99Hawk120

Ok, my tachometer is irritating me. I swapped a 91 gauge cluster into my 84 and can't get the thing working right for the life of me.

I have an MSD 6AL in the car, but the tach behavior seems identical whether it is bypassed or not.

I did finally got the thing mostly working, it turned out that that I did in fact have to bypass the inline filter with the 91 tachometer. Also, one of my tachometers appears to be bad, so I'm not using that one any more (it jumps rather than moving smoothly).

My current problem is that the tachometer is not accurate and the needle "floats" a lot (sometimes +/-250 rpm). I have set the needle at 1000rpm with diacom, but it is WAY off at higher RPMS. The RPM listed in diacom is within 50 or so RPM of the signal coming off the MSD tachometer sender (as measured by my DVM's frequency counter). I'm not actually using said tachometer sender from the MSD, it just proved to be a useful measuring point.

What I've noticed is the signal as measured at the lower left pin of the tachometer signal not only does not match the signal coming from the MSD box (nor is it "right", the frequency of it is off), it's also a very dirty signal. It floats around a bit and occasionally drops way down (the frequency counter is too slow to see what it's actually doing, unfortunately). Where does that signal come from on the stock ignition setup--from the post labeled "tach" on the distributor cap (coil-in-cap HEI)?

Also, can anyone tell me what's supposed to be at the three posts for the tachometer? The instructions in the Helms manual leave a lot to be desired. One is clearly the tachometer sender, but the other two come from that "chip" that's plugged in to the cluster and I don't know what that does.

Tim Burgess

Sounds like you may have a V6 tach, in which case the reading would be off by a factor of 4/3 = 1.33. The tach signal is taken from the "-" lead (white wire) on the coil in a factory system & should be taken from the "tach output" terminal on the side of the MSD box.

99Hawk120

It shouldn't be a V6 tach, it originally had a 5500 RPM red line. The one that's "jumping" around supposedly was, because it had a 6000 rpm red line.

I've got four tachometers, and I'm willing to try them ALL at this point. Ignore the MSD for the moment, the tachometer behavior is identical whether the MSD is hooked up or not. I modified the wiring harnesses so the the MSD can be completely removed from the equation by juggling around some weatherpack connectors, and the behavior persists even on the stock ignition setup (although the idle is definitely rougher without the MSD).

Do you happen to know what the signals should be AT the back of the tachometer? I'm trying to determine if something in the wiring and/or cluster is messed up before condemning the tachometer. How about whether or not the signals coming out of the cluster are the same for the older style tachs? I've still got my old tach ('86) and could hook it up with jumper wires if I thought it would do any good.

Tim Burgess

I never looked at the signals @ the tach meter, itself, but the only inputs to the tach circuit are ign power, ground & the tach input. You may want to verify that you have a good ground to the cluster.

99Hawk120

I can tell you right now the 3 signals on the tach itself aren't simply +12V, Gnd, and sender. The IC mounted in the corner of the cluster seems to receive +12V and GND from the instrument cluster plugs, but I have yet to figure out exactly what it's doing with them before sending to the tachometer itself.

The tests in the Helms manual are pretty bad since they mention measuring voltages at pins that I can't find. I just got frustrated and gave up.

Tim Burgess

Right. The tach "meter" receives some type of output from a frequency to voltage converter (probably the IC), but the inputs to the tach circuit are simply power, gnd & tach.