Bad Speedometer Ground?

[dbrownlfk]

I just acquired a 1987 911 Targa and drove it cross country home. It drove amazingly and we had an awesome time. The only nagging issue I had was with the speedometer when the headlights were on. The speedo would work great if I dialed the interior illumination to 0. As I increased the interior illumination brightness the speed reduced inversely proportional until the speed read 0 and the lights were at the brightest.

My gut tells me that something is up with the ground and that the gauge lights are using the speedo signal wire to ground back. I'm new to the 911 world and have been studying up on the Bentley. From what I can tell the gauge lights ground internally to the speedo. Does anyone have any other ideas?

[GH85Carrera]

Weak or bad grounds are 80% or more of the electrical issues on air cooled 911s.

The electrical system on G body or the air cooled cars from 70s up to the 964 were poorly designed, as they were just patched new features added to a car designed in 1963. The delay wipers is the classic example of add on features.

You need to start reading the posts on this thread on electrical issues. There are several circuits that the factory did not even fuse, that need fuses. The dash lights are one of them!

Good luck with your new toy. It is a fun big boy toy.

Check every ground on the car front to back, and read the electrical threads on this site. It will scare you at first, but soon you will see it is mostly easy stuff to fix. It will all depend on your particular car. If it was owned by a guy that fixed all the issues you are golden If it was ignored, you will have lots to tinker on.

[dbrownlfk]

I pulled the speedo out last night and the main ground had come completely unhooked from the speedo. The connector was very loose. I bent the connectors until they were tight again and put it all back together. As good as new!

[Walt Fricke]

Clean up the battery to chassis ground first. Most important for instruments, and easiest to reach. Get a voltmeter that plugs into the lighter socket to check voltage with high loads (like at night).

The sensor and its wiring isn't part of the problem. But the sensor ground is from the speedo ground, so if speedo ground has some resistance, could influence things. Signal is just a square wave from a reed switch opening and closing 8 times per wheel rotation. If that charges/discharges a capacitor in the speedo, I can see low voltage could do this.