Erratic Tachometer

Edit: This is probably tech. I guess posting automatically in general is a bit of a habit. Sorry, mods.


I am not good with electrical issues, and am about ready to farm this one out, but I thought I'd check in here first in case it's something obvious. Since I picked up the '93, the stock tach does this. The shift light started staying on constantly about a week ago, and I decided to try to diagnose it yesterday.

Relevant info:
- Stock Ford Distributor and Motorcraft TFI module.
o I replaced the TFI module a couple weeks ago when the original crapped out. The tach issue already existed, it continued when I stabbed a "known good" distributor/TFI in, and after I pasted a spare TFI module back onto the first distributor and reinstalled. So, I think TFI & PIP are very unlikely candidates at this point.

- Big Stuff 3 runs the show, and the RPM signal to it is solid.

- MSD 6AL runs an MSD Blaster TFI (P/N 8227) coil. Joe gave me an spare coil and swapping it on made no difference. I've ordered an 8253 HVC coil, and if it fits in the wheel well, I'll replace it, again. I've got an HVC coil on the black car and trust it a little more than the blaster series.

- I don't know exactly how the tach is configured, but I'm guessing it's from the 6AL somehow.

- The shift light runs off of optional "tach out" wire on the 6AL. I unplugged it and at least the shift light went out.

I've read that perhaps a frayed wire, or one near other wires is getting "voltage interference." I've also read that a bad ground is the next likely culprit. I'm not sure, at this point, what steps will be easiest to diagnose. I don't want to pull the dash if there are easier things to diagnose. Once the dash needs to be pulled, I'll let someone else who's more electrically inclined take over. lol

Haha! I have the racepak in my other car. There's something kinda nice about original analog gauges, though.

Somehow, I suspected that both the tach and the shift light were coming from the same source, because they were both acting up. So, this is the wiring instructions for this coil and MSD box:



The shift light came off of the tach output. I'm guessing "FACTORY FORD HARNESS" is where the BS3 enters the picture, and where the factory tach gets its signal. Still, the signal must be coming from the MSD box somehow, since the BS3 is stable.

GM cars need an adapter with the MSD box. Big Stuff uses a lot of GM style sensors... perhaps it runs ignition the same way, too. If so, maybe I need this: