SI Board

[Dig]

I have a 1988 325I, and I just replaced the SI board because the tach,temp, and mpg gauge were not functioning. The new board solved two out of three issues. The mpg gauge is working, but not correctly. At idle the gauge goes all the way to the right and the only time it shows the most mpg is when you let off the accelerator at highway speeds. The gauge is mostly in the zero to five mpg position. Does anyone know what the problem may be.

[dvmotorsports]

Originally Posted by Dig

I have a 1988 325I, and I just replaced the SI board because the tach,temp, and mpg gauge were not functioning. The new board solved two out of three issues. The mpg gauge is working, but not correctly. At idle the gauge goes all the way to the right and the only time it shows the most mpg is when you let off the accelerator at highway speeds. The gauge is mostly in the zero to five mpg position. Does anyone know what the problem may be.

I don't know exactly how they function, but i believe the older ones work on vacuum. I could be wrong though. But to be honest, they aren't accurate if that is what you are concerned about. But if you do want it working and you can confirm that is does operate on vacuum, start with the line going to the gauge. My 87 VW GTi had a vacuum line running right to the back of the gauge. Haven't had my E30 cluster apart, so i don't know if it's the same. Not much help, sorry.

[Dig]

Thank you, but I don't recall a vacuum hose or connection for one on the back of the cluster.

[gotKRAUT]

actually the BMW one is very accurate, it works via ambient temperature, fuel flow rates and throttle position

Completely different than the crude VW system

did you replace the battery to the board? if its got a new board and still off then the batteries are bad or the actual instrument itself...have you swapped injectors or engines?

[Dig]

Kyle, thanks for the reply. I put in a remanufactured board and I actually checked the batteries before I put the board in. They were almost completely charged. Do they recharge when the engine is running or whenever they're connected to a 12 volt battery? The board was rebuilt in September of 2007. There has not been any work done to the engine, it has less than 65,000 on it. The MPG gauge is basically doing the opposite of what it's supposed to do except when I'm at highway speed and take my foot of the gas. It's really weird.

[dvmotorsports]

Originally Posted by gotKRAUT

actually the BMW one is very accurate, it works via ambient temperature, fuel flow rates and throttle position

on the old ones? i thought that setup was only on newer cars.