What would adding a resistor and diode to position sensor switch circuit? - Page 1
Forum section: electronics / general

author: DougSpindler
author_rank: Super Contributor
timestamp: November 06, 2021, 05:30:22 pm
content: I have a Chevy Volt which has the "Shift to Park" issue which prevents the car from powering down when you park it. People are having to leave their cars "running" when they parked. In my case it also prevents the car from engaging the HV batteries and drive motor leaving me stranded.GM's fix is to install a 100 ohm resister and a diode which are in parallel. (Someone posted a pic, attached. I think they don't know how to use a multimeter to test a diode.) The resister/diode are in series with the shifter position sensor switch.I'm also providing a link to the GM's "fix"https://static.nhtsa.gov/odi/tsbs/2020/MC-10171895-9999.pdfWhat would a resister/diode do in this circuit?Thanks

author: Gyro
author_rank: Super Contributor
timestamp: November 06, 2021, 05:54:31 pm
content: Please don't double post.https://www.eevblog.com/forum/renewable-energy/any-chevy-volt-owners-with-the-shift-to-park-defect/

author: ataradov
author_rank: Super Contributor
timestamp: November 06, 2021, 05:56:25 pm
content: This looks more like an inductor, which would make a parallel diode make a bit more sense.But without looking at the schematic, it is hard to tell what this actually does.

author: DougSpindler
author_rank: Super Contributor
timestamp: November 06, 2021, 05:57:01 pm
content: It's not a double post. In one post I'm asking how the circuit functions. In the Energy post I'm asking if anyone is having the Shift to Park issue.

author: DougSpindler
author_rank: Super Contributor
timestamp: November 06, 2021, 06:01:10 pm
content: Quote from: ataradov on November 06, 2021, 05:56:25 pmThere's no schematic. What would an inductor do in a circuit with a switch? Sure looks like a resistor to me.

author: ataradov
author_rank: Super Contributor
timestamp: November 06, 2021, 06:04:33 pm
content: Quote from: DougSpindler on November 06, 2021, 06:01:10 pmNo idea, it depends on what the actual issues is and the circuit used. Maybe provide a current spike to break though the oxidation layer, something that would be more of a problem than over-current.Quote from: DougSpindler on November 06, 2021, 06:01:10 pmLook at the proportions compared to the diode. It is short and wide, just like axial inductors.

author: PlainName
author_rank: Super Contributor
timestamp: November 06, 2021, 06:05:05 pm
content: Without knowing what this connects to it's hard to say, but it could be switch debounce without adding a delay to activation. Presumably there is a capacitor (or capacitance) upwind, possibly as EMC mitigation.Of course, it's likely to be something else

author: bdunham7
author_rank: Super Contributor
timestamp: November 06, 2021, 06:09:34 pm
content: Without consulting a manual or schematic, my best guess would be that there isn't a PARK 'switch', but rather a PRNDL sensor and the add-in jumper acts as an electrical shim to compensate for a slightly out of calibration sensor that can't be adjusted or easily replaced.

author: PlainName
author_rank: Super Contributor
timestamp: November 06, 2021, 06:32:00 pm
content: QuoteThat would get my vote. Except... wouldn't that need just the diode?

author: DougSpindler
author_rank: Super Contributor
timestamp: November 06, 2021, 06:33:36 pm
content: As I recall the switch is only activated when the the button to move the shifter is engaged.

author: DougSpindler
author_rank: Super Contributor
timestamp: November 06, 2021, 08:00:45 pm
content: I pulled my console apart and it's a microswitch. Mechanically the switch in my car was working fine. Could it be the contacts are getting burned? Over voltage or current? The resister is to reduce the current and the diode to dissipate a voltage spike?Take a look in the this video.

