Tachometer issues

[riverdale21]

Alright, on with the insanity. I've got a 87 325is the tachometer was working every now and then. That engine blew and I swapped it with an M20 from a 1989 325ic. Now no tach, no econ gauge whatsoever. Thought it might have been the SI board so I swapped the cluster with a friend's that is known to work fine. Still nothing.

Any ideas?

[riverdale21]

Oh yeah, forgot to mention tried the ECU from the 89 325ic and still no tach or econ gauge

[jlevie]

Did you use the engine harness from your 87 or the one from the 89? The 89 harness is somewhat different from the 87 in regards to a few signals (the tach being one). While it is possible to modify a later harness to match your car, the easy solution is to use an 87 harness.

[nando]

Did you use the engine harness from your 87 or the one from the 89? The 89 harness is somewhat different from the 87 in regards to a few signals (the tach being one). While it is possible to modify a later harness to match your car, the easy solution is to use an 87 harness.

it must be the C101 connector that's different, right? the pinout at the ECU itself is the same.

[jeffw]

this may be wrong but when me and my buddy did a motor swap he had to run two extra wires to the ecu for tac and something else. Message dashboardmonkey for the specs on that if you want. I don't know if that applies to your situation though cause we went from eta to an i motor in his but its something to think about

[jlevie]

it must be the C101 connector that's different, right? the pinout at the ECU itself is the same.

There are three differences between an early (9/86-8/87) and later 325i engine harnesses. One is that Start Input, Tach, and Econometer signals are routed from the DME via C101 in a late harness and via a pigtail off the early harness. Another is that the engine side of Pin 20 of C101 in ar early harness is not connected. On a late harness the pin goes to the main relay and since the car side is gounded in a early car, you smoke one of the wires to the main relay. And the last is that the early harness has no connection for for the unloader function.

[jeffw]

When me and a buddy swapped his we had to run two extra wires from the c101 to the ecu one being tac the other being the fuel rate for the econometer so I am assuming you need to do the same. Get ahold of dashboardmonkey and he can give you the specifications on doing that.

[jeffw]

I just talked to him and that is definately what you need to do

[riverdale21]

Yes, I did use the wiring harness from the 89. It seems like a gigantic pain in the ass to swap out the harness to the 87. Sent a PM to dashboardmonkey to see if he could provide some info. Depending on how difficult this solution is I still have the old harness and can always take the time to do the old switcho chango. Thanks for your help everyone. I'll post an update once the ball gets rolling on this.

[jlevie]

The easiest solution to this is to use the 87 harness. If you want to use the 89 harness you'll have to cannibalize the white three pin connector from the 87 harness and splice it in to the 89 harness.

[dashboardmonkey]

OP, did you get what we talked about done?

[jeffw]

Did it work for you? Hope so good luck with everything

[riverdale21]

Yeah it all worked out. Dashboardmonkey, you inspired me. After looking up several wiring diagrams and a C101 pinout (http://www.e-owned.com/albums/userpi...E30_wiring.pdf) I was able to locate the wire on the 89 harness that carried the tach signal. Unfortunately with your description I couldn't find the wire, my neighbor had to point out where it looked like it had heatshrink on it.

Wound up doing pretty much what Jlevie said. Cannibalized C104 and spliced it with the 89 harness. I wouldn't have had to do that if my neighbor had shown up earlier. In spite of the compromised setup the tachometer is working great.

Next up is trying to get the fuel consumption squared away. The C101 pinout I linked to shows it as being a black/white wire from pin 8. Unfortunately there are two identical black/white wires that come into the glovebox in the 89 harness from the firewall. Now that I think about it a simple continuity test could show what is what. I'll be back in a few after I check it out.

If you guys haven't heard of this website check it out: http://www.armchair.mb.ca/~dave/BMW/ tons of great wiring diagrams. Has really helped me troubleshoot the little electrical gremlins that plague my car.

Just to confirm on C104 black is the tach (confirmed), white/yellow is fuel consumption (about to trace and splice), what is the green/red?

Again, thank you to everyone for your help.

[jeffw]

glad it worked out.