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Troubleshooting tachometer M20


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Hi everyone, looking for some guidance on troubleshooting the tach in my 73 2002. I've done the M20 swap - car runs - fuel, water temp, and speedo all work except the tach.  I am getting voltage from the Motronic ECU. Voltage does drop when I rev the engine, however the tach need never moves.  I can't confirm if the tach worked when I got the car since it was a POS when it arrived. 

 

Any help appreciated. 

 

Thanks

 

 

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7 minutes ago, TurkisM2002 said:

Hi everyone, looking for some guidance on troubleshooting the tach in my 73 2002. I've done the M20 swap - car runs - fuel, water temp, and speedo all work except the tach.  I am getting voltage from the Motronic ECU. Voltage does drop when I rev the engine, however the tach need never moves.  I can't confirm if the tach worked when I got the car since it was a POS when it arrived. 

 

Any help appreciated. 

 

Thanks

 

 

 

Ack, I think I just figured it out - the tach signal from the ECU just isn't compatible with the 2002 tach.  

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7 minutes ago, TurkisM2002 said:

Ack, I think I just figured it out - the tach signal from the ECU just isn't compatible with the 2002 tach.  

Yep, sorry! E30 tach is driven from a 12v square wave sent from the ECU, while the 2002 tach is driven straight from the flyback voltage spike coming of the negative side of the coil every time the coil fires.  You could hook the 2002 tach to the negative side of your M20 coil of course, but then with 6 pulses vs. 4 it would always read 50% too high.  You might be able to get some sort of tach adapter, but honestly I think the easiest route is to just swap out the tach for one meant for 6 cylinders.

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Yes,  I did.  However, the sales were not enough to justify ongoing production.  I handed it off to Andrew at IE, who now keeps production going.  Can confirm that they are on the shelf over there.

 

Tachometer Adapter – 2002/E21 – Ireland Engineering | Racing & Performance Parts for BMW & MINI (iemotorsport.com)

 

Not a fan of swapping out to a non-stock tach.

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So I ended up grabbing the adapter from Ireland and got it wired up per the directions but I'm having an issue with the tach needle jumping around all over the place at idle.  See video below.

 

Assuming the adapter is not bad, I must not be pulling the tach signal properly.    Directions only say to take the signal from the low tension side of the coil, so I ran a wire from the negative post (correct?) of the coil to the input of the adapter, which is what the motronic uses as an input for timing I believe.  +12v for the adapter is coming from a switched source. Grounds are clean. 

 

TIA

 

 

 

 

 

 

 

 

On 9/19/2021 at 4:41 PM, AceAndrew said:

Yes,  I did.  However, the sales were not enough to justify ongoing production.  I handed it off to Andrew at IE, who now keeps production going.  Can confirm that they are on the shelf over there.

 

Tachometer Adapter – 2002/E21 – Ireland Engineering | Racing & Performance Parts for BMW & MINI (iemotorsport.com)

 

Not a fan of swapping out to a non-stock tach.

 

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