Gauge problems

dynamical-duck · 12-28-2003, 12:33 PM

I fixed my car's starting problem with a new Optima battery and a new tranny ground strap. But when it fired up and ran, the tach was completely dead (turn signals/hi beam lights work). The speedometer also is shaky, but meaures approximately the right speed. The fuel gauge is stuck at 1/2 full, but it has been like that ever since I got the car, and is probably the fuel sending unit. The tach and speedo worked fine before I put in the new battery.

I did not touch any wiring in the cabin, except to replace all the 8-amp fuses. I did move the batt. neg ground from the middle of the engine to the little 10mm bolt on the extreme right of my 1972 engine, because the Optima had the posts reversed.

I pulled the gauges, and everything is (relatively) clean, unbroken and attached.

Any ideas on where to look? Will just moving the ground back solve the problem? I don't see any obvious reason why the ground shouldn't work where it is, but I'm not an electrican.

philinjax · 12-28-2003, 01:12 PM

The battery negative cable connection must be made solid to the chassis, not the engine. The negative ground post is on the bulkhead inside wheel well immediately adjacent to the battery.

Phil

dynamical-duck · 12-28-2003, 01:22 PM

There are two battery neg. cables on my car (and a disconnected blue wire I'm not sure about). One goes to the chassis, the other to engine.

dynamical-duck · 05:01 PM

Just tried out the new voltmeter I got.

Pos. batt. cable (attached to fuse box bracket) to all 3 tach wires: 12 volts

Pos. batt. terminal to tach wire at coil: 12 volts

Pos. batt. terminal to #7 on the wiring harness at relay board from up front: 5 volts.

I'm not sure if the terminal to harness check made full contact, though. I'm thinking maybe dead internals in the tach.

Dave at Pelican Parts · Dave at Pelican Parts's Garage

The black/purple wire from the tach should be hooked up to the (-) terminal on the coil. The purple can be tough to see after 30 years of fading. Make sure it didn't get hooked to the (+) side of the coil.

The speedo is mechanical. Changing the electrical system won't change it at all, unless somehow the starter was trying to use the cable as its ground path. In which case, you can melt the cable, which can give you the symptoms you mention. (I haven't heard of the speedo cable serving as a ground, just the throttle cable or clutch cable.) Chances are the speedo problem will be cured by a new cable.

--DD