I apologize for the long winded post:)
I own a 1992 740 wagon that falls within the two or three years where the speedo would die. I replaced the 220µF capacitor on the speedo circuit board as stated in the FAQ and the speedo still didn’t work. The main reason I want the speedo to work is so I can use the cruise control. So, I pulled the speedo apart again and looked closely at the components. I found three electrolytic capacitors that had failed and had leaked what I assumed to be electrolyte. These three capacitors were next to two 28 pin each chips and the black electrolyte had flowed between the pins of the chips and a few other components. I removed the three capacitors, in addition to a fourth electrolytic capacitor as a precautionary measure, and cleaned as well as reasonable the electrolyte from between all the contacts. This wasn’t easy since the black electrolyte seemed to be burned onto the board (kinda like the bottom of a pan when you really burn something). I even used Acetone and it only helped slightly. I finally got a sewing needle and meticulously scraped off the electrolyte from between the pins.

Anyway, after soldering in new capacitors, the speedo worked but the needle wouldn’t go above 45mph even when doing 70mph (I used my GPS to check my speed). Between 0mph and 45mph it worked fine. The speedo needle connects to a pin connected to a flat piece of metal that is wound or tightened when the needle turns thus causing the needle to return to zero (the flat piece if metal looks like the coiled bimetal in a mechanical home a/c thermostat). I figured the coil may be getting too tight and not allowing the needle to read above 45mph so I put the needle back on in the 0mph position with negative tension on the coil. Well, the speedo would go above 45mph but would read too fast at all speeds (not enough tension). I tried several different tensions to no avail.

My question is; does anyone know the proper tension on the coil for needle replacement and/or adjustments so that the speedo is accurate and will read accurately above 45mph (at least up to 80mph)?

The four electrolytic capacitors I replaced are as follows:
10µF 25V
6.8µF 25V
22µF 50V
47µF 25V
If you replace these, make sure you get the same value for the capacitor at the same or greater listed voltage.

I am happy however that I can use my cruise control!

Michael
Melbourne, Fla

1985 740 460,000 mi (original motor)
1988 780 Coupe V6 181,000 mi
1989 780 Coupe V6 149,000 mi (parts car)
1992 745 190,000 mi