Test Tacho how to?

[NeverEnough02]

Ps, those with the m50, mpg gauge is pin #17 on the dme.

[NeverEnough02]

Hey sorry for the delay. Everything is hooked up and working perfectly! As promised, here is how you check if your tach is getting a signal.

Pull out your gauge cluster (plenty of how tips around the forum). You do not need to pull the steering wheel or unplug the gauges. Lay a cloth down on the steering column to protect the window from scratching. Mine was a spare so I didn't care about scratches.


You can see to the right that there is a blue plug. On the side facing passenger, there is a black wire. That is your tach wire. I shaved a very small amount of plastic off mine for easy of testing with doing other things but you are able to poke through it with most multimeters.

Ground your multimeter somewhere and set it to ac 20. While your car is running, poke through the wire coating and you should see your multi meter start getting single digit numbers that increase with revs. There you have it. Your tach is getting signal. If it is not working, you may have to reflow some connections on the tach itself or replace/repair your SI board. There is a vendor on the forum who supplies reman SI boards.

[NeverEnough02]

I did find the black wire coming from the pin you suggested and chased it up to a good tapping point. I was not able to test the tach last night. I apparently threw away too much of my scrap wire and need to go buy some tonight that will match this application.

I looked at a 1991 525i wiring diagram, looks like pin #24 would be the one for my mpg gauge but, I am still not sure. It is black/Violet wire and awfully thick compared to the gauge wires. Any advice here?

[NeverEnough02]

Ok great, and do you know what pin for the mpg? This is a non vanos swap as well.

[ForcedFirebird]

Yes, that link has accurate info, except you have a swapped car, so the ECU pins will have different numbers. There will be at least 2 black wires if memory serves correct. I have only done one m50 non vanos swap over the years, and it was a long time ago.

The ECU connector comes apart. Take off the tamper-proof metal tie and screw on the end of the black part of the connector. You then can slide the white part through it exposing the backs of the connector pins. This is also how you do electrical testes with the ECU plugged into the harness.

[NeverEnough02]

Some things are left unplugged from the factory. That rectangle open connector is not needed.

You need to get that black wire to the black wire in the main ECU connector (pin #74). When looking at the engine harness connector unplugged from ECU with the longest row on top, the #1 pin is the top right and counts to the left, #56 is on the bottom right, #88 bottom left, etc.

Is there a way to tap the pin itself. I did not see a way that would allow me to pull back the rubber/metal boot on the ecu to see what wire goes where. I could always do a continuity test to be sure if there is in fact more than one black. I was hoping that there would only be one black in the bunch and use wire taps.

I am also going to do the same for the mpg gauges based on the instructions in the link I attached earlier in this thread. (http://www.strictlyeta.net/technical/328i_8.html) That thread said #6 black for tach and #32 Yellow/white for the mpg.

[ForcedFirebird]

Some things are left unplugged from the factory. That rectangle open connector is not needed.

You need to get that black wire to the black wire in the main ECU connector (pin #74). When looking at the engine harness connector unplugged from ECU with the longest row on top, the #1 pin is the top right and counts to the left, #56 is on the bottom right, #88 bottom left, etc.

[NeverEnough02]

Third pic. White connector, black is tach. You found it :)

Thank you! I did see from my search that you have a long history of helping people out with wiring issues. Much appreciated. Do you know what those other 2 random pins are as well as the black harness that is not connected to anything? Curiosity more than anything.

Tonight I will be bridging those wires from my harness under the glove box to the plug for my tach signal and testing. If all goes well I may just continue tearing down and replace the dash since I am already half way done with it at this point.

[ForcedFirebird]

Third pic. White connector, black is tach. You found it :)

[NeverEnough02]

Did some more digging and found the 3 prong plug is wired into this. I tested continuity to the top half of the plug (colors all align) and then tested the top half to the blue cluster pack. All good there. I followed the bottom half all the way down into the drivers side under the lower dash panel. Seems like the plugs go into a larger white 16-20 prong plug. All looks factory wrapped.

[NeverEnough02]

Alright, so I pulled the glove box and the cover.

Here is what I have for a box:


Here is what is also coming from the engine bay:


Here is what I found stuffed in the top coming from the stereo area:


And this was wrapped up behind the HVAC controls. Purple and black wire. I know it is not related but what is it?

[vpilarrt]

I think I had mine on a low level DC setting

[NeverEnough02]

Just found this link with instructions on bridging the cables from the plug going into the ECU in the glove box to the cluster itself. It also gives all of the pin numbers on the ECU as well. http://www.strictlyeta.net/technical/328i_8.html

Where does the black wire on the gauge come from? I thought it would have connected directly to the ECU which is where it got its signal from. Apparently not if a bridge is needed all the way from passenger glove box to the gauges for this swap. Otherwise, you could just bridge right there in the glovebox. I thought the signal traveled like this motor>coil output>ECU>gauges>OBC. It seems like the bridge bypasses something in between the ECU and gauges. What am I missing?

5 more hours and I will have some pics of the current ECU.

[NeverEnough02]

It's a swap car, that changes things. Who knows what the previous owner did.

Yes, the white rectangle plug under the plastic on the top of your glove box. Late model wiring has 2 black connectors.


I have seen a lot of swap cars come in the shop where the tach doesn't work, 90% of the time it's because the person doing the swap didn't realize the tach signal came from the dash, not the c101 in the engine bay.

Yes to the correct test or yes to the referenced plug?

After work today I will take the panel out and see what plug is going into my box. Once this is sorted out I will take some pictures to post on here for those searching in the future.

[ForcedFirebird]

It's a swap car, that changes things. Who knows what the previous owner did.

Yes, the white rectangle plug under the plastic on the top of your glove box. Late model wiring has 2 black connectors.


I have seen a lot of swap cars come in the shop where the tach doesn't work, 90% of the time it's because the person doing the swap didn't realize the tach signal came from the dash, not the c101 in the engine bay.