Tachometer Works then Doesn't, Then Does!! Grrr!!

[momack55]

Hey y'all!!

Rallye dash '70 Challenger Tachometer, works fine sometimes, then dont!!  Then I can be driving and it comes to life again! WT??  Have fiddled with wiring, tried everything except pull out the cluster!!  It just seems to have a mind of its own.  When I just think its finally gone to heaven, it starts working again!!   
I sooo hate elec problems!!

Anyone got the same thing goin' on???

Mozz in Oz!!   

[dodj]

No I haven't had that happen to me, but it is a loose connection. It may be on the circuit board of the tach, a problem where it goes through the firewall, connection on the coil itself, the connector on the tach, etc. Not an easy one to find.
If you had another tach to sub in for a while you would be able to determine if it is the tach itself or external wiring connections. 

[momack55]

Fanx Dodj,

Time to pull the cluster!

Mozz! 

[MEK-Dangerfield]

Well, the tach gets it's ground connection from being installed in the gauge pod that is connected to the dash frame. So that isn't your problem.

So we now have to wires connected to the back of the tach. The blue wire is the "+", connected to a stud. The gray wire is the signal from the "-" side of the ignition coil. It's connected to the tach by a slip on spade connector.

A suggestion would be to make sure the gray wire connection is tight at the coil.

My fear is you might have a small crack in the circuit board of the tach. It might be dry rotten by now.

[momack55]

Cool!!

I have good background in electronics. I just thought there may be simple common problem fix from the outside!  Its not too hard to just pull the cluster and check wiring and the circuit in the tach itself!  Then run new wire if I have to. A lot of the wiring in this car is old and dying!  I fixed the main board in my electronic gas injection system, so this should be simple as there is not a lot to how the tach works.

I'm pulling it out tonight

Mozz!