Tach not working

Mike Hausner · #1 (**permalink**) 09-18-2007, 07:17 PM

My tach quit working. The needle started jumping even though I was crusing at a steady speed. Then the next time I ran the car it didn't work at all. I have heard of others having this problem - something to do with the MSD box.
I have a 331 SBF with an MSD distributor and 6a box. The tach is SW. Is this just a grounding problem (I hope)?

nevermind65 · #2 (**permalink**) 09-18-2007, 07:51 PM

I had the same problem. The needle started jumping all over the place. 2 weeks later, the module in the distributor took a hike. I replaced the distributor (it was cheaper to replace the distributor than buy a module) and lo n behold, the tach works great again.

Mike Hausner · #3 (**permalink**) 09-18-2007, 08:01 PM

Did your motor quit when the distributor module gave up? I thought the tach got the signal from the MSD box. The engine runs fine. However I always thought the tach was wrong, especially at low rpm. This seems to be a comon problem with MSD box and the tach.

SantaFe66 · #4 (**permalink**) 09-18-2007, 10:22 PM

I had a real problem getting my Smith's tach to work with the 6AL MSD. Finally went to a shop that specialized in restoring old European instruments (MO-MA in Albuquerque) and they replaced the innards with a euro gauge and it works fine now. Of course, it had to be wired correctly, which they also helped me with.

Paul

nevermind65 · #5 (**permalink**) 09-19-2007, 05:22 AM

Sorry, I should have explained a bit more. I had a UniLite distributor, not an MSD.

I've actually had this happen two separate times on different vehicles. The other vehicle and a regulator in the alternator going bad, which was causing voltage spikes. Again, the tach jumped around wildly. Both times the vehicle ran fine with no indication of problems.

Bryan Wilson · #6 (**permalink**) 09-19-2007, 05:41 AM

I've had this problem a couple of times where the tacho starts jumping around just before it stops the cause was a bad ignition coil the car still ran although a little rough.
Cheers,
Bryan

chopper · #7 (**permalink**) 09-19-2007, 08:56 PM

One of the problems encountered with the Unilite distributors is that they are supposed to have a ballast resistor installed in the wiring in series with the power wire. The distributor is not designed to run without the ballast resistor. See ERA's website:

http://www.erareplicas.com/427man/wiring/msd.htm

Without the ballast resistor, the unit will run for a while and then the module will burn up. If your engine is still running, that's probably not the problem.

More likely, depending on the specific models of ignition box, distributor, and tachometer, you may need a tachometer adaptor:

http://www.msdignition.com/1troub1.htm#anchor335804

Hope this helps.