Instument Cluster

Post Mon Jan 17, 2005 1:26 pm

I've just swapped my 320 Instrument cluster for a 325 one.

The speedo is now over reading by about 10%. Compared to previously.

I drove next to my Dad's car before, and it was showing the same speeds as that, but now it's well out.

I still have the old dash, and I noticed there is a chip in the front. Is this model specific? So can I swap it and it will read as before? Or do I need to do something else? Or is there nothing I can do?!

At least the fuel gauge isn't flickering when it gets near empty now!

Post Mon Jan 17, 2005 1:48 pm

From what I remember:

The coding chip doesn't effect the speedo - just the MPG, tach and fuel guage I think.

Look on the face of the speedo - there is "K = xxxx" written on it. This is the number of pulses required from the diff before 1 distance unit is clocked (be it miles or Km).

You can just open the instrument cluster and swap that single clock.

I think you've just confirmed my suspicion that the pulse rate is dependent on the diff ratio: the senor must be on the input shaft.

Post Mon Jan 17, 2005 1:58 pm

Yeah, I thought xxxx was something to do with it.

Dunno what to do now. Is it easy to swap just the one dial?

Post Mon Jan 17, 2005 2:08 pm

Simple job. About 15 screws on the back of the cluster (The bigger ones - not the small ones behind the clocks themselves). Split into two by pulling directly apart. Undo the 2 or 3 screws behind the speedo and lift it out.

Out of interest - what are the k numbers from your two?

Post Mon Jan 17, 2005 2:16 pm

I've left my 320 cluster at home, but I can find out for you.

I've put the chrome dial rings in this speedo now, and the speedo needle has broken on the other one. I think I may have foooked it because I was playing around with the mileage (didn't think I'd need it again) and the numbers don't line up now. Can I sort that?

I thought there were only about 8 of the bigger Phillips screws?

edit: Can I swap the needle easily?

Post Mon Jan 17, 2005 2:50 pm

Again, straining to remember this: Only the circuit board will be different I think. You might get away with swapping that off the old speedo. Or if you are good with a soldering iron, you might be able to swap the duty cycle setting resistors by the ITT chip.

To be honest, I never investigated how the odometer and speedometer are actually driven, but I imagine all the diff ratio dependent stuff is electronic rather than mechanical.

Post Mon Jan 17, 2005 2:52 pm

I assume it must be. As there's a sensor and just wires rather than a worm drive (?!!!). Must be magnets?

My Dad is good with soldering and electrics so maybe I'll get him to sort it when I get the other cluster back!

Post Mon Jan 24, 2005 12:01 am

Just figured out that the pulses come from a multi-tooth interrupter on the output side of the diff. That means all E30's probably use K=7784 (for miles) and your speedo issue was simply a bad/different clock.

The odometer is a stepper motor and should be dead accurate, but the needle is just a drive coil with spring backpressure. I could probably dig out the datasheet again and figure out how to calibrate the speedo if you like.

These things aren't meant to be accurate. The nominal design is to overread by 5-10% I think. That way, even the weakest spring and greatest RC resistor tolerance won't underread.

Ever noticed that the OBC speed limit gongs way after the speedo goes past the limit? The OBC is a quartz clocked interpretation of the speed pulses and should me accurate.