Tach, Mileage and Temp Gauge acting erratically

[fastair]

New E30 owner here. Looked for over 3 years and bought one that is beautiful and seemed very well cared for. Have done all the usual, new odometer gears, rebuilt alternator, new hoses, water pump, thermostat, timing belt, radiator, fan clutch, rotor, cap, wires, plugs, checked ICV, MAF sensor, rebuilt complete front suspension, etc. Saturday I replaced the O2 sensor and the car was running perfect, best and smoothest it has run. Sat night the alternator light came on. Sunday I adjusted the valves, changed oil and filter and put the old alternator on. The old one had been charging but it's front bearing is bad. As I was taking the large wire off the rebuilt alternator I accidentally touched the block with the handle of the wrench and caused a spark. I had not thought to disconnect the battery. After disconnecting the battery I replaced the rebuilt alternator with the old one. When the car was started the battery light went out but the tach, mileage and temp gauge all were acting erratically. Not sure if the fuel gauge is effected or not as it was reading and steady. Wiggled all connectors I could reach to see if I had loosened something to no avail. Any thoughts would be appreciated. Car is a 1990 325i, manual coupe. Sorry for the long story.

[tomstin]

Try cleaning fuse 12 contacts and replace the fuse.

[fastair]

Thanks, am out of town at the moment but will try as soon as I return.

[Gregs///M]

Unfortunately, when you unplugged the vehicle battery, those 30 year old SI board batteries will never be able to hold a charge again. Therefore, your tach, temp, and MPG gauge no longer function correctly. The SI board powers and controls those gauges.

Time for an SI board refurbish.

[fastair]

Over the four months I have owned this car I've disconnected the battery several times, timing belt change, radio update, etc, and my gauges have always worked afterwards. This time the cable was only off for approx 3 hours. Is this typical?

[Gregs///M]

Yes, your batteries are still 30 years old, cant expect them to survive.

[fastair]

Well, it looks like I have been lucky. Cleaned fuses 10, 12 and 21, started the car and everything works as it should. I may send the board to Greg to be repaired/updated at a later date but at the moment, everything is fine. Thanks for the replies.