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M42 swap, tach not working


gli_ryan

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Alright so I've swapped to an e36 obd1 m42. Its done, for the most part. Put maybe 20 miles on it so far. I'm working on a couple fairly critical things yet. The first couple have to do with wiring. I can't get the tach or temp gauges to work in the cluster. I've got the tach hooked up, but the gauge isn't reading anything at all. I've verified with an ohm meter that I have the black wires (two together coming off the back of the tach, but one at the harness connection) hooked up to the correct pin number 20 on the e36 wiring harness.

Also I'm having an issue getting the temp gauge to work as well. There are two wires coming off the e36 harness that are hooked into the stock cluster, but just the one for the 2002 cluster.

Any ideas on those wiring issues? I've got an aftermarket temp gauge hooked up, and the interesting part is it doesn't ever read over 80 Celsius or 160ish Fahrenheit. Is that how cold these motors usually run?

Another issue I'm having is I must have mounted the tranny up a little too high as when I shift into 5th gear the linkage is rubbing on the guibo. So I need to lower the tranny a bit to try and alleviate that issue as well.

pic for clicks

IMG_1596.jpg

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You need to install an M20 sending unit in the M42's head (two pin fuel injector style connector with only ONE pin in it) BMW p/n 12-62-1-710-512

As for the tach you'll probably need an MSD tach adapter. I did. HTH

Budweiser...It's not just for breakfast anymore.

Avatar photo courtesy K. Kreeger, my2002tii.com ©

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Temp Gauge: Install a 2002 sensor in the head in place of the M42 sensor. Run a wire separate from the harness or cut and change the harness connector if you want.

Then, use the lower temp thermostat if you're reading higher than 1/2 way up the gauge.

Tach Signal: Use an MSD 8920 Tach Adapter unit. It cleans up and amplifies the modern (weaker) tach signal and gives the older tach a strong input. Wiring is really simple and it should only cost you about $50 with shipping.

41V1AD7TSVL._SL500_AA300_.jpg

See my project blog for details (www.tx02.blogspot.com)

Also, what are you doing for a speedometer? Mine's electronic but looks stock:

IMG-20110702-00019.jpg

ClayW
1967 1600-2 - M42 - 1521145          Follow my project at www.TX02.blogspot.com          E30 DD Project Blog

 

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Tach Signal: Use an MSD 8920 Tach Adapter unit. It cleans up and amplifies the modern (weaker) tach signal and gives the older tach a strong input. Wiring is really simple and it should only cost you about $50 with shipping.

the cap costs $0.10. Mine is running with a 0.22uF, but these old tachs aren't really consistent so you may need to try a few values to find the right one.

drmattsevilworkshop.com

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Thanks for the help. I'll look into the tach adapter and the capacitor for sure. Maybe try the cap first since they are super cheap.

As for the electronic speedo which are you running? I don't have one currently, but I've been riding my bobber since last year with no tach or speedo and have grown accustom to it. I do want one for the 02 though, just not there yet.

I dont have the ETM here right now but normal is to have one sensor to cluster and another sensor to ECU.

Can you find 2 sensors. older sensors was one blue and one brown, not 100% sure if its same on the M42.

I'll have to check, I don't know for sure where even one temp sensor is for this motor though. But according to a couple of the threads i've read two wires go to the cluster for the temp sensor. Not sure why but thats what I've been reading. I don't have an ETM, nor do I know where to get access to one. If you figure out that situation that would be amazing.

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The M42 temp sensor is a one wire design. It's under the intake on the head and it has a modern connector (with just one spade inside). The problem is that it operates on a different range than the 2002 temp sensor, so the 2002 gauge will read way low. the sensors are the same thread, though, so it's a matter of swapping them out.

I also saw were an E30 m42 guy drilled and threaded the thermostat housing to accept a sender (he wanted a separate temp gauge). That might work for us, too.

ClayW
1967 1600-2 - M42 - 1521145          Follow my project at www.TX02.blogspot.com          E30 DD Project Blog

 

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Dont know what year you have on the M42 so i justed watched one ETM and that year there where as you said 2 wires to the cluster to one temp sensor. also found a second sensor for the ECU.

find the right ETM here (for free): http://wedophones.com/BMWManualsLead.htm

I also got a electronic speedo with my S14, tach was also changed when the old one didnt really like the Schnitzer ECU.

post-18739-13667646736382_thumb.jpg

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As if you needed another tach adapter solution, you may want to follow what they advise with Megasquirt. First step is trying a capacitor then the following steps

http://www.msextra.com/doc/ms1extra/MS_Extra_Hardware_Manual.htm#tachoout

Note that I didn't like the solution involving removing a coil from a relay, so I adapted a solution with standard parts

http://mybmw1600-2.blogspot.com/2011/07/high-voltage-tach-adapter.html

I believe a m42 is a wasted spark coil setup, right?

1968 BMW 1600 US VIN 1560713

manufactured on October 2nd, 1967

http://mybmw1600-2.blogspot.com/

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The m42 is a wasted spark setup....well two coils fire at the same time.

You can also use the circuit that the MS crowd does. I did that on my m42 car originally but then I used a different tach (black face) and it worked without an adapter or anything...... just works

The other m42 cars I have done I always use the MSD adapter. Works everytime.

1985 e28 w/LS1/t56 and a bit more...

1970 2002 w/ m42 swap

-Contact me for m42 mounts or e28 ls1 mounts-

www.classicdaily.net

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  • 2 weeks later...

You are going to need the high voltage tach adapter. If you look at the e30 tach. It takes a 5V square wave from the ecu.

http://www.r3vlimited.com/board/showthread.php?t=107720

The 2002 tach was intended to take signal directly from the coil. The stock coils output is over 10,000V. So the signal has to be boosted up quite a bit from 5V to 10,000V.

By the way... The M42 has individual coils. There is a control lead for each coil from the ECU. Also there is a Cam position sensor. In a sequential spark configuration, you typically have to get a tach lead from the ECU as a 5V square wave.

http://repair-manuals.blogspot.com/2011/07/bmw-318ti-1995-electrical-repair.html

If it was wasted spark there would be 2 coils (each with two spark plug wires) and no cam sensor.

By the way, my MS3 outputs 5V square wave tach signal. I can verify that my adapter works to convert 5V square wave to stock 2002 tach.

1968 BMW 1600 US VIN 1560713

manufactured on October 2nd, 1967

http://mybmw1600-2.blogspot.com/

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