Tachometer not working at all.

[GoMangoBoys]

Now that the engine is running, the tach is not working at all.  Before I assembled the dash, I tested the tach on my tractor.  It is a 4cyl, so the rpm value was wrong, but it did work.  I believe that I have it wired correctly.   I have verified continuity from the coil- back to the tach.  I have measured 10.9VDC on the blue power wire to the tach.  Later o will figure out why it is a little low.  Or, maybe the lower than normal voltage is the reason it isn't working??? The only other thing that is coming to mind is that I added the electronic distributor instead of a points distributor.  Any thoughts would be welcomed.
Thanks.

[Blowout]

Is the gauge supposed to be grounded?

[Glenngomango]

The tach wire should be connected to the coil DIST terminal along with distributor

[GoMangoBoys]

The tach connects to the negative terminal on the coil right?

[MoparCarGuy]

Yes, the tach signal wire is the gray with red tracer wire that goes to the coil NEG terminal.
Your tach appears to be wired correctly at the tach studs. Check those stud isolator washers really well to make sure you do not have a grounded tach signal on the gray/red wire connection.
To get started troubleshooting the tach signal path, you will need a multimeter set to ohms to make continuity checks from the tach gray/red wire out to the coil NEG. Disconnect the Battery at the NEG cable/post to remove 12V from all the wiring harnesses.

The tach signal starts at the coil NEG and goes through the engine harness to a bulkhead plug (see diagrams). The bulkhead plug side is part of the engine harness.
The terminals you are concerned with is the male and female connectors labeled #20. The plug goes into the bulkhead connector. Again, #20 which goes through the dash harness up to the tachometer gray/red wire on the tach stud. The 1970 Challenger Rallye gauge diagram is below. Same for 1971. It clearly shows this tach signal path through the bulkhead.
Continuity checks:
1. Check for corrosion at the bulkhead connector #20 and the bulkhead plug #20. Your tach signal could be lost if the connection is poor. May need cleaning with contact cleaner. Visually check those terminals. They should fit tightly when plugged in. No bent brass on the female side.
If the connector and plug look corrosion-free, go to Step 2.
2. Leave the plug unplugged and check for continuity from the bulkhead plug #20 to the coil NEG wire. You may need to remove the wires from the coil NEG to get a proper low ohm to zero ohm reading. If this checks good for continuity, go to Step 3.
3. Check the gray/red tracer wire from bulkhead connector #20 female connector up to the tachometer stud for continuity.  You need to unplug the gray/red wire from the tachometer and ground it so you can use your meter out by the firewall to check the bulkhead connector #20 to ground. If close to zero ohms, then the wiring inside is good as well.
These three steps should find your issue. If they all check good, you will need to check the Dark Blue wire from the tach to Ground (GND). The 18DBL wire is grounded along with many other devices like the radio. If you have a good ground on 18DBL, then the only thing left is a possible bad tachometer.

Let us know what you find out.

[Challenger in NC]

I thought that the factory tach was not compatible with electronic ignition without either modifying the gauge or adding some type of converter? There is some good FAQ at this site:  https://www.rt-eng.com/index.php/RTE_Tach_Faq.html

[MoparCarGuy]

I thought that the factory tach was not compatible with electronic ignition without either modifying the gauge or adding some type of converter? There is some good FAQ at this site:  https://www.rt-eng.com/index.php/RTE_Tach_Faq.html

You are correct about the tach not working with aftermarket electronic ignition systems but the factory tach works perfectly with factory electronic ignition.
For aftermarket, the MSD or Summit tach signal converter will solve any problems.

[Jay Bee]

I did the factory electronic ignition conversion on my car and the RTE tach upgrade, both work fine.

[GoMangoBoys]

So I started this thread a while back, but moved on to other things.  I am now back on trying to slove my problems with the tach.  Firstly, I DID debug the wiring from the coil back to the tach.  I did this by 1 using an ohm meter and 2 by pulling the wires off the tach and connecting them to an aftermarket tach on the floor.  That tach worked just fine.  I have now pulled the tach out of the dash.  I connected it directly to the battery, ground and coil wire.  It works some, but it does not read the right RPM and it does not track the changing RPM.  When I rev the engine, it goes up and then stays there a bit before slowly falling.  I also connected it to my 57 Chevy and it pretty much behaves the same.
I am going to say at this point, I need a new tach.  What is my best option?  Is there someone who does rebuilds on these with a fast turnaround?  Anyone who sells rebuilt ones?  Are the reproduction ones any good?
Thanks

[JS29]

RT engineering makes a replacement board for our mopar tach's.  RT-eng.com  Not to bad to install. 

[7212Mopar]

Classic Industry sells repro gauges, not cheap. Don't know how much for RT or Redline to do rebuild.

[Skdmark]

RT engineering makes a replacement board for our mopar tach's.  RT-eng.com  Not to bad to install. 

X2 on the rt-eng.com tach board.
I had one installed when my gauges were refinished.

[Aloha Randy]

Mr. Heaterbox has a very good Tach Replacement board for about $100. Very good quality, turnaround and tech support. https://mrheaterbox.com/

[GoMangoBoys]

So i bought and installed the tach board from RT Engineering.  I got the one with the built in calibration.  I did the cal procedure and had problems with the tach sticking and not going back down.  The guy from RT eng advised me to loosen the screw in the center by 1/4 turn.  It helped a little bit.  I now have it 3/4 of a turn looser than when I started.  The tach seems to stick still.  Any suggestions o what else i can do to make it work correctly? This is the original tach that was in the car when i bought it.  Probably hasn't been driven in 20 years i am guessing.  It was a pain to pull out of the dash, so i want to be certain that it is right before i put it back in.  See this video.
Thanks,
https://youtu.be/RFuPLHboBdo

[MoparCarGuy]

The tach's meter movement could be contaminated with metal particles or shavings which wreaks havoc on the needle reading correctly.
The companies that restore instrument clusters and gauges may be able to repair the movement.