ATV speed sensor

[oliver602]

Hi,

I have a speed display from a 2000 Honda Fourtrax.

I have to replace a zener diode used to regulate the supply voltage to a 3 wire speed sensor. I don't have the speed sensor to check it. The dash puts ~4.5V on the signal wire so I'm assuming the sensor is a open collector output type? There is a diode, some passives and a transistor between the signal and the mcu for the display.

It is ZD4 in the picture. There are two parallel 240ohm resistors in series with the zener. They are connected to a regular rectifier diode to battery voltage.

It is marked '12 1A' so it must be a 12V zener?

[oliver602]

The signal from the speed sensor goes through this circuit before the mcu. Can someone help me understand whats going on here? Perhaps the transistor should be PNP?

[fzabkar]

To me the circuit only makes sense if the transistor is PNP.

[floobydust]

TR2 SMD marking "M7" is PNP 2SA812. The speed sensor appears to run off battery voltage according to the service manual, and the output has 5V seen so it's an open-collector hall sensor probably.
Then the diode is likely a TVS for transient protection but I wonder what burned it up? There doesn't seem to be enough energy with 2x240ohm resistors. Is there an R70? or something going to the connector pin that goes to the cathode.
Guessing SMAJ12 which 13.3-14.7V @ 1mA.

[oliver602]

Like this? Or have I got that upside down.

I tried measuring noise on a older fourtrax some time ago, the spike is from the ignition module. Don't remember where I connected the scope but perhaps that could have killed it?

R70 was unpopulated.

[floobydust]

no, flip it so the emitter goes up to +5V. Is D4 supposed to be ZD4 the one that burned up, or is the clamp somewhere else?

[oliver602]

ZD4 is somewhere else, I've added more to the schematic

[floobydust]

It's not making sense to me unless ZD4 is on the other side of R57/R58 (not the sensor power side).
Because (as drawn) R57/R58 don't need to be power resistors, and they can't supply enough current to cook ZD4. I see a need for ZD4 to also protect the 5V regs.
The sensor power is supposedly battery voltage.
So I still think it's a 12V TVS because a 12V zener plus D2 drop is going to load down the battery key-off. The "12V" TVS has higher min. breakdown voltage of 13.3V
C35 if populated would go on the sensor power side?

[fzabkar]

I think the power resistors are needed to account for external shorts to ground.

ZD4 is just a zener regulated linear power supply.

I agree that there is no logical way that the zener could be damaged by an onboard fault. However, I'm wondering whether a mechanic may have injected battery voltage across the sensor (because the service manual implies that this is the expected supply source).

[oliver602]

floobydust, I can't find C35, but C37 is on the sensor power side.

I've updated the labels, they weren't very clear before. The sensor isn't powered when the key is off.

Why are there little triangles scattered around the flood fills?

I'm wondering whether a mechanic may have injected battery voltage across the sensor (because the service manual implies that this is the expected supply source)

This sounds plausible. I bet the speed sensor is faulty... on second though if they back probed the sensor connector with battery voltage then the zener would have gone open circuit like a fuse?

Anyway I've had to go with the 12V zener because it's what I've got and I need to give the display back. It didn't occur to me it could be a TVS when I ordered the selection of zeners.