C104 to C101 tach connection

[MR 325]

As most of you know, I prefer to use the 87 i wiring harness when doing an i swap in to a pre-88 car. Well since a friend of mine is not doing things this way, I've got a quick question...

Now I know that everyone says that the black wire from C104 needs to go to black (pin 9) at the C101. What I am wondering, couldn't this be tapped near the ECU connector and achieve the same results without running a bunch of wire? I believe I read that the black from C104 can be connected to pin 6 at the ECU but I haven't found enough info confirming this.

Thanks fellas.

[jlevie]

If you are using a later (from 9/87) harness in an early (up to 8/87) car You need to graft the C104 connector onto the engine harness. That connector carries the tach & econometer signals as well as the START signal to the DME. The alternative to that is to add those wires to C101.

[MR 325]

If you are using a later (from 9/87) harness in an early (up to 8/87) car You need to graft the C104 connector onto the engine harness. That connector carries the tach & econometer signals as well as the START signal to the DME. The alternative to that is to add those wires to C101.

This absolutely did not answer my question.

I am only concerned with the tach signal, nto the others. I asked if it could be tied in at the ECU plug VS the C101 plug.

[BenM]

I'm guessing you're doing an "i" swap into a pre-88 car w/ 88+ wiring harness , yes? It should work if in fact the Pin 9 tach signal on the c101 does go to Pin 6 on the DME.

What harness is he using, I'll look at the wiring.

Edit - Just checked the 1989 325i ETM, looks like it should work fine!

Double Edit - While you're at it, DME Pin 32 (white/black) is for the Fuel Economy Gauge

[MR 325]

Yes sir, it's an 87 Eta car with a late model i harness. I am unsure of the harness year.

[jlevie]

This absolutely did not answer my question.

I am only concerned with the tach signal, nto the others. I asked if it could be tied in at the ECU plug VS the C101 plug.

I thought it did answer your question. If you graft a C104 connector onto the engine harness, like it is on an early I harness, The tach signal will reach the cluster. That is done by picking the tach signal, econometer signal, and START wire out of the harness near the DME connector.

[BenM]

I think all the 88+ harnesses are relatively the same.

You also have to bypass the OBC Relay box on the 87 "i", or I believe the car won't start or at least run rich. The easiest way to do that instead of going through the C104 would be to run a wire from the 15+ side of the ignition coil to DME Pin 27 (Green) which is start input.

Hope this helps.

[MR 325]

This car is a 325e base model, it doesn't have a full OBC.

Not only that, it's running a Megasquirt computer. Hence why I only asked about the tach signal and not the others.

I will ask the question again... Why do I need to run a wire from the chassis side C104 all the way to the C101 in the engine bay? What is the difference in doing it this way VS. tapping the tach signal at the ECU plug's wiring which is already in the glove box.

[BenM]

I thought it did answer your question. If you graft a C104 connector onto the engine harness, like it is on an early I harness, The tach signal will reach the cluster. That is done by picking the tach signal, econometer signal, and START wire out of the harness near the DME connector.

The early "i" and "e" cars C104 engine harness connector is connected in exactly that way, by the DME connection in the glovebox, which is right by the chassis side C104. On the later cars/harness the tach, fuel economy gauge and start wire are wired from the DME to the C101 chassis plug. Does that make sense?

[MR 325]

So Ben, what pin at the DME plug is the tach signal wire and can I tap it from there directly to the chassis side C104 and get the tach working?

Do you see what I am getting at, avoiding running wire to the C101 plug when I don't need to.

[Jean]

Yes, you can just run it from the DME to not run the wires under the engine bay from C101.... way cleaner. The ETM and I think even Bentley shows the pins you will want to tap into :)

[BenM]

So Ben, what pin at the DME plug is the tach signal wire and can I tap it from there directly to the chassis side C104 and get the tach working?

Do you see what I am getting at, avoiding running wire to the C101 plug when I don't need to.

Ha I knew what you were getting at, guess we were just talking in circles.

The DME Pin 6 is a Black wire and is referred to as the "Engine Speed Output" in the ETM. This is your Tach Signal Output from the DME.

Refer to pages 32, 37, & 137 in the 1989 325i/325is ETM for more info, and it will also show you the correct orientation of the DME Plug (Motronic Control Unit Mating Face) so that you splice into the right wire! :)

Yes, you can just run it from the DME to not run the wires under the engine bay from C101.... way cleaner. The ETM and I think even Bentley shows the pins you will want to tap into :)

^exactly!

[MR 325]

Thanks guys!

[MR 325]

Ben, got it working just after my last reply. Thanks.

[BenM]

NP, glad it worked!