General > General Technical Chat
What would adding a resistor and diode to position sensor switch circuit?
DougSpindler:
This guy is saying the microswitch is what's causing the "Shift to Park" issue. He's just shorting the switch to ground and claims that fixes the issue. Stupid, because now the car's computer doesn't know if the shifter is in park or not. I can see "fix" as causing an accident.
If the a GM's fix is a resistor/diode that's just going to protect the switch from high current/voltages in the future, but the switch has been damaged. (That's why there's a "Shift to Park" error message. Shouldn't GM also be replacing the switch as well? As someone pointed out cycling the switch 50 times may clean dirty/damaged contacts but it's not going to "fix" them. Seems like GM is going for the "temporary" fix.
Skip ahead in the video to 5:00 to see the switch fix.
https://youtu.be/TFqKzOM5Bwg?t=497
PlainName:
If it were a 2 second job I'd probably go for the ground fix, but since he's dismantled it enough to get to the microswitch anyway it would make more sense to just replace that and be done (and add the official bodge to prevent future hassle).
floobydust:
No wiring diagram for the Volt on the Internet? Even just the BCM input Shift to Park switch?
Cars are built to be manufactured, not repaired. Simply replacing the microswitch is not can option because it's an entire subassembly 84955381 of the shifter - the BTSI solenoid, wiring harness, connector are all a prefab piece. Suppliers having a lot of extra stock of that, or the entire shifter assembly who knows what was agreed to.
For those that can solder, it's no biggie to replace the switch but that does nothing for the root cause, back-EMF spikes.
I would say it's two problems - the vehicle trashing the microswitch with overcurrent or arcing or both, and a now dead microswitch. But "Shift to Park" switch failures are common - in Ford Edge, Cadillac STS etc. many other cars too.
The GM fix looks wrong to me - my guess is the solenoid(s) are in series with the microswitch, so the back-EMF trashes its contacts. You'd have to do an autopsy on one to see the contact failure.
There should be components at the solenoid(s) to quench the back-EMF, but as a diode this ruins the "reverse battery" automotive requirement so I see some cars missing it. Even a 1k resistor across a 12V solenoid, or RC snubber lowers that spike well enough. But the costs... too much.
Some (GM service bulletin) car models the park switch is on the high-side, others low-side to GND. The universal "fix" might not make sense for the Volt as it switches to GND.
On my old '60s Chevy the Park Switch was in series with the ignition switch and starter solenoid, so it saw 40A during cranking 8) Better modern cars run the switch only into a BCM digital input.
PlainName:
If it really is back EMF from the solenoid, you gotta wonder why they let mechanical engineers loose on the electrics.
Someone:
--- Quote from: dunkemhigh on November 07, 2021, 07:48:46 pm ---If it really is back EMF from the solenoid, you gotta wonder why they let mechanical engineers loose on the electrics.
--- End quote ---
Cars are engineered down to skin and bones, this setup probably passed the lifespan testing (where lifespan is anticipated use of that part up to end of warranty). "Odd" decisions are made in automotive electronics, you often seen bizarre/convoluted multiplexing to avoid running an extra wire (+pins in connectors) as they are super sensitive to cost of manufacture.
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