Triumph Repair: Spitfire Tach, voltage drop test, battery cables

Question
My 1500 tach flips over to max RPM whenever I am driving.  Zeros out when off but goes to 50k when if first turn the ignition (before starter engages) then pegs out at 70k while engine is running.  Any thoughts on what may be wrong and how to fix?

Answer
Hi Pat,

I would look at the ground on the back of the tach to see that it is a good ground. Also test the charging system to confirm that the charge rate is not high (13.8v to 14.5v).

The tach counts the number of pulses the ignition goes through. I was lucky to have a scope avaiable at most of the dealerships I worked in and I did find that faulty ignition systems can drive a tach crazy. I also found one car that was having several unknown strange problems in the electrical system that several mechanics missed and it was a leaking diode in the alternator. The charge rate was ok but when I put the charge voltage on the scope I spotted a small amount of AC current in the charge voltage and this drove the tach crazy. Even a poor or loose connection anywhere in the positive circuit (battery cables, alternator power etc) can drive a tach crazy as current always looks for the easyest way to go and when a tach is in a circuit it can try to go through the tack. This includes the ground circuit. (a poor or loose ground cable on a battery or on the body or engine to frame ground strap.)

The ground circuit is easy to test. Just take a volt meter and set it on a low DC volt scale and put the positive test lead on the engine block and the negative test lead on the Negative battery post (not the cable end but the post itself) While the tach is reading wrong, note the voltage. It should read less then half a volt. And it should be steady. This is called a "voltage drop test" used to test connections. The engine has to be running to run the test. The positive circuit can be tested the same way. Put the positive test lead on the large post of the alternator and the negative test lead on the positive post of the battery (not the cable end) while the engine is running and the tach is reading wrong and you should not see more then half a volt DC.

It can also just be a bad tach. Without a known good tach you will have to run all the tests you can first and a bad ground on the back of the tach is the first thing to look at.

Howard