VDO Speedometer Burnt Resistor — Identification, Odometer Problem, Speedo works.

[snbush67]

Reference VDO speedometer, 1978 911 SC.

Please help me identify the resistor, I need to replace the one in the photo attached.

The speedometer works but the odometer stopped turning. I had hoped that it was a simple gear replacement, but that’s not the problem.

I checked the signal from the transmission, I get about 8 beats of connectivity per tire revolution.

I noticed a fried resistor in the circuit board the color rings are unidentifiable. I removed the resistor to see if the bottom had any paint or markings but it did not.

I checked a working odometer in my car and it works perfect.

The cause of the burnt resistor is likely that I hooked the speedometer up wrong after I had removed it to do some work behind the dash.

I have searched the threads and have found many pictures but none that match my board.

[Jonny H]

Judging by a photo in this thread it looks to be a 33 Ohm resistor. (Orange, Orange, Black)

Need Speedometer Calibration Help

[snbush67]

Thanks Johnny. From the thread. The board is the same but the resistor on my board that is burnt, might be different, I guess I will try it and see if it works.

Originally Posted by E Sully

[florio]

You can try this: Unsolder the resistor and scrape it clean. Then, connect the ohmmeter to one lead and run the other probe along the resistor noting the highest value you get before it drops. Do the same from the other side and add the two values. This will be your (lower) starting point for the replacement resistor. You can guess the wattage from the diameter, a little extra being better than not enough. What happens is that the resistor burns in one location, but is good the rest of the way. By doing this, you can (sometimes) read the value of the remaining two unburned halves.
Good luck,

[snbush67]

Originally Posted by florio

You can try this: Unsolder the resistor and scrape it clean. Then, connect the ohmmeter to one lead and run the other probe along the resistor noting the highest value you get before it drops. Do the same from the other side and add the two values. This will be your (lower) starting point for the replacement resistor. You can guess the wattage from the diameter, a little extra being better than not enough. What happens is that the resistor burns in one location, but is good the rest of the way. By doing this, you can (sometimes) read the value of the remaining two unburned halves.
Good luck,

Thanks I'll give that a try.

[30westrob]

I found this. Looks like orange, orange, black, silver, or gold,gold, black, silver. Rob Rob

[snbush67]

Thanks rob rob that’s orange, orange, black, gold which matches everything else I’m seeing, indicating 33 ohms.

The problem is that the rest of the resistors don’t match up to mine, and I’m measuring 42 ohms on my old resistor, using the method that florio posted.

Which is safer to start with a 33 or a 42?

[Jonny H]

Use 33 Ohms. Btw, the schematic posted above does NOT match your circuit board so is misleading.

[mysocal911]

Originally Posted by Jonny H

Use 33 Ohms. Btw, the schematic posted above does NOT match your circuit board so is misleading.

One needs to determine why the resistor burned, e.g. shorted capacitor, before assuming that just a resistor replacement will result in a working speedo.

[snbush67]

Originally Posted by mysocal911

One needs to determine why the resistor burned, e.g. shorted capacitor, before assuming that just a resistor replacement will result in a working speedo.

I had installed the speedometer wrong, the wires were on the wrong post. The only visible damage is the burnt resistor.