author: bdunham7
author_rank: Super Contributor
timestamp: November 06, 2021, 08:36:49 pm
content: Quote from: DougSpindler on November 06, 2021, 08:00:45 pmOK, so reading more closely, I see that step 3 in the service procedure has a 'park switch contact cleaning procedure'. So yes, there must be some issue that arcs the switch contacts and this is their patch. Very sketchy, IMO. There is also a release solenoid in the circuit that only allows you to shift out of park under the correct circumstances. If you dig up a schematic it will probably all be much clearer. There should also be PRNDL sensor of some sort somewhere, but it appears that this isn't implicated in this problem.

author: floobydust
author_rank: Super Contributor
timestamp: November 06, 2021, 09:21:40 pm
content: What century is this where a microswitch causes so much hassle and mayhem going back 5 years, across multiple models: Chevy Volt, Malibu, Blazer, Traverse, Acadia. Corporate denial a problem? leading to a class action suit."The cause of the condition may be the park switch in the transmission control (shifter) assembly not pulling BCM signal low to electronically show Park condition."Theswitch requires soldering to replace,so it looks like the entire shifter gets replaced instead.Nobody's ohmmeter readings of the "fix" GM stub harness 84733196 make sense. How do you get "basically a short" across a 100R resistor and diode? Need to see the Volt wiring diagram to fully understand what is going on.Mechanic "operate the switch 50 times". Microswitches aren't usually that crappy, the contacts do wipe... unless this is super cheap i.e. copper leafs only. Would look like either not enough wetting current 1mA and contact cleaning/wiping action to overcome oxide buildup- for the case it's only switching a simple digital input to the BCM or ECU.Or, some car models the Park Switch also feeds the Brake/Transmission Shift Interlock (BTSI) solenoid, so higher current could flow switching that. The switch is rated only 2A. In the case somebody at GM forgot the solenoid back-EMF diode, contact arcing would occur and likely damage the relevant BCM/TCM/ECU digital input as well. Inrush from any big capacitor filtering the DI seems not likely, EMC caps are 47nF or less.The stub harness tries to cover all makes/models affected but it looks like the basics of having a TVS is not there?Original microswitch is Omron D2HW-BL251H. The chinese are on this, sellingreplacement switchon Aliexpress for $3 lol.

author: NiHaoMike
author_rank: Super Contributor
timestamp: November 06, 2021, 09:49:40 pm
content: My guess is they had a bypass capacitor too high in value on the control module and the series resistor is a workaround so it doesn't pit the contacts.

author: floobydust
author_rank: Super Contributor
timestamp: November 06, 2021, 10:21:16 pm
content: But what's the diode doing? 100R is nothing for a DI that it warrants a shunt diode. The switch is carrying or breaking high/inductive current for some reason, which is killing it- if this oddball GM fix is correct and actually works verses replacing the switch/shifter.My guess is it's the BTSI solenoid, usually controlled by the BCM for more safety and anti-theft etc. but the park switch can be in series for extra safety.Then it looks like the "break after make" is the microswitch actually switching the solenoid (current), not the BCM because it is slow and later to act, in firmware, with its digital output.Hard to tell because this is about 5 years of cars and at least than five different models.

author: DougSpindler
author_rank: Super Contributor
timestamp: November 07, 2021, 12:07:12 am
content: 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

author: PlainName
author_rank: Super Contributor
timestamp: November 07, 2021, 10:23:20 am
content: 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).

author: floobydust
author_rank: Super Contributor
timestamp: November 07, 2021, 07:30:28 pm
content: 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 crankingBetter modern cars run the switch only into a BCM digital input.

author: PlainName
author_rank: Super Contributor
timestamp: November 07, 2021, 07:48:46 pm
content: If it really is back EMF from the solenoid, you gotta wonder why they let mechanical engineers loose on the electrics.

author: Someone
author_rank: Super Contributor
timestamp: November 07, 2021, 09:08:36 pm
content: Quote from: dunkemhigh on November 07, 2021, 07:48:46 pmCars 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.

author: DougSpindler
author_rank: Super Contributor
timestamp: November 08, 2021, 03:30:45 am
content: In some industries it might be about the Washingtons. In the car industry it's all about the Lincolns.