Automatic Transmission Fluid Pressure Switch

MarkVII88

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2012 Mazda5 Sport AT
On Monday, I took our 2008 Mz5 to the shop for a transmission issue. Car was not shifting between gears smoothly, especially 3rd to 4th. There would be a bit of a lurch, especially if giving it the gas like to merge on the highway. Car was holding revs too long when shifting 1st to 2nd as well. No check engine lights.

Last Sept I did a full 9qt. Trans fluid flush and replaced fluid with Valvoline Import M-V (multi vehicle). I have also replaced both engine mounts and the trans mount.

Turns out it was a bad trans fluid pressure switch and the car was running the trans at higher than normal pressure, per one of the codes the car threw for the mechanic. Replaced it and it drives fine now, $240 later.
 
I'm glad you got it repaired for a reasonable cost. What you are referring to is the EPC, pressure control solenoid.

https://www.ebay.com (commissions earned)

I'm not sure how reasonable that price is, but the car needed fixing. I could get one of those fluid pressure switches/solenoids from Rockauto for about $63 so unless it was hella-difficult to access and replace, I probably could have done it myself and saved $150. At least this way though, I didn't spend a Saturday morning doing it.
 
Was the code P0841 by chance?

If it is, the really crappy thing about this code is it doesn't turn on the check engine or AT lights. I've had this code for a while and I'm trying to remember if it turned on after I replaced 9 quarts of the AT fluid with Castrol Import or not. The one thing I cannot figure out is if you have to drain the ATF first or not. Any ideas Aaron?
 
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I think there's some confusion here

There's a pressure control solenoid that is related to code P0758, turns on the Check Engine Light and AT light, and requires a pan drop
There's also a pressure control switch that triggers code P0841, does not turn on the lights, and does not require a pan drop but may require draining the ATX first
 
I'm not sure how reasonable that price is, but the car needed fixing. I could get one of those fluid pressure switches/solenoids from Rockauto for about $63 so unless it was hella-difficult to access and replace, I probably could have done it myself and saved $150. At least this way though, I didn't spend a Saturday morning doing it.

Any chance you can find out what code it was?
 
Odd. AlldataDIY and Torque say otherwise. I have a 2009 if that matters

Upon further reading, the workshop manual has conflicting data regarding the MIL/AT lights.

I don't have personal experience with P0841, so I can't speak to the causality except what the workshop manual states. It is a NO switch that provides ground to TCM when pressure is applied and can simply be checked for continuity to determine malfunction.
 
I see that. Might have to try testing it again this weekend

On-vehicle Inspection (harness inspection)
1. Remove the battery duct and battery cover.
2. Disconnect the negative battery cable.
3. Remove the under cover.
4. Disconnect the cord assembly connector.
5. Verify that there is no continuity between the cord assembly terminal and ground.
If there is no continuity, inspect the cord assembly.
If the cord assembly is normal, inspect the oil pressure switch.​
6. Connect the cord assembly connector.
7. Install the under cover.
8. Connect the negative battery cable.
9. Install the battery duct and battery cover.


On-vehicle Inspection (oil pressure switch inspection)
1. Remove the under cover.
2. Disconnect the cord assembly connector.
3. Start the engine, with gear position in N or P.
4. Reaffirm that there is no continuity between the cord assembly terminal and ground.
5. Firmly depress the brake pedal with the left foot.
6. Shift the selector lever to D range.
7. Verify that there is continuity between the oil pressure switch terminal and ground.
If there is any malfunction, replace the oil pressure switch.​
8. Connect the oil pressure switch connector.
9. Install the under cover.


Off-vehicle inspection
1. Remove the oil pressure switch.
2. Apply air pressure at (5863 psi)
3. Inspect continuity the between the oil pressure switch terminal and screw part.
If there is any malfunction, replace the oil pressure switch.​
4. Install the oil pressure switch.


OIL PRESSURE SWITCH REMOVAL/INSTALLATION

Warning
A hot transaxle and ATF can cause severe burns. Turn off the engine and wait until they are cool before replacing the ATF.​

1. Remove the battery duct and battery cover.
2. Disconnect the negative battery cable.
3. Remove the under cover.
4. Disconnect the cord assembly connector.
5. Remove the oil pressure switch.
6. Install the oil pressure switch.

Tightening torque
(12.716.2 ftlbf)​
7. Connect the oil pressure switch connector.
8. Install the under cover.
9. Connect the negative battery cable.
10. Install the battery duct and battery cover.
 
Thanks for the help Aaron. The oil pressure switch does have continuity in D so it appears to be working properly. I realized tonight that I never did the line pressure test and after re-reading the troubleshooting steps for P0841 I see that the solenoid could be one of the causes if the line pressure is too low or too high, among other things

Still think that is the way to go or not so much?
 
The workshop manual lists only the oil pressure switch circuit being the cause of P0841, either the switch, the harness, the wiring to the TCM, or the TCM itself. If you have instructions that list otherwise, I'm sorry I can't help with that.

Did you perform the harness inspection?
 
The workshop manual lists only the oil pressure switch circuit being the cause of P0841, either the switch, the harness, the wiring to the TCM, or the TCM itself. If you have instructions that list otherwise, I'm sorry I can't help with that.

Did you perform the harness inspection?

I did test for continuity between the harness and ground and there was none. The wires appeared to be in good shape at the connector as well

Looks like next steps are checking the wiring to the TCM, checking the harness, and praying it's not the TCM itself
 
Got in there today. Did not have a small enough probe for the TCM. And with that wire being in the middle of the harness I wasn't able to do much unfortunately. Sucks. I did disconnect the harness from the TCM and did not see anything wrong with the connector

I also realized that I was testing the switch incorrectly. I was using Ohms but because the switch puts out voltage I should have been measuring volts instead. May double check that one more time but I'm almost out of patience with this
 
The pressure switch does not have a reference voltage, it is switched ground only.

Place 1 probe on metal grounded surface and other on part being tested. Set meter to Ω (ohm):
  • When testing the switch, you should have infinite resistance (no continuity) in Park, and 0 (no resistance) in Drive.
  • When testing the connector, you should have infinite resistance (no continuity).

Also, check continuity through the harness, 1 probe at the TCM (harness disconnected), and other probe at the switch connector.
 
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Thanks Aaron. Working on tracking down some back probes. I'm sure my mechanic buddy will have some
 
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