Speedo issues.
 

After about 150 miles in the car (glad I put a heater in it), the speedo ceased to function.
It looks like the proximity sensor at the back has gawn west.
I'm going to try and use the BMW pick up in the diff.
I had a go at using this when building the car with no success.
I couldn't get a signal but that could be that I wired it up incorrectly.
Does anybody out there know how the original pick-up functions so I have an idea of how to hook it up?

Hi Rob,

I used the e30 speed sensor on the diff for my speedo and it works fine.

I ran a twisted pair of cables from the diff. sensor all the way up to the speedo. I think I grounded one side, near to the dashboard, to avoid the whole thing acting as an aerial. Initially, I got some intermittant readings (luckily not during the SVA test!) which turned out to be a poor connection between the sender and the plug in connector. I took the sensor out and soldered some leads to the terminals and then filled the void with silicone rubber. No issues since doing this, but not many miles since this mod.

I used an american made Autometer speedo, but I think Stuart has also sucessfully used the BMW sensor and I think he has the Marlin supplied dials?

cheers

Peter

I used the E30 pickup in the diff too, just a pair of wires out and connected up as per instruction that came with the dial. Got the dip setting wrong first time as I was a clot and left a part of the calculation out! IIRC there are 9 pulses per rotation.

Great.

I know that the speedo worked in the donor when I drove it home and that I left the wires in marlin when I first had a go at using the diff sensor, so it should be easy.
I'll get back to you on this.

We didn't think ours was working either initially but it turned out that we would have need to have been doing 10 times more speed to get a normal read out.

Not good so far.

There are two wires on the back of the speedo that are used for the input and both don't seem to work with the BMW sensor.
I've tried entering different numbers into the speedo but nothing. not even a flicker (flat out, in fifth on axle stands. Sounds awesome in the garage.).
An oscilloscope, connected up to the sensor wire, tells me that there is a pulse, but not very big.
Could be that the sensor is stuffed, but worked enough for the BMW dash.
Other than buying a new sensor, I'm out of ideas.

The BMW sensor is, I believe, a Hall Effect device. Basically there is very little that can go wrong with it. (It's just a disk with some cutouts and a coil of wire) If you are sure of your wiring I would more suspect the other end - the Speedo. It may be new but it is electronic and far more likely to fail than the sensor. It may be worth contacting the speedo manufacturer and see if they have any idea's.

The best test would be to try the sensor with something you know that works - the BMW dash if you still have it would be good.

With all electrics and electronics fault-finding is often a process of elimination (Sorry if I am taching you to suck eggs..)

Robin

Thanks for that, Robin.
I never assume that I am always right.

I have checked the wiring which seems okay.
I've checked and cleaned the connector at the sensor.

What I could do with is someone to stick a meter across the contacts of the sensor and come up with a reading.
Mine reads open circuit, which could mean it's let it's blue smoke out.

That could just mean that the toothed wheel is in a 'tooth' position near the sensor at its current resting point, and is therefore open?

I was lead to believe that it's either going to be open or closed, depending on the toothed wheel, thus generating the pulses required.

Try rotating the wheel, and see if the reading changes?

We had the back plate off Richard's new diff yesterday. Its a wheel with cut outs that rotates around through the sensor causing a pulse.

Red is the metal disk with cut outs, black is the sensor seen from the top:

I don't think it should be open circuit. Assuming it's a hall effect device the sensor is basically a coil and I would guess it should measure a few tens to a few hundreds of ohms. The rotating toothed disk produces a small pulsing voltage which is detected by the speedo. If it is open circuit there will be no voltage!

It might be worth asking on the E30 zone what the resistance should be. Or ask someone else building a car with access to the diff still.

Robin

I'll try measuring tonight. If needs be, I can jack the rear end up and rotate the wheels to see what the pulses look like on the multimeter.

speedo sensor
 

I believe from memory the speed sensor on that model is a reed switch. The hall sensor has 3 wires, feed, earth and signal. The other common sensor type is inductive which has 2 wires plus a screening wire.
good luck

Brian
Sportster Sierra cosworth (born april 1998)

No, i'm pretty sure Robin is right it is a hall effect sensor. I don't think you'll see any pulses on a multimeter but you should be able to measure its resistance which should be measurable on the multimeter. I'm sure i've seen a reference to its expected value in one of my manuals but can't rememeber where?
If I can easily find the connector under my dashboard (:noidea:) i'll try and measure mine, but i'm sure open circuit suggests it is kapputen.
I'd keep an eye on ebay as i'm sure somebody will have a spare ?

I have an answer for Caigauge about speedo inputs which may prove handy.

Does your sensor have 2 or 3 wires?
If 2, the sensor generates a sine wave shaped signal that can be measured
with a voltmeter on ac setting. The signal needs to be greater than 1 volt
ac to trigger the speedo.
3 wire senders generate a square wave signal, measuring this signal with a
meter is more error prone.
If you have an oscilloscope, look at the signal with the speedo connected to
confirm that the signal amplitude is greater than approx 1 volt above
ground.

When I connected the sensor up to oscilloscope I only got a ragged signal of 0.5vac.

Off to the local scrap yard methinks.

Problem solved!
I bought a new sensor from BMW (£18) and cut the old one apart, only to discover that it's a common or garden reed switch.
There are two types of speedo offered by Caigauge. An older type which is adjusted by the means of 'DIP' switches, and a newer type which is adjusted by a code fed into the LCD display.
The Caigauge website gives you the installation for both types of speedos and the wiring.
Looking at both wiring guides made more sense that just the one supplied.
I wired the two wires from the sensor, one to earth and one to the speedo head.
Because the speedo is looking for a pulse, earthing the sensor produced no pulse for the speedo (obviously).
By swapping the earth for a 12v feed the problem was solved.
So yesterday, with a working speedo, we trotted of to a place (For Dorset people,the road running past Monkey World near Wool) where the local police check their speedos by the means of mile markers and got the speedo dialed in.

speedo
 

congratulations on getting your speedo working, I did say in an earlier post that I thought it was a reed switch. This type of sensor is pretty rare I think most being inductive type. Brian