Hey guys. 76 regal 455, tach has always worked perfect (auto gage). Just started happening above 4,000rpm the needle jumps around, car is running perfect but the tach jumps around a little and never did before, I checked connection at distributor and checked ground. Any ideas? Thanks Ray
Personally I would go over the connections. As in unattach them and check for corrosion. Also check the wire(s) themselves for any kind of problems. Beyond that I'm sure others will chime in.
Are you running an H.E.I. Distributor by any chance? If so, a bad pick up or one on it’s way out will cause this.
Put new coil in, tried different battery. Still jumps when driving. In driveway I rev it up and no jumping only when driving
Okay, I know very little about cars, HEI distributors, or most anything like that, but, I am an Electronics Tech. From reading the thread, I suspect you have some interference when you are moving. I would try to put a choke on the power cable from the distributor, just before the tach. If you have interference, that might either minimize or eliminate it. Sorry I cannot help more, but, I am kind of out of my element here. If were a telephone switch now, I would be all over it.
Put new coil in and tried a different new tach. Still no cure, anything else in the distributor that could cause this?
Like what Ray had said and I. Did you change the pickup? Mine also did this. Putting in a new pickup fixed it.
Checked all connections probably 3 times. New pick up coil, tried different battery. Tried different brand new tach. Tried brand new plug wires. Still does it.
Check power on distributor/coil. There clearly is interference and based on RPM it's as more load is applied. Next check plug wires and tach wire routing. Plug wires can be dirty rf generators.
I have my rpm gauge and other gauge cluster (water/oil) together in panel by feet. I guess I can try n seperate them, just don’t know why it would matter since I’ve had it this way for 3+years. And have now taken apart and checked
Something had to change. Im not suggesting you separate gauges, I am suggesting that you eliminate potential causes by moving wires away from potential noisy sources. 4000RPM is roughly 66Hz. Though, just maybe there is enough scuffing of tach wires to begin compromising insulation or potential a wire was stretched over time. Or, directly wire a tach from under hood not using or running the same path as the prior wires, tape to windshield for test drive or if you can get to misbehave with only RPM change (no load) then just in the drive way would be worth a check.
That was my next trial was to rewire gauge seperately, when I tried the other gauge to eliminate it being a gauge problem I tapped into same power. I used a different ground and obviously different green (tach) wire to dist.
Traditional HEI coil did you reinstall the ground under the coil? These use to get left out all the time.
