what I mentioned here.

Ron