88 325I - installed 89 325is engine - rpm and tach issues

[MikeL91]

Question: Are the ECU wire harness from a 88 325i (auto) ok to use with a 89 325is engine? I am having rpm and tachometer issues after the engine change and i am wondering if its because i kept the old wire harness or if maybe the old one is bad.

other thread on it:

https://www.r3vlimited.com/board/forum/e30-technical-forums/general-technical/10020044-88-325i-installed-89-325is-engine-rpm-and-tach-issues

[Gregs///M]

Not too sure about the wiring harness different but I know at the cluster itself, all clusters, early vs late vs standard vs M3 are all interchangeable with each other. This means the pinout is exactly the same.

In your white connector, brown/red wire should have a pulsed signal that increases with vehicle speed, speedo input. This line is known to fail at the rear differential, check for broken, oxidiated or cracked brittle wires.

In your blue connector, black wire, is the tach signal input. Also a pulsed signal that rises with engine speed. This comes from the DME.

Both speedo failure and tach failure are extremely common. Speedo failure is much more common in the vehicle wiring but tach failure is almost always related to a failing SI board. If your cluster is not refurbished, assume it could be bad.

[nando]

There are minor differences in the wire harness, but not functionally. The DMEs are basically the same as well (although the 88 has the dual layer board, they are compatible and functionally identical). Unless you have a bad harness or DME, I don't think that is you r issue.