syoo8

Just make sure that the problem is electrical and not mechanical. In both of my cars the odometer itself (the numbers) had some slack and bound up. All the gears were fine, so I thought it was electrical, but actually all I needed was a thin washer.

Ian S4

The PCB should look something like this. Check the resistor nearest the bottom of this pic. It should measure 22 ohms.



This is a schematic showing a "typical" application, which is a little bit different from the 928's implementation. But the motor windings can clearly be seen on the right. The green wires are on pins 3 and 5.

paalw

It looks like that,yes, and the resistor measures around 24 ohms.
Thanks for the schema, unfortunately it didn't do much else for me than making me feel like a retard.

Ian S4

It's easier than you think! The signal from the rear sender arrives on pin #1, and the chip counts these pulses. When enough of them have been accumulated, it turns on one winding, then the other, to advance the stepper-motor a little bit. It does this by alternately pulling pins 3 and 5 low, so that the current in each winding will look a little like in the following image. So, you can fake the input signal by pulling pin #1 to ground repeatedly, in which case both the speedometer and stepper-motor should respond; or you can fake the output by alternately connecting pins #3 and #5 to ground (i.e., one at a time). In the latter case, the speedometer will not move, but the stepper-motor should.

James Bailey

there are something like 7,500 pulses per mile given the number of magnets in the aluminum wheel in the differential and the circumference of the tire so getting the motor to move much can take a lot of pulses like 750 to log a ten of a mile !

Ian S4

6673 pulses per mile, as it happens :-)

But remember there's a very significant mechanical reduction in the odometer gears. The stepper-motor should advance one step for somewhere between 32 and 128 input pulses, I don't have that exact number to hand.

James Bailey

Thanks I just wanted to be sure everyone knew that 10-20-30 pulses might not do much

Ian S4

It's very easy to build a "sender simulator", which is basically a switch that closes/opens at the equivalent of 32mph (27mph in Europe), with a small transformer, 1N4001 diode, and a cheap reed-relay. :-)

NickT

Just to chime in a bit, my trip odometer was not working, had a cracked gear so I replaced that. It work intermittently after that and was puzzled as to why.

Long story short, what happened is the gears were still working but the "number" could move slightly to the left and disengage. I fabricated a small "C" washer out of some fiber board (very thin) and inserted it on the shaft at the left end to apply a slight amount of pressure to the right and ensure the gears were always meshed.
Been like that for 5 or so years and has worked ever since.

paalw

So, for me to test operation I would have to build one of these?
I guess I'll be looking for a replacement speedo, if there isn't any way of fixing the electronics.

Ian S4

Not necessarily, You know the input from the sender is good because the speedometer works. You can test the motor separately, as described above. That part needs no special equipment. If the motor works, then the chip is probably fried and will need to be replaced. Are you handy with a soldering iron?

One other thing to try: Remove the motor and check that the nylon "screw" part is not loose on the motor's steel shaft.

paalw

Okay, but where do i apply power to the motor, the solderings on the print board?? Or do I detach the wiring from the motor end(will the wires detach?), then plug some 12V into it? Test with motor in place or remove motor?(please excuse me for my "dumb" questions..)

Don't think the nylon screw is loose, when i turn it i feel the step resistance of the motor like it should be I believe.

Ian S4

Connect a 12 volt battery to "Bat +" and "Bat -", as shown in this next photo.

Using a short piece of wire connected to "Bat -", alternately touch the free end to pins #3 and #5. The motor should move each time.

Be extremely careful not to touch that wire to any of the other pins on the chip!

James Bailey

The collective knowledge here is just awesome !! Well done Ian

Ian S4

Thanks, James! I've "taken" so much from this forum, it's about time I gave something back.