When I discovered my 1970 BMW 2002's temperature gauge pegged in the red with no real sign that the engine actually was that hot, it turned out to be an electrical malfunction. The article by Louis D'Amario on page 12 of the February 1977 Roundel paid for most of my first Year's membership in the BMWCCA.
Lately, however, both the temperature gauge and the fuel gauge-started jumping and pegging in harmony. They could be fixed temporarily by blowing the horn, flashing the lights, or running the wipers, but people were starting to stare, so. The final fix is something that should be done on your car even if you don't have any such electrical problems, just to forestall them.
First, check to see that the ground strap from the battery to the chassis is secure and that the ledge on which the battery rests is securely mounted to the car. Remove the instrument cluster by first unscrewing the knurled nuts in the back and disconnecting the speedometer cable. Unplug the tach wire. Unplug the large white multi-connector plug, and then remove the instrument cluster.
Check the ground wire from the white plug to the chassis. On the back of the heat/gas/idiot light assembly there are two slotted nuts that screw onto studs from the gauges. Underneath each is a brass rivet through the panel which is the gauge's ground connection. One or both of these rivets will have come unsoldered. When this ground connection is broken, the only current return for the gauge is through its sensor, and this disproportionate flow pegs the needle. Solder the rivet where it has broken free. Use a small (25-40 watt) soldering iron on this circuit board and electrical solder and don't cook it. Mr. D'Amario's fix, connecting these terminals together, worked because one of his grounds was still good. In my case, neither was.
Assuming that if it broke once, it will break again, you may want to also perform the following step. Locate a black rubber block impaled on three pins sticking out of the panel and remove it. These pins are the connections for a clock if you have a tach, and vice versa. The center pin is a ground. Solder a wire to the base of it, connecting the other end of this wire to both gauge studs, or put solder lugs on the studs and solder to them. This will provide a redundant ground. Put the rubber plug back on. BMW 2002 Tii owners will have to sneak the wire under one plug or another, as they have both tach and clock. Take this chance to replace any burned-out bulbs in the panel and to clean the face of the panel.
Recheck everything, making sure there are no shorts in your wiring, and put it all back together again.
I was pleasantly surprised to discover how much thicker the latest Roundel is compared to the February 1977 issue. It seems that there are a lot of members who are writing in now who did not before. Guess I'm one of them.
Author: Gary Koerzendorfer