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I've searched and searched other platforms and have come up with nothing.

 

Every time I turn the key to on (acc and run) and while the car is running the tach pegs all the way around.   I've taken the cluster out a few times and tried looking for corrosion or broken wires inside the tach and nothing.   

I have checked grounds 

added more grounds 

Checked all the wiring to the cluster and nothing seems off or shorting out.  

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Find the tach wire at the coil (it should be black) is it hooked to the - terminal with the ign points?  Remove it from the coil and check the tach, does it still peg?  The engine does run when it does this?  

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27 minutes ago, Preyupy said:

Find the tach wire at the coil (it should be black) is it hooked to the - terminal with the ign points?  Remove it from the coil and check the tach, does it still peg?  The engine does run when it does this?  

Yeah I've unplugged it from the gauge and as long as the positive side (green wire I think) of the gauge is hooked up it pegs.   Yes the engine does run when all of this happens.  

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You sure the green wire is +12v? 

I've seen this happen on cars before. 

Tach is getting a signal that is wrong from somewhere? 

You didn't say if tach returns to normal rpm when engine is running? 

Don't have wiring diagram in front of me. 

 

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8 minutes ago, m5bb said:

You sure the green wire is +12v? 

I've seen this happen on cars before. 

Tach is getting a signal that is wrong from somewhere? 

You didn't say if tach returns to normal rpm when engine is running? 

Don't have wiring diagram in front of me. 

 

No it does not return to normal when engine is running.    The positive (green) does show 12.xxv when hit with a multimeter.   And on the back of the tach has a prong for a + wire going to it.  If I turn the car to the accessory position it pegs just like in the video.  No matter the signal wire(black) attached or not. The brown wire (ground) does seem to change how the tach acts.   But if I unplug the green wire the tach drops back to zero. Even with the other two wires attached. 

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My guess is a fault in the tach then. If it pegs with just the 12v power hooked to it on the right terminals ( there are only 3 and they are clearly marked) then it’s a gauge issue. North Hollywood Speedometer is my go-to place for BMW instrument repair if you can’t find a good one somewhere easy. 

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18 minutes ago, Preyupy said:

My guess is a fault in the tach then. If it pegs with just the 12v power hooked to it on the right terminals ( there are only 3 and they are clearly marked) then it’s a gauge issue. North Hollywood Speedometer is my go-to place for BMW instrument repair if you can’t find a good one somewhere easy. 

Yeah that's what I was fearing. Thank you for all your help ! 

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Did you try bypassing the whole stock wiring harness, and just run a wire from the tach to the coil?

 

Weird stuff goes on with that tach wire.  Somewhere 12V power can short into the tach wire and disrupt the spark signal, preventing the engine from running.  It happened to me intermittently, as well as another guy on the FAQ that sent me a giant Snickers bar for figuring out his problem (there are two of us!  two of us!).

Your problem doesn't sound the same exactly, but just saying that stock wire is not trustworthy.  I ran my own wire to replace it.

 

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33 minutes ago, KFunk said:

Did you try bypassing the whole stock wiring harness, and just run a wire from the tach to the coil?

 

Weird stuff goes on with that tach wire.  Somewhere 12V power can short into the tach wire and disrupt the spark signal, preventing the engine from running.  It happened to me intermittently, as well as another guy on the FAQ that sent me a giant Snickers bar for figuring out his problem (there are two of us!  two of us!).

Your problem doesn't sound the same exactly, but just saying that stock wire is not trustworthy.  I ran my own wire to replace it.

 

I'll try that today.    Do you think I should run a new 12v and ground wire as well ? 

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50 minutes ago, pullacowonaski said:

I'll try that today.    Do you think I should run a new 12v and ground wire as well ? 

 

I'm personally not a fan of any ~50 year old wiring on these cars, but thats your call.  Losing the 12V or ground to your tach shouldn't leave you walking, but I know a bad tach wire will get you using up your AAA tows if you don't know the cause.

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15 hours ago, pullacowonaski said:

If I turn the car to the accessory position it pegs just like in the video.

This is telling, as the +12V from the green wire should ONLY be there when the key is in the start or run positions, and should be 0V in the accessory position! I suspect you either have some miswiring at the fuse panel or you some crossed wiring somewhere such that your green wire is getting 'polluted' somehow.

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