The Dodge Challenger Message Board

I sent the (stock) tach from my 70 R/T to a guy here in the U.S. for repair. It had not worked since I bought the car in 1975. The problem with the repaired unit is that it will works fine, (although a little slow responding) for about 5 minutes after initial start-up then slowly stops reponding at all. If I stop the car for even a couple of minutes say to get gasoline. It will do the same thing except stops working in less time then the initial start-up.

I have sent it back to the repairman once whereas he tested it for 20 miniute and it worked fine. He suggested that there is a problem with my wiring or ignition. I have the Mopar dual point to electronic ignition conversion set-up along with a new Accel Super Coil. I changed out the ballest resistor as recommended with no change in results.

Any help would be appreciated.

Jon

Check the tach is getting a constant and good 12v from the battery and then check you have a good ground to chassis in the negative side. Both things could cause problems.

The signal that drives the tach comes from the negative side of the coil of course.

Thanks for the information Dave. I think the negative wire is correct as I have traced it to the - coil pole. I also have attached a separate ground wire to the back of the tach housing. The blue wire that feeds the 12 volt positive may be the problem. Do you know off hand if it passes through the voltage regulator on the instrument panel? I will also try checking the wiring diagrams provided on this site.

Thank you,
Jon

Don't know if this is an option for you but at the recent Monster Mopar Weekend in St Louis MO USA there was a vendor selling electronic replacements for tachs and clocks. Don't know anything about them though. http://www.rt-eng.com/ for details.

e body tachs get their power thru the bark blue wire- this is from the red radio wire plug!

John Mac

Disassembled the dash (again) to get to the tachometer wires and check the voltage as Dave suggested.

I ran the engine for 30 minutes and could not get the tach to malfunction! It was still slow responding but reluctantly followed the RPM curve and did not fall to "0" as before.

I am pretty sure that this is a heat related issue. Possibly the cooler weather and the open dash kept things in the right range.

As for the voltage readings:

At idle it is approximatly 12.2 v on the positive side and 4.5 v on the negative (coil wire).

At 3000 RPM the voltage is 14.3 positive and 7.4 negative.

Are these normal values on the positive side? I assume the neg. would go up to affect the reading on the tachometer (?) I also checked a few other positive wires on the wiper circuit and they also show 14+ volts at higher RPM's. Could there be excess voltage affecting the system? Such as the positive voltage raise at RPM working against the negative.

Thanks,

Jon

The tachometer did fail again after a long drive in warmer weather so I contacted Greg at www.rt-eng.com. He was very helpful and I ended up ordering one of the replacement circuit boards from him.

The instructions that came with the board were easy to understand and the installation required simply soldering 4 wires. The tach now works fine and is actually a little more responsive than the repaired unit. (When it worked)

I will try to send a picture of the new unit.

Oh and thanks FAL308 for the great tip!

Jon

what happenes is the old electronics ( Usually the capacitors) dry out and the tach signal dies out. 66-67 chargers have this problem as they are over 30 years old!!!!!

When I start the next project in waiting ( 67 Charger) I will get the RT tach board!