wiring up the rpm gauge

[kman_e30]

i did my m20 swap almost a year ago now, ever since the swap my rpm gauge hasnt worked, id really like it to work now lol. ive heard there is a way to hard wire it or something like that, but i havent found anything. can someone help me please

[Gregs///M]

Engine speed output comes from the ECM. Pin 6.

[kman_e30]

Speed sensor in differential output signal wire is brown/red. That wire needs to be connected at differential and to the cluster / tachometer input.

If you are using an aftermarket tach (which you did not specify,) you will need to run power and ground, and tap into signal wire mentioned above.

sorry, ya i am running the stock cluster. ok im a little lost here, how is the speed sensor on the diff going to pick up engine rpm?

[Gregs///M]

sorry, ya i am running the stock cluster. ok im a little lost here, how is the speed sensor on the diff going to pick up engine rpm?

Opps, was thinking speedometer input. I edited my post above. Signal comes from pin 6, ECM.

[Gregs///M]

Its also a black wire in the blue connector C1, pin 7.

[kman_e30]

awesome, so would i just run a wire from pin 6 to the rpm gauge. sorry im horrible with electrical

[Gregs///M]

Your RPM gauge should already have signal if the blue connector on the back of the cluster is plugged in. You can test the signal from the ECM by setting multimeter to AC. You should see between 1.5-6 volts AC.

[kman_e30]

Your RPM gauge should already have signal if the blue connector on the back of the cluster is plugged in. You can test the signal from the ECM by setting multimeter to AC. You should see between 1.5-6 volts AC.

could it have something to do with my square to round connector conversion? sorry again i forgot to mention that my car is an 84 318 with an 87 b25 engine

[jlevie]

Originally a three wire connector was pigtailed off the engine harness near the DME connector. That went to a connector on the body harness. The DME START input, tachometer output, and econometer out were routed through that pigtail. 325is cars built from 9/86 to 8/87 had an engine harness like that, but all later E30's route those signals through C101. The cheap fix is to add a pigtail to the harness for the tach and optionally the econometer.

Hmm, I just saw that the car was an 84 318. Dunno if it is exactly the same, but it will be similar.

[kman_e30]

would that 3 pin happen to be white with a green and purple, a red and blue, and a brown?

[kman_e30]

still really stuck, help guys! lol

[kman_e30]

ok so there are two plugs off the harness in the ecu area, i guess they are both called the c104, the white plug has the 3 colored wire i was talking about in my post above, the black plug has one black wire, im starting to think this is it, if it is then all i would need to do is connect that those two black wires together and that would solve the problem?

[Gregs///M]

I suggest you download the ETM for your two wiring harnesses. BMW electronic troubleshooting manual. I has the wiring diagrams for everything. There are threads on this site that have the download links.

[kman_e30]

this is turning into more of a pain in the ass then i thought, how hard is it to wire up an aftermarket tach?

[Gregs///M]

Power, ground, engine speed signal from ecu. Done.