69 Charger electrical question

[sixt9rsx33]

I have a 69 Charger RT 440 auto with air PS and rear defroster. I restored the car and upon completion I have an electrical gremlin. When I turn my fan on for the AC/heat and at idle my tach will move up to around 2000 rpm’s. When I accelerate the tach will settle to the correct reading. The tach works otherwise when the fan is not on or while driving. Seems at a low voltage situation the tach acts up. Any ideas?

Thanks

Lawrence

[thehornworks]

Probably a bad or missing ground.

[sixt9rsx33]

Thanks will try and find that.

[njsteve]

On the old Mopars I usually have to add extra dedicated ground wires. Behind that gauge pod is a big foil printed circuit. If it is missing any of the little nuts or screws, the ground suffers and the back feed into the gauges.

Another familiar glitch is the tail light system. Sometimes you see the reverse lights dimly light up when you step on the brakes, or one of the turn signals get dim when you step on the brakes. An extra ground wire from the tail lamp stud to an existing bolt on the trunk panel cures that.

[sixt9rsx33]

I was considering installing LED bulbs in the taillight housings due to the amperage draw from the incandescent bulbs, but have not looked into it yet.

There was no resto to the dash, I just removed it cleaned it up well and installed new Voltage limiters as I recall. Has been about five years since I did this work. I will check the rear of the dash for loose grounds. I very well may have missed one. One thing to note. I removed the clock and installed a repro tach/clock. So don't have a before or after with the tach function. Thanks for the help and will let you know what I find.

[njsteve]

I do recall that Mopar frequently used specifically located screws that were the grounding screws for the dash. The pass through metal tabs on the printed circuit and gauge pod frame to the dash metal. IIRC it is one of the upper two screws of the four screws that hold the lower square panel below the steering column.

[sixt9rsx33]

Thanks Steve. I will check that out. I have that piece installed with the four original fasteners. I will make sure they are grounded.

[Rsconv68]

Does the tach needle change with moving the fan speed lower or higher?

[sixt9rsx33]

Originally Posted by Rsconv68

Does the tach needle change with moving the fan speed lower or higher?

At low rpm, with the AC on and fan speed on med or high the tach needle will go up to about 2K - 2200 rpm and just sit there. When I move off of idle it will settle down to where it supposed to be and the amp gauge will go from negative to charge as normal.

[Tinkerman]

Originally Posted by sixt9rsx33

At low rpm, with the AC on and fan speed on med or high the tach needle will go up to about 2K - 2200 rpm and just sit there. When I move off of idle it will settle down to where it supposed to be and the amp gauge will go from negative to charge as normal.

I have a 68 Charger with 440 non-AC. The older Mopar alternators are notorious for not being able to keep up with the electrical load at low RPM.

I was running a 40 amp electrical cooling fan and while at idle the Amp Meter would peg to the battery side as the alternator could not keep up with the load to run the cooling fans. It would even out once I got above 2000 rpms.


V/r,
Tinkerman