HarrisonGillis

Last week my car started to not shift into 4th gear. Usual blinking D and code p0843 (Pressure switch A). So what I'm trying to do is figure out if it's my switch, or the wiring leading up to the switch.

My question is how can I test the wiring on this switch? I know it's directly connected to the ecm, so I don't want to put any voltage on it and fry the ecm. Anybody have any suggestions on this?

ezone

2nd gear pressure switch stuck off or open circuit.....top of trans under air filter box.


Erase codes. Drive it up to 30 MPH or so so it shifts though several gears. If the code does not reset after the trans has shifted to 2rd a couple times, replace the switch.







If it recodes immediately during the quick test drive, THEN you will want to check wiring and whatnot.
Check for voltage on the wire (key on, engine off). If there is voltage found, then it's connected to the computer. Not a wiring problem. Replace switch.

HarrisonGillis

Thanks, I was more asking about the actual testing procedure of the wiring itself. But between many youtube videos and your response I figured out its only one single wire, with a constant 5v, so it should be relatively easy to test.

Probe the wire, find a ground(casing of the transmission?). Should be a definitive test.

Thanks again!

ezone

This is dynamic testing: the pcm will probably give you the answer with almost zero effort!


Why it can:
The code will reset within 2 seconds of detecting the failure, and most switch fails are intermittent.....so if it does not recode by the time it has shifted to 3rd gear, continuity from switch to computer is verified without having to open the hood. Order the switch and gasket.

Much time spent learning how and why these things work, now saves me a ton of time in diagnosis.

If you really desire getting dirty, then your thinking is correct. A single wire pcm input like used for that switch will generally have either 5v or 12v from a controller, and the switch pulls that to ground when closed. (Battery ground is usually much cleaner and easier to connect your meter to)


Of course if it set the code again on the first try, either the switch is continuously bad or the wire has a problem...... now you need to get dirty to check the wire....rodents are a problem in my area, they love wires.

HarrisonGillis

It's definitely not intermittent as the code is always comes back, even after clearing it. I've been driving it, just pissing a lot of people off on the highway, and not once has it gone away.

I'm leaning more toward the switch because of how it happened. I drove about 10 minutes to the store, everything was normal, went in the store, came back out and the problem immediately started.

Im hoping a mouse chewed it off near the connector because it's a cheaper fix than a $90 switch, but I'm also hoping it's the switch cause I know depending on where it was chewed, it could be a real hassle to find.

Anyway, going to take the airbox off tomorrow and have a look. I'll update back here just incase anyone runs across the same problem.

ezone

Most mouse damage I see is in the area between the main harness bundle and the component connector. They usually chew one or two easy wires, not 30...

If it's chewed it is usually pretty easy to find once you can see where the wire should be.