Speedometer with Microsquirt?

[Drscottsmith]

All -

I have not had a working Speedo since the car has been running. I pulled the hall sender and cleaned it today and reinstalled - no change.

So started looking at the schematics. The hall sensor has three wires, and one is labeled ‘HG’ and one labeled ‘H+’. Almost seems like the H+ is supposed to get voltage from somewhere to generate the signal, and I am wondering if I lost that voltage somehow when I disconnected the old computers.

Any of you guys have to bridge pins somewhere when you put MS in?

All other gauges are working fine (except boost which is my next issue to solve).

Thanks as always!

-scott

[Tom]

All -

I have not had a working Speedo since the car has been running. I pulled the hall sender and cleaned it today and reinstalled - no change.

So started looking at the schematics. The hall sensor has three wires, and one is labeled ‘HG’ and one labeled ‘H+’. Almost seems like the H+ is supposed to get voltage from somewhere to generate the signal, and I am wondering if I lost that voltage somehow when I disconnected the old computers.

Any of you guys have to bridge pins somewhere when you put MS in?

All other gauges are working fine (except boost which is my next issue to solve).

Thanks as always!

-scott

If this is for your '86 951, I think the ECU is a red herring, as the speedometer signal goes straight from the sensor to the cluster without any involvement by the DME. The sensor should have power and ground to make it work, but the signal it produces is just a digital ground pulse. Each revolution of the rear tire generates 8 ground pulses (and 8 equal length periods of open signal). Read more about that here. The signal is on the blue wire with red stripe in the cluster harness. You might start by making sure you have continuity from that wire's pin in the edge connector all the way to the sensor connector. Then, if you probe that wire/pin and turn the wheel slowly enough, you should be able to see the signal pulse to ground on a multimeter. If you don't see any signs of life, then the sensor or wiring is bad. If you do see it pulse, but the speedometer still doesn't work, then the problem is in the cluster/connector, flexible PCB, or speedometer itself.

[whalenlg]

See my post here for my speedo fix

viewtopic.php?p=36661&hilit=Burned#p36661

[Drscottsmith]

Ok -

So need to do the continuity test (raining outside right now). But I put the meter on pin 4 (blue and red wire) on connector B and get 12v with ignition on. Turning the drivers side rear wheel resulted in no change in voltage at all - so the voltage is getting to the sensor and back but not being pulled to ground 8x per revolution, correct?

The gear is fine in the diff, so bad sensor itself internally? BTW yes this is the 951.


Thanks!

-Scott

[Tom]

Ok -

So need to do the continuity test (raining outside right now). But I put the meter on pin 4 (blue and red wire) on connector B and get 12v with ignition on. Turning the drivers side rear wheel resulted in no change in voltage at all - so the voltage is getting to the sensor and back but not being pulled to ground 8x per revolution, correct?

The gear is fine in the diff, so bad sensor itself internally? BTW yes this is the 951.


Thanks!

-Scott

You get 12 volts on that wire? I could have sworn it was open unless grounded, but would need to check that. As for spinning, it would be safer to put the car in gear and creep forward to check for the pulse. The inside of the transaxle is a black box of mystery to me, but not sure spinning the wheel by hand also spins the speedometer gear -- if you don't see the driveshaft spinning when you do that, I'd be suspect...

[Drscottsmith]

Correct - 12v with ignition on. I guess the sender could be stuck in that case and not grounding?

I have never been in the transaxle either. The sensor gear inserts about where the output shaft would connect, so I assumed that would register. I’ll see what else I can find and maybe some else will chime in.

Can I test the speedometer itself by grounding or sending voltage from the edge connector?

[Drscottsmith]

Update -

Tom it does appear that the car has to be moving. I definitely feel like I see a pulse when creeping. It’s digital, but the reading goes up and down (kind of blips) at a regular rotation.

So pulled the instruments to see what I can find in there.

[Drscottsmith]

@whalenig -

I pulled my Speedo and that resistor is a little crispy looking as well. It measures 8 Mohms in circuit.

You mentioned two caps as well - what all did you end up replacing on that board?

I count two caps and 6 resistors I believe.

[whalenlg]

Definitely both caps and the bad resistor.

Check all other resistors.

[Drscottsmith]

Ok - I got the 10mf and 47 mf caps. What did you replace that resistor with? Mine is too far gone to Id?

Thanks -

-scott