Automotive Switches: Resistance?

[Ferrino]

I'm trying to troubleshoot an annoying electrical fault with my daily-driver: a 2010 Mazda 3, which has been flawless, except for a schizophrenic front passenger door latch switch. Basically, the car periodically thinks that door is open, when it's closed. The locks operate fine, but about 60% of the time, the "door ajar" light comes on when the door is firmly closed. This is nothing more than an annoying warning when I'm driving, but when the ignition is off, the "courtesy lights" illuminate, draining the battery a little. It has never been a problem until COVID, since before, I use the car each day and there is not enough down-time to drain the battery substantially. Well, the other day the battery got down to 8.5V after several days sitting, so my hand was forced. I figured out the parasitic circuit is controlled by the "ROOM" fuse, which gives power to a bunch of interior loads (door locks, interior lights etc.).

With the passenger door panel removed, I can wiggle the harness connector that attaches to the door latch assembly and provoke the computer to switch between thinking the door is open and closed. I checked the connectors and all pins seem clean/intact. When driving, you can see it flipping back and forth - almost as if a wire is shorting or losing a ground. I removed the latch assembly from the door (the switch is part of the entire locking/latch assembly, not an independent switch) and bench-tested it. When the latch is open, I see no continuity between the switch terminals. When I close the latch, I am seeing resistance readings of 5-10 k-ohms with the switch apparently closed. It's actually jumping around quite a lot, but my auto-ranging multimeter is staying in the k-ohm scale.

So my question is: how may ohms should I see on a simple door latch switch when it's closed? I naively assume it should be reasonably low, since these are just metal-to-metal contacts? And should I assume that the above reading is indicative of a switch with too much resistance, presumably due to buggered/corroded contacts?

Thanks! Obviously I could just go out and buy a new latch assembly, but I'd like to understand the failure modes of such switches.

[john70t]

Switches should have almost no resistance.
Electric amperage load adds ohms.

They should flip easy and solid.
Use WD-40 to clean and loosen it up. Wipe off exterior.
Replace if either doesn't happen in a day.

[sc_rufctr]

My daughters Holden (GM) Cruz had a similar problem recently. Interior light wouldn't switch off and her radio kept playing. She'd have to open and close her door a few times before it would work properly. I thought about opening it up and checking the micro switch but eventually decided not to bother. These are not designed to be opened up and serviced... She bought a new lock from the dealership for around $200. I fitted it a few days ago and it's been perfect ever since.

This is the original 8 year old lock. I may open it up just to have a look inside but at some point this will be going into the recycling bin.

[nota]

CRC eltra-clean spray can or even WD-40 can fix a stuck switch

I would give that a try before spending 200 on a new switch

[sc_rufctr]

Originally Posted by nota

CRC eltra-clean spray can or even WD-40 can fix a stuck switch

I would give that a try before spending 200 on a new switch

Temporary at best... Just replace the thing and be done with it.

How much is you time worth and how many times can you take off the door trim panel without breaking something?

[asphaltgambler]

^^^he's right ^^^^& also that is likely attached to a body control module. I would replace it, move on. It's not like an old school full load switch.

[Ferrino]

Thanks all! I wanted to open the thing up and investigate, but the plastic shell is built like some sort of aviation black box and I didn't see an obvious way to pry it apart reversibly. So I thought I'd shoot an impossible quantity of lubricant in there, hoping to drench the switch (wherever it is). No change - still getting a weak contact when closed. So I will now proceed to order a new complete assembly for $100, to replace what is likely a $0.50 switch! Naturally, I will hack the old one open and report back for those of you following this drama.

[Ferrino]

Got the replacement OEM part in and measured zero ohms on the new switch. Here's to another 10 years of service. I will do an autopsy on the faulty switch another day. Thanks for your help.

[Bill Douglas]

Maybe do the autopsy first. that way you may find you can clean the contacts, AND with a bit of luck get it back together. Then return the part and spend the $100 on alcohol as a way of congratulating yourself on a job well done.