Erratic Tachometer

FastDriver

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#1
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
 
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GroverDill

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#2
If I remember getting the stock tach to work with the BS3 was a big PITA. I hate to say it but a bad connection or gorund is likely the problem.


Just get the racepak :)
 

FastDriver

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#3
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:

TFI coil wiring.jpg


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:

Might need.JPG
 
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GroverDill

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#4
you know the shift light had been acting up for a while if that is same source for tach then I think you are on the correct path.
 

HotFox

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#5
I can't speak for the Big Stuff but I run a MSD 6AL. I bought the car with a non working factory tach. Since the engine harness is long gone, I spliced into tach from the back of the dash right to the tach input. It is shared with the aftermarket AutoMeter tach. Both now work. The Ford tach requires a square wave signal that is why you don't need a adapter. This is BTW a 1992 mustang, looks like the early ones may require a adapter.
 
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JMGlasgow

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#6
The MSD 6AL needs a tach adapter to work with a Ford tach. I had to install one on my '85 GT to work with a 6AL. I have one I'd sell if you need it.
 

90sickfox

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#8
My 88,89,91, and 93 did not need anything to run MSD 6al. Tachs all worked.
 

TOOLOW91

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#9
My 88,89,91, and 93 did not need anything to run MSD 6al. Tachs all worked.
With a stock ecu .
Once you pull the stock ecu you need to feed the tach some how .


@FastDriver

I have mine fed from the tach out on my msd box .

Is it possible you have a break in the wire where it’s tapped to get the feed from the bs3?

Obviously not coming from the msd because you disconnected the light and the tach still works irratic.

It almost looks as if it’s picking up some type of inductive voltage somewhere in the video . You figure a broken or frayed wire it would just drop out totally and come back to life.


Wondering if you have the cluster out of your other car you could try to use as a diag tool to see if it could be the tach itself
 
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FastDriver

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#10
Unfortunately, I do not have the old cluster.

Inductive voltage hmm? I guess the wires just have to be inspected throughout to see what can be seen.

I'm a little confused as to how the tach signal is ultimately sent. Do you know if it goes from TFI to Big Stuff, then to MSD, or if the TFI sends signal to both the MSD and Big stuff separately?

Seems like whatever the case is, the BS3 is getting a solid signal. So if there's a common signal from the BS3 to the MSD, that would make sense. I think I'm following your logic here. As to how the tach is wired, I really haven't the faintest. I guess I should pull the dash and take a look.
 

TOOLOW91

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#11
Unfortunately, I do not have the old cluster.

Inductive voltage hmm? I guess the wires just have to be inspected throughout to see what can be seen.

I'm a little confused as to how the tach signal is ultimately sent. Do you know if it goes from TFI to Big Stuff, then to MSD, or if the TFI sends signal to both the MSD and Big stuff separately?

Seems like whatever the case is, the BS3 is getting a solid signal. So if there's a common signal from the BS3 to the MSD, that would make sense. I think I'm following your logic here. As to how the tach is wired, I really haven't the faintest. I guess I should pull the dash and take a look.

I’m not sure on the big stuff 3 how it would be wired . If the signal is clean though like you said you know you don’t have a distributor issue . I’m wondering if they maybe used one of the coil wires to grab the tach signal . I’m pretty sure that’s a nono once you add an msd in though .

Best bet is going to be pulling the cluster to see where the wire is tapped in and runs too
 

TOOLOW91

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#12
Or I am wondering if I am overthinking this completely and one of the 2 grounds that feed the cluster are just bad . Still wouldn’t explain the shift light doing it too .
 

Noobz347

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#13
This might be a good time to temporarily splice in an inductive pickup over the coil wire and feed it t the tach (or whatever else) to see if it's the component(s) or source.