Auto Trans Snow/Power Switch

[4rrow88]

Hi Guys, looking for some info on how the snow/snow switch works from the electrical side of things or some kind of manual that shows the transmission wiring etc (don't see anything in the workshop/engine manual).

Vehicle is a C34 Laurel, originally rb20e auto (3speed+OD). The switch used to always light up orange when switched to power mode.

I swapped the engine to a rb20det, ever since then the snow mode still works, but the power mode doesn't (at least 99% of the time but weirdly it did work the other day out of the blue for about 5min then stopped again).

Wiring wise:

- all of the wiring from the trans to the body/transmission computer are unchanged from factory.

- wiring FROM engine loom/ECU TO the trans control unit (related to throttle position) are hooked up

- wiring FROM trans control unit TO the engine ECU (which tell the ECU what gear it's in) are not connected as it's a manual ECU

Transmission operates and shifts fine - just the power mode doesn't seem to engage, assume something with the wiring but weird that it did actually work briefly the other day which got me wondering if there is some other variable at play. 

Anybody out there have any ideas or know where to find a workshop manual for the transmission?

[GTSBoy]

It's just a digital input to the TCU. There's no secret magic. Break out the multimeter and start probing that input with the switch on and off and see if the circuit is connected properly. If it is reliably detectable (that you have turned the switch on) and the TCU doesn't activate power mode, then there is either a problem in the TCU or there is some other input that the TCU is not happy about.

If the only change you made was to the engine and ECU, then it is remotely possible that the ECU is not providing some signal that the TCU wants to see to be happy. This is more common with later ECU/TCU combos than early. They talk to each other on a data bus, not discrete signals.

The other possibility is that you have simply created a wiring problem when you pulled it all apart and put it back together. A broken wire or faulty multi-pin connector perhaps.

[Duncan]

Yes, very possible for it to be wiring problem or possibly even led in the switch....you need an answer to that first question about what that switch actually does to know if the trans is responding to it or not.

I understand that Snow should lock our first gear (ie, you move from a standing start in second) and Power should allow it to rev to redline before changing (while normal probably shifts at 70-80% revs) but I am not certain

[GTSBoy]

Oh, Power mode will also make it kickdown earlier, possibly make the shifts a little faster. But none of that really matters. It will do what it does, and because it's an auto it will still suck. But right now it doesn't appear to do anything.

[4rrow88]

Thanks for the suggestions - will get out the multimeter and do some more digging, the way it briefly worked the other day makes me wonder if it is indeed just a broken/loose plug - or even could be just the LED not working - will have the car road legal soonish so will actually be able to test out properly if it changes anything by having the switch on/off.

When sorting out the wiring was going off of the wiring schematic for an r33 rb20e auto (in the engine/workshop manual) which I'm pretty sure is more or less an identical setup to c34 - there was no data bus between the ECU/TCU just some pretty basic inputs/outputs

- throttle position feeds into the TCU which when not hooked up made it shift at really high rpm even on light throttle - once connected works mint

- signals from TCU to ECU for what gear it's in - my understanding is this affects ignition timing on the auto ecu

Edited February 10, 2023 by 4rrow88