Speedometer not working

[Grackle]

Hey all, I have done searches but a lot of the info I keep turning up is older and the pic links are broken so please bear with me.

All my gauges work fine except for the speedometer and odometer (VDO). I replaced the gears but it still isn't functioning (though best as I can tell the gears only fix the odometer, not the speedo). I tested the speed sensor in the diff and it is working, the batteries in my SI board are reading 3.3 volts (two lithium cells). I read about the blown cap problem but mine don't look to be ruptured, I tried to take a pic, but it may be hard to make out.



At least it isn't leaking out of the bottom, I don't know about that stuff around the top of it. Any ideas where I can look next?

[101]

Check the connector on the rear cover of the differential and the wiring coming from it. That's what gives the speed signal to the speedometer. That wiring and connector can be damaged easily.

[Grackle]

Check the connector on the rear cover of the differential and the wiring coming from it. That's what gives the speed signal to the speedometer. That wiring and connector can be damaged easily.

The rubber boot is a little cracked up, though the connector looked fine. Is there a way to verify the wiring at the cluster with a regular dmm (like at the plugs)?

[jlevie]

From memory, the speedometer isn't dependent on the SI board. In this case I'd suspect a bad sensor, bad connector, or a wiring fault. You can check the wiring for continuity or shorts (with the sensor and cluster connector disconnected) with a DMM. The factory wiring diagrams (http://wedophones.com/BMWManualsLead.htm) will be helpful.

The other possibility is failed electronics in the speedometer. Swapping in a known good unit will tell if that is the case.

[Grackle]

From memory, the speedometer isn't dependent on the SI board. In this case I'd suspect a bad sensor, bad connector, or a wiring fault. You can check the wiring for continuity or shorts (with the sensor and cluster connector disconnected) with a DMM. The factory wiring diagrams (http://wedophones.com/BMWManualsLead.htm) will be helpful.

The other possibility is failed electronics in the speedometer. Swapping in a known good unit will tell if that is the case.

Oh nice. Thanks, that gives me somewhere to start.

[rturbo 930]

You might try reflowing the circuit board. That's what fixed my speedo.

[Grackle]

You might try reflowing the circuit board. That's what fixed my speedo.

I touched it up, but I was just sort of guessing where. Didn't have any effect.

[Grackle]

This may seem like a silly question, but car the car run without the gauge panel being in the car? I mean are there any adverse affects?

[jlevie]

Yes the car will run, but to be safe open up the cluster, remove the speedometer, and re-install the cluster.

[Grackle]

I have a silly question regarding the fuses. I see a lot of folks recommend replacing the fuses when there is an issue with the gauge cluster. Mine ohm out OK, but these are some funny looking fuses. Can you use regular plastic spade fuses in these cars? Or should I stick with the odd looking open element fuses?

[jlevie]

You can use either type of fuse.

[Grackle]

OK thanks. I unplugged the sensor connector from the diff. I have continuity from the brown/red to the brown/red that plugs into the cluster (white housing), but not the solid brown wire. I'll have to get the car up in the air and trace the wiring to see if I can find a break.

ETA: I can't tell from the schematic if it matters or not, but the PO unplugged the O2 sensor because it went out. This wouldn't have any affect on the speedo would it?

[jlevie]

The O2 sensor won't have any affect on the speedometer, but a good O2 sensor is critical for proper operation of the engine. It is a scheduled maintenance item with a replacement interval of 100k. Use only an OE or OEM part.

From memory, I think the brown wire (which is a ground) may run to the ground point on the right rear fender well under the rear seat back.

[Grackle]

Scratch that. I found a second brown wire. I was able to hook both my probes up to them and push the car around. The continuity buzzer on my fluke pulsed with the movement of the car. The problem is in the gauges. Yeah I plan to replace the O2 sensor, just haven't yet.

[Grackle]

Will an older 318 Speedo that is a VDO speedo work with the rest of my cluster? I found one for sale out of an 84 or 85 that is supposed to be good.