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Discussion Starter #1 (Edited)
Hey guys.

So I just had my mustang in the shop for subframe connectors being installed. It's all done, good to go, so I start up the mustang and notice right away, the tach is being weird!

So right when I start the car, the tach shows 900rpm, and then it just stays there, doesn't matter if I rev the engine, doesn't change if I'm driving, just shows 900rpm, it doesn't even fluster or move at all, then it goes back to 0 when the car is off.

I've been trying to look this up, and look at all the wires, but I just can't figure it out!

I have MSD Digital 6A PN 6201 ignition box
and MSD PN 8920 Tach Adapter
and MSD Pro Billet Distributor PN 8578
Stock Gauges!

I have been running this set up for about 2 years now and it's always been perfect, so I know the wiring is correct.
I asked the guy who installed the subframe connectors about it, and he just shrugged and said he didn't do anything near there.

Any ideas?
 

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I would be highly suspicious of the tach adapter.

I toasted two on my car while running an MSD box. The first time, it actually smoked it to the point it was melted...so easy to diagnose. The second time it looked fine but quit working.

Now...the only reason I'm not 100% certain is, in both cases my tach quit working all together....but still....IIWY I'd get a new tach adapter and try it. If that isn't it, keep it for a spare....because these are kind of notorious for lasting 3 or 4 years. Especially susceptible to damage if the car is ever jump started.

Did the guy WELD in the subframe connectors? May have inadvertently put high current into a piece of sensitive electronics.

Phil
 

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Discussion Starter #3
I would be highly suspicious of the tach adapter.

I toasted two on my car while running an MSD box. The first time, it actually smoked it to the point it was melted...so easy to diagnose. The second time it looked fine but quit working.

Now...the only reason I'm not 100% certain is, in both cases my tach quit working all together....but still....IIWY I'd get a new tach adapter and try it. If that isn't it, keep it for a spare....because these are kind of notorious for lasting 3 or 4 years. Especially susceptible to damage if the car is ever jump started.

Did the guy WELD in the subframe connectors? May have inadvertently put high current into a piece of sensitive electronics.

Phil
Thanks!
So I just found an old post of mine, and it was actually 3 years ago that I installed all this MSD stuff, so it could be that 3 year expiration of that tach adapter.

And yes the subframe connectors are welded in, it crossed my mind that something could have been fried, but I don't know how it could fry anything..
But I think thats a good idea to just get a new tach adapter anyway.
 

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Since it all worked good going into the shop, and didn't upon exit, I agree with "HoosierBuddy", the welding operation did it in. Perhaps, it was marginal. But then, isn't good practice to disconnect the battery when welding on vehicles? The shop guy should have known this.
 
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Discussion Starter #5
Since it all worked good going into the shop, and didn't upon exit, I agree with "HoosierBuddy", the welding operation did it in. Perhaps, it was marginal. But then, isn't good practice to disconnect the battery when welding on vehicles? The shop guy should have known this.
Ok here's some more information that could be relevant.
While this guy was doing it, there was a small piece of floor I wanted replaced, and while he did that, he cut through a fuel line, so new fuel line was installed.

So when I picked up my car, I had to crank and crank and crank to get the fuel back up.
So could all that cranking have fried something in the Tach Adapter?
 

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A good friend of mine owned a body shop for 30+ years. He always disconnected the battery before welding unless he was gas welding. Electric current can feed back to electronics and damage them. Dave R.
 

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I still think it's the poor safety practices. Especially, when you stated the tech cut through a fuel line! You are darn lucky your mustang wasn't "fried"!
 

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Time for a new shop...
 

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Discussion Starter #9
Ahhh ok makes sense...
I was wondering why my hood wasn't fully latched when I picked up the car.

I'm assuming he did disconnect the battery.
 

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Discussion Starter #10
So today I replaced the tach adapter!

Still no change.

But it is such a cold day that when I started it up, it barely stayed alive, I had to keep my foot on the gas to keep it alive, and while I was doing that, I could see the rpm's going lower than 900 when it was going really slow, then when I rev it, it goes up like normal, but it's like it's pinged out at the top at 900..
Still totally confused why it would be maxing out at 900..
 

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It sounds like something got fried in the tach.
 
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