I have a tach in a TR8 that works then stops working then works again. Trying to clean every contact surface that the signal wire passes thru is not cutting it. I need to find exactly where the problem is. I know the tach gets the signal from the negative side of the coil, but thats about the extent of my knowledge on how a tach works. My guess is somewhere along that wire's route, I have a bad contact or a broken wire. Problem started after I spent a couple of weeks chasing another electrical issue. How do I test the signal wire? Is continuity the only way, or is there a way to see if I'm getting the ground signal from the coil going on off several thousand times a minute?
You are correct, the tach gets its signal from the '-' side of the coil. Try this: run a separate wire from the '-' terminal to the tach trigger, then drive the car. If it keeps happening, it's probably the tach. Doesn't have to be anything fancy, you can even run it carefully out of the back of the bonnet and in through the drivers door.
This can also be indicative of a failing coil, in my experience.
In reply to SlickDizzy:
That would make sense, breakdown causing unwanted voltage on the - terminal.
That might explain some of the weird electrical issues I was having. Kept getting voltage readings on my ground wires. Wasn't the full 12 volts, but more like 3 or 4 volts. I chased wires to the full pump, fuel sender, initial switch, OP switch, come to think of it just about everything that got switched power. Don't know exactly what I "fixed", but when I was done, everything worked except for the tach and the speedo. I managed to snap the end of the cable where it plugs into the speedo.
wbjones
UltimaDork
7/20/14 3:49 a.m.
naw … the weird electrical issues are because … LBC ...
Not to threadjack my good friend, but I was going to start a similar thread, so why not consolidate? Just over the last week or so with my 79 X-7 the tach has been acting strange at times.
At idle the engine note doesn't change, but the tach will jump from 8-900 rpms to around 2500-3000, and then back down sort of like a metranome. Then while cruising with the tach around 2500 it will jump back and forth from there to 5000 rpms, again like a metranome. The engine is running just fine, but the tachometer is acting strange.
Coil, tach, ignitor, gremlins? FYI I am running an 83 ignition system instead of the 79 points system. I have had this system in the car since I bought it in 2005, and have had zero issues until now.
LBC, maybe, but the coil is German, the tach is French, the fuel pump is American made in Japan. Chris, I'm having enough trouble diagnosing a car with one coil. I'd be twice as confused with that dual coil, spinning triangle, buzzsaw you have. When are going to give up on that truck and let me throw a Rover V8 in it?
The red truck? If so I just started taking it apart, and it is in much worse shape than I had thought. You are welcome to put a V8 in it if you want. Of course you will have to help me repair the rotted floor boards, and such.
Chris
