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I have an autometer tach on my steering column. I've been trying to figure out why it is jumpy. At one point, I thought I had it narrowed down to a bad ground...played around with it a bit and it was working fine. Once I put the dash back in and drove the pickup, it was jumping again.
FINALLY, I figured out that it begins jumping when the gauge cluster is plugged in. I can't seem to figure out why. I have 2 wires spliced to the tach on the plug. One is for power, and the other for lights (I believe this is correct, but will check). Since the wires are spliced in on the plug side, they are connected regardless of whether the plug is plugged into the back of the cluster. This has me a bit confused...I don't understand why plugging it in would affect anything if these wires are always connected.
It has got to be something to do with interference coming from the dash once plugged in.
If you are trying to power the tach from the gauge cluster you are possibly having your power go through the ICM which provides the gauges with a "pulsed" current. You need to get your power from a 12 volt "hot when on" source.
The power doesn't come from the cluster itself, it comes from the plug that goes into the cluster and works fine until it is plugged in. I've attached a simplified diagram that I just made in paintbrush to show what I'm saying. I will pull the cluster out again in the next couple days to let you know the exact PINs used, but from what I can tell the power should be clean.
The ground is well secured.
BTW, this is on a 74 F250. I see that I didn't mention the vehicle details.
Just a guess but I would speculate that the dash cluster ground(s) may be somewhat flaky. Not enough to affect the dash but enough that the extra load might be just over what they can handle? I'm not an electrical guy but it sounds logical.
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