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I have a factory stock tach in my 67 RS/SS. I replaced my distributor with a ready to run MSD. After replacing the distributor the tach now reads half of the RPM scale. So if the engine is doing 4000 RPM the tach reads about 2200 RPM. I sent the tach out and had it rebuilt. They tested it and said it is working fine. It still does not work correctly. I called MSD and they said to replace the ground straps in the engine compartment. I did this and tested them to make sure I was getting a good ground to the engine and to the frame and body. Everything checks out. The tach still will not work properly. Last night I took the car out as I had just finished putting in new front head lights. I normally do not drive the car at night but took it out to test the headlights. As soon as I put the head lights on the tach starts working correctly. When the running lights or the headlights are on the tach will work properly. When they go to off the tach will not work properly and only gives me a half scale reading once again. My questions are= 1. is the back of the tach supposed to be grounded and if so where do you attach the ground to the tach and where at on the body? 2. If not a bad ground, could the circuit board be bad and when the headlights come on it some how allows more power to go to the tach? 3. Or is it something totally different I am not thinking of? Thanks for any help [email protected]
 

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I am having MSD box problems and have checked out the circuit, an easy test of your circuit board will show the power for the tach loops through your indicator lights and ends at the tach. Start at the plug for the cluster with a VOMeter and check each exposed terminal at each point. There is a ground screw on the circuit board to the housing of the back of the cluster. Could be a bad connection, ground with a jumper to test for bad ground. I think to complete the circuit a ground wire exists in the plug at the printed circuit.
 

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tell me more about your car. What year is it? what kind of ignition did you replace? Exactly what tach? original, OER, or aftermarket? Are you running a stock charging system with a mechanical reg, or? Have you tested the voltage? Check it at the horn relay, both with headlights on and off. Has the car been painted recently, and was this done with the front clip off?

That it works with the headlights on makes me suspect a charging system issue, perhaps related to voltage and load.

When I recently fired my car after detailing and painting everything under the hood, my charging system was putting out 18v! Found that all the new paint was leaving me with no ground to the rad support, and therefore no ground to the regulator.
 

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I have a MSD 8360 ready to run with a Blaster SS coil and my car, a 69Z still has it's factory original tach and it works fine. If you have an aftermarket OE tach or if it's been reworked/rebuilt you will need a tach filter. (available from MSD or make it your self) Your problem does sound like it may be power or ground problem. I believe your tach has three wires on the back. One is ground, one is power and one goes to the negative side of the coil or if you have a MSD ignition box it would go to the tach terminal on the side of the box. To check the ground just use a jumper from any bare metal body area under the dash and clip it to the ground on the tach. It's interesting that turning the headlights on makes the tach work properly. It's possible that your tach may either not be grounded or it's not getting proper 12 volts. Some how the 12 volts for your headlight/parking light circuit is finding it's way to the tach. Check your wiring to make sure your distributor/coil is getting a full 12 volts and is not being supplied by the resistor wire or a ballast resistor is not in there anywhere. Check the wiring on the back of the tach to see if it was added later. If it was, it may not be wired properly.
Jim
 
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