SPEEDOMETER CALIBRATION


Subject: BMW: Speedometer calibration
Date: Fri, 26 Dec 1997
From: Wes Jackson <wjackson@mail.island.net>

> Does anyone have an idea on how to calibrate the speedometer?

I went through this back in June '95. A Prez named Chuck Hawley, who I haven't seen on the list for a long time, told me how to do it. I then wrote it up and passed it on to several others that year.


Adjusting Motometer speedometer on airhead BMWs from '74 on:

As we all know, BMW speedos are pretty well all about 10 mph or kmh optimistic. This easy adjustment will make it correct.

Removing and opening the instrument cluster

Getting the instrument cluster off is easy. Remove the two bolts holding it to the steering head. Turn it over and you'll see two small screws holding a large rubber plug with the wires. Unplug it and you can take the whole assembly over to the work bench, where the fun begins.

Remove three screws and lift the bottom cover off the cluster. Remove four screws and the T-shaped plastic piece. Remove three screws and the printed circuit strip will swing out of the way on its wires. Remove three screws and the speedo lifts out.

Opening the speedometer (educational - but not necessary)

This is where I had trouble with Chuck's instructions, not being able to visualize well and had to remove the speedo face plate so I could see what's going on. If you want to do that, with two small screwdrivers gently pry the speedo needle off its spindle. Remove two screws and lift off the face plate. Under it you will see a hair spring around the spindle, one end secured by a strip of brass with one end bent over. Rotating this brass strip around its pivot puts more or less tension on the hair spring, raising or lowering the zero point of the speedo.

Making the adjustment

If you're now okay with how it works, replace the face plate and the needle. Bend a small hook on a piece of thin wire (paper clip works fine). Lift the needle gently over its stop, allowing it to hang free. Slide the hook you made along the under side of the face plate and rotate the brass strip a wee bit. Rotating toward the odometer numbers will lower the speedo reading and vicey versey. As you move the brass strip you'll see the speedo needle move with it. You have to guess about how far to move it, depending on your speedo error. Mine was out about 10 kmh and it took a needle movement of about 1/8 inch. Lift the needle back over the stop and put everything back together.

Checking your accuracy

The best way to check the speedo is on a measured section of highway. I've seen them all over the place, lines painted across the highway every 1/2 km or mile, used by police for aerial observations. The section I use is 2 km long. Travel this section at whatever speed you want, holding the speed as steady as possible and using a stop watch to keep track of the seconds. At the end of the section, divide the number of seconds you took into 3600 and multiply by the number of km or miles. This gives your actual speed, which you can compare with the observed speed.

I like to do two runs each way and average the results to compensate for throttle variations. Thus, if you ran a 2 mile section in 30 seconds, your actual speed was 3600/30x2=240mph. If you're only reading 55mph, you know something's wrong! If, after testing, your speedo is good, fine. If not, take it apart and try again. It's not hard once you've done it once. I've done a few others since I learned how. BTW, I was lucky and mine was within .01% so I left it alone. Good luck!

Wes Jackson
Nanaimo BC


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