I have an old Sun tach that I put on my car. When idling, it reads about 1600-1800 rpm and its jumps around a good bit. When i put the car in drive the rpms go up a little to around 2000-2200 and the needle becomes even more erratic. When i accelerate the needle will drop to 1100 rpm and then seem to function smoothly and correctly.

I purchased a new tach to replace the old one because I thought it was faulty. The new one is behaving exactly the same.

I put a test light on my coil and when idling the light appears to flicker. I put the test light on the alternator and it gave a constant bright light. I don't know if this is normal or not? Would this cause the erratic behavior of the tachs? Would my cam cause this? (Comp 264)

Green-Negative Side of Coil
Black-Ground
Red-Switched Power
Blue-Illumination

Most tachs. come with a switch to select 4-6-or 8 cylinders. Make sure you changed it to a 6cyl.

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All in a days work

It is set to 6 cylinder. One of the first things I checked when I started. Good point though.

I wonder if your coil is shorting out or starting to go bad. Have you tried another coil yet? You are attaching the green to the negative side of the coil correct?
Where are you getting the power from?
I hooked mine in at the 12 volt side of the resistor.
My tach is steady as a rock.

That particular cam would not cause a problem.

Let us know..

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Aggressive Ted

http://cid-32f1e50ddb40a03c.photos.live ... %20Swinger


74 Swinger, 9.5 comp 254/.435 lift cam, 904, ram air, electric fans, 2.5" HP2 & FM70 ex, 1920 Holley#56jet, 2.76 8 3/4 Sure-Grip, 26" tires, 25+MPG

I have heard that without the capacitor on the coil tachs will act odd. Do you have the capacitor on the coil?

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Part of Tyrde-Browne Racing

I do not, it came loose when i hooked the tach up. I'll give that a shot. Thanks.

I hooked the capacitor to the positive side of the coil. Initially, this did not seem to make a difference. As i backed out of the driveway, the needle seemed to settle down and show a smooth idle around 800 rpm while i was in gear. It showed an idle of about 1000 rpm in park. I

drove off and noticed the tach was sticking at 2000 rpm, something it has never done before. By the time I got back from my drive, the idle had become erratic again with the car in gear. As soon as i put the transmission int park or neutral the idle immediately smooths out and stays at 1000 rpm.

My tach was jumpy until I started fixing the harness problems like adding a second supply wire from the alt to the battery and using the old harness to power relays that are hooked directly to a battery feed. I'm running a sunpro and no capacitor on the coil (73 elec ign)

The documentation from sunpro states that the tach may read high if the supply voltage is low.

I would guess that the problem is not in the tach or the ignition system but is in the charging system or old wiring.

To test take a 12v wall wart (power supply) and plug it into an extension cord and power the tach with that and see how the thing behaves