Hello,

Was hoping to get some advice on a problem I'm having with my '69 Camaro (Z28).

Tach does not register any reading at all unless the car is run for a while (20 minutes or longer) and allowed to get good and hot. Once it's real warm the tach will spring to life - although it bounces around considerably once it starts to display a reading.

Here's some background.

I've recently made some changes to improve the way the car runs. Previously I was getting a weak spark at the coil and the car would either not start consistently or would shut down once the car got hot. We determined that the resistor wire (original) was failing (it got incredibly hot). So we replaced the resistor wire from the bulkhead/firewall to the coil with a new section of 12 gauge wire. We installed a ballast resistor between that lead and the coil and replaced the coil with a new Mallory unit (29216 - does not contain internal resistor). The car now runs real well, but the tach no longer works (or does not show a reading) - until the car gets real hot (let's say after it's running for 20+ minutes and the engine compartment gets real hot - like if I let it idle in my driveway for a while after taking it out for a ride). Once the tach does start working, it bounces around considerably (like from 500rpm to 2,000rpm and everywhere inbetween). Even before the changes, although the tach worked, it still bounced around a bit (maybe from 500rpms to 1,200 rpms).

I'm stumped. The coil is new, the ballast resistor is now, we've replaced the e-spark module in the distributor (Mallory Unilite) as well as the rotor. I know a lot of folks do not like Mallory, but I don't want to change to an MSD at this time. I want to try and fix what I've got.

Here's some more background information When I test the voltage in the ignition system, I getting 13+ volts at the incoming lead on the ballast resistor, 11.5V at the outgoing side of the ballast resistor and the same 11.5v at the + lead on the coil (that's the only wire on the + side of the coil). On the negative side of the coil I'm getting 8.5v to 8.9v - which Mallory says is ideal. I've got the green wire to the distributor coming off the - side of the coil as well as the brown wire (which should be going back to the tach). The red lead for the distributor comes off the incoming side of the ballast resistor.

Anyone ideas why the tach is not working?

Thanks,

Mike

 

Factory or aftermarket tach?
Have you tried messing with the tach wire (bending, pulling in different spots) with the engine running to see if the needle jumps? For all you know, the wire could be broken or shorting out anywhere between the coil and tach.
A stepped down power supply to the coil was necessary for points and condenser ignition, but not for all electronic types. Does the Mallory distributor require a lower voltage supply? That wouldn't affect the tach reading though, just curious about the main hookup.

 

Thanks for the quick response. Tach is factory stock Z28 tach. Tried wire jiggle (brown tach wire) where it connects to coil - no change in tach behavior.

Hookup is as per Mallory Spec - ballast resistor between ignition switch and coil (coil does not contain built in resistor) - Unilite gets power off green wire from - side of coil. Confirmed set up with Mallory tech support (they have no clue as to why the tach does not work).

I've not traced the brown wire all the way back to the tach, but it seems that if it were bad it would never work. The fact that it works "only" when the engine gets real warm tells me that something is changing in the coil output once the coil gets really warm (like the current is dropping) and the tach seems to like the lower output better. Just a guess - don't know.

Any other ideas?

 

The tach is reading coil discharge pulses. If juice wasn't there for a tach to read, the engine wouldn't run. Have you tried an aftermarket or tuning tach, if you have one?
Also, as things heat up, so does wiring that has electricity flowing through it. Just a thought as to why things could change. It could also be that your factory tach is just shot.