Tach Trouble

[jshcobra]

The tach on my ghia recently went south. It seems to work fine until I turn on the signals, headlights, or heater fan- then it jumps around and when it does settle down, it reads 2x the actual rpm. I replaced the tach adapter with a good one to see if that was the problem but it made no difference.

Any ideas? Does the voltage regulator on the instrument panel have anything to do with it?

 

[Wart]

Isn't it a bitch?

The tach is seperate from everything else on the dash.

Except maybe the grounds.

 

[jshcobra]

Thanks Wart. I'll start checking the headlight, blower, etc circuits for shorts. Obviously current is getting into the tach circuit somehow.

 

[Wart]

I'm stripping a Ghia, I'll see if I can find the grounds when I get the dash out.

The headlights are grounded at the headder, I don't know where the dash lights or turn signals in the dash are grounded. THe fan has me scratching my head ... AC or Non AC? I wonder if htere is a potential for feedback?

The '75 cluster circuits ground dash illumination and turn signals through the 4th tab from the bottom on the right side of the dash socket when looking at the back of the dash, that would be the 4th up from the bottom left when looking at the plug.

Least thats how it is with the D5 ribbon on the D4 cluster I have sitting here on the desk.

 

[jshcobra]

I suppose it could be picking up the errant voltage at the coil. I'll start there. Thanks for the input.

 

[Wart]

The only power at the coil should be from the resistor wire.

I got part of the dash loome disected. THe ground runs to a connector that has several ground wires ran to it.

Interesting thing is the green yellow wires insulation was fused to other wires inthe loome, as were the remaing two wires. I guess it's possable for the wires to have done a bit of cross circuiting, specially if the harness has been moved around a bit.

Now you got me curious, I don't need the harness, I think I'll disect it.