Test Tacho how to?

[NeverEnough02]

Hi everyone,


Tach and mpg do not work but I want to be sure they are getting a signal before buying a new SI or taking apart the cluster to re-flow. I have tried 2 clusters that I have with no luck.

How do you test the tach is getting a signal. I have searched for a few days and have not found any complete instructions. Some people say test plug 104c on the board but I do not know what method of test. ohms maybe?

Would like to test the wire going into the cluster if possible. Which wire would I test and what settings on the multi meter?

[vpilarrt]

Pin 7 (black wire) on C1 (blue connector) on back of cluster. Its probably a frequency variation or PWM (if they even used that on E30s) but I've checked this with a DVM (digital volt meter) before and IIRC the voltage reading will vary with engine speed.

[ForcedFirebird]

Pin 7 (black wire) on C1 (blue connector) on back of cluster. Its probably a frequency variation or PWM (if they even used that on E30s) but I've checked this with a DVM (digital volt meter) before and IIRC the voltage reading will vary with engine speed.

Only on late models. Early models have the three wire c104 connector next to the ECU plug that feeds the tach/econometer.

[vpilarrt]

Only on late models. Early models have the three wire c104 connector next to the ECU plug that feeds the tach/econometer.

NeverEnough02's profile shows that he has an 87 325ic. Pages 6210-1 thru -5 shows that there is a Motronic on the I engines that feeds pin 7 on the cluster and yes it feeds through C104. I'm not sure what point you're making.

[ForcedFirebird]

NeverEnough02's profile shows that he has an 87 325ic. Pages 6210-1 thru -5 shows that there is a Motronic on the I engines that feeds pin 7 on the cluster and yes it feeds through C104. I'm not sure what point you're making.

Just making it a point for people searching in the future who may come across this thread.

87ic will have the c104.

The OP even implied in the first post that he was told to check the c104 wiring which would mean early model wiring.

Easy way to tell (specially on cabbies as they were the chassis is always a year older than the hard tops) is to see where in the engine bay the expansion tank is. If it's on the passenger side and cylindrical, it will have the early model wiring.

If that c104 is unplugged or damaged, the tach and econogauge won't work. There also could be a late model engine harness installed which would require tapping in the ECU pin #6 for tach wired to the black c104 wire, pin #32 would need to go to the white wire - or - do it like the factory did for late models and run them through the c101.

[ForcedFirebird]

OP, here is what the c104 looks like:

http://s2.photobucket.com/user/bimster/media/host/labeledecuwires.jpg.html

To add: With a late model engine harness, the car will still start when the green wire is disconnected. If it has an early model engine harness the car will not start without the green wire plugged in as that's the power feed to the ignition coil.

[NeverEnough02]

Thank you for the help so far. Yes it is an 87 cabrio with an m50 swap done by the previous owner. I believe that box you referenced with the 3 plug is under the glove box ceiling right? I will have to take a look.

As for testing, here is what I did: set the multi meter to ac 20. Red lead to to black wire in the blue cluster group. Black lead to the neg battery terminal. Started it up and no signal. Was that the right method of testing with a mm?

[NeverEnough02]

Also, expansion tank is the rectangle on the drivers side. What does c104 look like? Is that the 3 wire plug pic you provided above?

[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.

[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.

[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.

[vpilarrt]

I think I had mine on a low level DC setting

[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?

[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.

[ForcedFirebird]

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