Tacho wires

[hzbloke]

1986 R65. Can anyone tell me the correct order for the three wires that go to the tacho? Clymer seems to indicate (top to bottom) green/brown/black but I'm not sure if that is just for the sake of keeping the schematic neat.

Yes...I know, I should have written it down when I took it apart.

Just as a matter of interest....what happens if they're connected wrongly?

Cheers
Ray

[Kurt in S.A.]

I don't have the wire order, but I suspect the brown wire is ground and it needs to find the ground pin. Otherwise, you'll be putting power on one of the other two wires directly to ground...not good.

Kurt in S.A.

[Jeff in W.C.]

I have it at home. When I get home, I'll remember to post their proper order. I've heard if you put them on wrong, you can fry the tach. I posted it here a few years ago when I ran into this issue, but I can't find my post. When I get home, I'll let you know what Oak says.

Edit: my info is for the RT, and I don't know if the instrument clusters are the same. Sorry. If you send me an email, I forward the email Oak sent to me, but it might not be appropriate for your model.

[Kurt in S.A.]

I found a message that Oak wrote about this. Be sure and get some more verification for these connections. But he said:

- pin 8 is RED
- pin 10 is GREEN
- pin 12 is BROWN

He provided this for an '84 RT but said it's the same for '78-on. The important thing is to find pin 1 in the row of 6 pins...pin 7 will be across from 1.

Kurt in S.A.

[Jeff in W.C.]

Here's how Oak responded to my email. I wanted to email you the PDF, but your email did not come through. Email me at jlperlow at hotmail.com and I will send you the PDF.

I hope you did not get the tach connections wrong. If you did, may very likely cause unfixable tach failure. There are TWO different wiring schemes depending which year is under question. For your 1988 model would be the later wiring scheme.

Attached is a PDF you can download and print. It is a wiring diagram of the cluster. Note there are pin connections labeled for the mating plug. Here is how to trace out where the tach wires connect.

1)---Put opened up cluster on work bench.Make sure ALL 3 TACH WIRES are DISCONNECTED from the circuit board lugs to which they normally attach.

2)---Using the PDF diagram and help with an ohmmeter on lowest scale ( R x 1 ) find pin 10 at the connector. Connect to one of the ohmmeter leads. With the other ohmmeter lead, test each of the three tach lugs available. ONE and ONLY ONE will read zero ohms. THAT lug is for the GREEN tach wire. Mark it with a green marker pen and make yourself a drawing.

3)---Now find pin 8 of the connector. Connect to one of the ohmmeter leads. With the other ohmmeter lead, examine the two remaining lugs for the tach. One of them should read zero ohms. Label that one for the RED tach wire. Use a red marker pen.

4)---Now find pin 12 of the connector. Connect to one of the ohmmeter leads. With the other ohmmeter lead, examine the remaining lug for the tach. It should read zero ohms. Check against the green and red to make sure all is correct. The remaining lug is for the tach BLACK wire. Mark it with a marker pin.

5)---If the tests show correct, make a drawing of the wire colors and lugs where they go. Staple it to the PDF I sent and keep it with your tech info. Reassemble the unit. Should work perfectly.

Hopefully your tach did not get damaged from improper connections. I will never understand why BMW did not color code the three lugs. How can they expect a mechanic to service the unit as there is no schooling on the subject. I suppose they expect the mechanic to make a sketch. Understandable as mechanc's rule number 1 when working the unknown !!

[hzbloke]

Thank you Jeff and Kurt (and Oak in absentia). The correct order from top to bottom is brown/green/red and the corresponding pin numbers are as Kurt says. I couldn't get a result using the step-by-step procedure but the pin connections on the diagram made it clear where the wires go. The tacho is now working fine.

I actually lied in my original post. I did follow Mechanic's Rule No 1 and note the connections. However I forgot to follow Mechanic's Rule No 2 which says "Don't throw out the bit of paper that you make notes on".

Thanks again