Speedo trigger wheel/ring

[TriggerFish]

Hi all,

So my ongoing, painful saga continues in trying to replace the backplate on my 325i's diff. I've finally got one (an e36 reproduction part off of eBay) that fits with minor modifications.

The speedo in the car has always been a bit inaccurate (30mph indicated = 22mph on GPS), and I've never known why.

I saw this yesterday when rotating the diff., checking for clearance against the cover. Is this likely to have any ill effects aside from skewing the speedo reading a bit? (They're not uniformly spaced now.) Before I took the backplate off, there was no indication that anything was wrong.

Thanks!
Joe

[Brianmoooore]

The E30 speedo. sensor doesn't rely on some complex eletro magnetic phenomenon, such as the Hall effect, or even a more simple one, such as a pulse of current being produced when a magnet passes a coil of wire. The sensor is a simple leaf switch, which is normally sprung open, but closes when it's magnetised by a nearby magnet. The vanes on the disc partially complete the magnetic circuit enough for the switch to close when they pass through the gap in the sensor, and the switch opens again in the spaces between the vanes.
This means it's extremely easy to test. Simply connect an ohmmeter, or a continuity tester across the pins in the socket, and you should get zero ohms, or continuity, every time a vane is between the jaws of the sensor, and infinity ohms, or no continuity every time a vane is not between the jaws. The vane doesn't have to be moving for the switch to close or open - it just has to be between the jaws or not between the jaws.
I would have thought that a twisted vane like that in your pic. would have destroyed the sensor - there's not that much clearance between the jaws.
Your speedo. should over read by 5% - 10%, so a genuine 22 MPH should indicate between 23 and 24.2 MPH

[TriggerFish]

Thanks”‹, as ever!

I'm not sure when this happened, but I'm sure it's before my ownership, so whatever caused this could well have destroyed the original sensor, and then had the fins bent flat (enough) again. They do fit in the sensor as things stand though, but it is close.

Would the fins being unequally spaced due to the bending change the reading at all? My thoughts suggest that it won't because it'll be working 'an average'. I'll check the resistance output On each vane before refitting the plate and will bend to shape if needed.

Weirdly, it seems to get a bit more accurate (certainly in %age terms) as speed increases.

[Brianmoooore]

Would the fins being unequally spaced due to the bending change the reading at all? My thoughts suggest that it won't because it'll be working 'an average'.

The switch is used in conjunction with a voltage supply and a resistor (on the speedo. circuit board) to produce a square wave which is processed to drive the odometers and the speedo. The slight difference in the spacing of the square wave won't cause any problems, and if a vane wasn't triggering the switch for some reason, it would reduce the speedo. rading, not increase it.

Weirdly, it seems to get a bit more accurate (certainly in %age terms) as speed increases.

It's possible that the needle has been removed at some time, and not put back on in the correct position. This would affect the reading by a greater percentage at low speeds than at high.
Have you checked the accuracy of the odometers?
IIRC, there's an adustable pot. on the speedo. circuit board, but I've no idea what it does. Mark its original position if you decide to play with it.

[TriggerFish]

Thanks. I haven't checked the odometers, no - good point. I'll give that a go and see. Failing that, I'll have a search around to see if anyone else's had good (or otherwise) success with the pot on the speedo.