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Flatlander
05-26-2011, 02:49 PM
I am trying to troubleshoot problems with the tachometer on my 77 Spitfire. My old tach was not working then finally I got it to work somewhat. It would seem to work properly during idle, registering 1100 rpm. When the gas pedal is pressed the needle would drop to 0 and remain there until the gas pedal is releasded and the engine returns to idle at which time the tach returns to 1100 rpm. I have tried using another tach out of a parts car with the same result. (I also thought about buying a cheap tach from Harbor Freight in order to rule out the tach.)

My thoughts are that either both tachs are bad and need to be rebuilt or there is something wrong with the voltage stabilizer. Any ideas? I have pulled the tach out of the dash and direct wired to the battery ground, power in dash and negative lead on the coil in order to rule out bad connections. Any way to check the working of the voltage stabilizer with a volt meter?

Thanks,

Gary

dklawson
05-26-2011, 03:11 PM
The tachometer is not powered through/by the voltage stabilizer. If it is currently hooked up that way, change it so the tach is getting full battery voltage. However, since you have hooked the tach directly to the battery for testing, that doesn't really explain what is going on.

If you have a tune-up tachometer or engine analyzer, I would try that before investing even in a Harbor Freight tach. It seems odd to me that two tachs would have the same failure mode. I'd hate to see you buy another tach only to find out the problem is somewhere in the car.

What is really strange is that the tach is dropping out at higher RPM BUT the engine still revs. If the coil were failing I would expect you to report a bouncing needle at speed or that the engine misfires at higher RPM.

TR3driver
05-26-2011, 03:16 PM
What do you have for ignition? I think a 77 would have originally had one of those funky Lucas 'electronic' distributors, in which case I would suspect the electronics.

Doug is exactly right about the stabilizer, it should only supply power to the smaller gauges, not the tach.

Flatlander
05-26-2011, 08:23 PM
I looked at the wiring diagram again and the power for the tach is shown going through the voltage stabilizer but not acted upon by the stabilizer. (I assume I am reading this correctly in that the "green" wire coming to the stabilizer is the "un-stabilized" voltage and the "light green" wire leaving to the fuel and temperature gauge is the "stabilized" voltage. So the voltage stabilizer has no impact on the tach.

Also the electronic ignition system has been removed and a points ignition system added.

Not sure what to try next.

DNK
05-26-2011, 11:13 PM
Prolly just piggy backin of of the other green

dklawson
05-27-2011, 06:56 AM
In Lucas wiring schemes, the "B" terminal of the voltage stabilizer is the "Battery" connection and is at full battery voltage whenever the ignition is on. That terminal is often used as a junction for other circuits. If your green wire goes to stabilizer "B", that is not going "through" the stabilizer.

Regardless, your test connecting the tach directly to the battery indicates that the problem is either in the distributor electronics, or (less likely) you have two tachometers that have failed in exactly the same manner.