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Discussion Starter · #1 ·
Hi,
So my son's Mazda 3 started throwing code P0758, transmission was on limp mode. I cleared the codes, and nothing, as soon as I put the car into D, codes came back.
I research a little online and most of the people pointed to the TCM module being bad. These 2.0l cars don't have a TCM, the tranny is controlled by the PCM.
Before I went ahead and ordered a replacement PCM I performed a wire continuity test from the PCM plug to the Plug in the transmission, test passed, then I checked the resistance of the solenoids, and the test passed, then I supplied 5V to the solenoids and I could hear the solenoids click, so the test passed.
I ended up ordering a re manufactured PCM and after dealing win the damn PATS (My son only has 1 key) I was able to start the car. Codes were gone, and car was shifting perfect in the driveway. I took it for a test drive, and everything was good.
My son left, and 20 minutes later, he called me and said that the car is doing the same thing again. Transmission in limp mode, AT and MIL lights on.
I still have to pull the codes to make sure the code is be the same. But I am pretty sure it will be.

My question is, can a "faulty" solenoid keep blow one of the lines in the PCM? Or could be something else?

What I am thinking is that the solenoid might get "shorted" maybe intermittently and blow the PCM line?
Should I replace the solenoid before playing with a new PCM?

There is nothing else in between the PCM and the solenoid to be causing this issue. Just the wires.

Any suggestion will be appreciated.
 

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Couple things...

Not saying this is your issue, but our 2007 3S with the FN5A transmission had the connector into the transmission have a failed silicone seal and water got in and eventually rusted away one of the pins into the transmission altogether. I performed similar tests to you like harness continuity checks before I found the missing pin. That is shown below as "TRANSAXLE CONNECTOR WIRING HARNESS-SIDE CONNECTOR".

From the 2007 Service Manual...

Font Parallel Rectangle Number
 

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Discussion Starter · #3 ·
Couple things...

Not saying this is your issue, but our 2007 3S with the FN5A transmission had the connector into the transmission have a failed silicone seal and water got in and eventually rusted away one of the pins into the transmission altogether. I performed similar tests to you like harness continuity checks before I found the missing pin. That is shown below as "TRANSAXLE CONNECTOR WIRING HARNESS-SIDE CONNECTOR".

From the 2007 Service Manual...
Thanks for the answer and the suggestion.
A couple of questions, how do you remove the connector that is in the transmission? or you access it from the inside?
Was your car also blowing the PCM?
I disassembled the original PCM, checked all the transistors and resistors and found all the transistors to test fine but one of the R10 resistors to be bad which is in the circuit of the pin 1E corresponding to solenoid B according to the diagram you posted.
This is driving me crazy.
 

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Thanks for the answer and the suggestion.
A couple of questions, how do you remove the connector that is in the transmission? or you access it from the inside?
Was your car also blowing the PCM?
I disassembled the original PCM, checked all the transistors and resistors and found all the transistors to test fine but one of the R10 resistors to be bad which is in the circuit of the pin 1E corresponding to solenoid B according to the diagram you posted.
This is driving me crazy.
The 3S 2.3L has a separate TCM, so the situation isn't exactly the same. The PCM was fine. It turned out the TCM was fine too. The 3S 2.3L has the FN5A transmission and you have a 3i 2.0L with the FN4A transmission. Regalrdless, the are related and there is s a connector form all the electronics that go into the transmission.

I pasted something form the service manual on how to get to the connector for the FN4A. If this is the same as the FN5A, this view is what the orientation of what you would see if you are standing in front of the car, just about even with looking into the drivers window.

On our car, the pin for the transmission oil sensor pin was rusted away and part of the pin was stuck in the wiring harness side. I ended up having to drop the transmisison oil pan and valve body in order to replace the wiring harness inside the transmission. I also went to the junk yard and got a wiring harness side connector and soldered it in place of the failed connector.

Font Parallel Rectangle Elbow Paper
 

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Discussion Starter · #5 ·
The 3S 2.3L has a separate TCM, so the situation isn't exactly the same. The PCM was fine. It turned out the TCM was fine too. The 3S 2.3L has the FN5A transmission and you have a 3i 2.0L with the FN4A transmission. Regalrdless, the are related and there is s a connector form all the electronics that go into the transmission.

I pasted something form the service manual on how to get to the connector for the FN4A. If this is the same as the FN5A, this view is what the orientation of what you would see if you are standing in front of the car, just about even with looking into the drivers window.

On our car, the pin for the transmission oil sensor pin was rusted away and part of the pin was stuck in the wiring harness side. I ended up having to drop the transmisison oil pan and valve body in order to replace the wiring harness inside the transmission. I also went to the junk yard and got a wiring harness side connector and soldered it in place of the failed connector.
I would love to have that service manual. hahaha
My question regarding the connector was how to remove the part of the connector that is in the transmission body. Do you have any info on that?
I assumed you had corrotion in the part of the plug inside the transmission body, which could be the reason my son's car is blowing the PCM if there is a short there. Also, the solenoid coil could be intermittently failing causing a short and blowing the PCM. (just guessing)
 

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Check you PMs

In order to remove that connector (on the FN5A anyway) you have to drop the valve body and it comes out through the top of the transmission. It connects to the solenoids and temp sensor and whatever else is in there. Not a big wiring harness, but not possible to replace without dropping the valve body.
 

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Discussion Starter · #7 ·
Check you PMs

In order to remove that connector (on the FN5A anyway) you have to drop the valve body and it comes out through the top of the transmission. It connects to the solenoids and temp sensor and whatever else is in there. Not a big wiring harness, but not possible to replace without dropping the valve body.
I have never dropped a transmission valve body, is it hard? Will I need torque specs to put it back together?
BTW, I appreciate the message in the PM.
 

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I have never dropped a transmission valve body, is it hard? Will I need torque specs to put it back together?
BTW, I appreciate the message in the PM.
It isn't hard, but you have to be careful of a couple things. You must use a torque wrench since these bolts go into aluminum. On the FN5A, a couple springs will drop out and they are not the same and must go back exactly how they came out. If I remember correctly, there is a part that has to do with putting the car in park (sometimes called the parking pawl) that must be properly seated. The transmission oil cover is on with RTV and not a regular gasket, so you'll need to use that -- either get it from Mazda or I used AC Delco in a caulking tube stuff (both are some version made by Three Bond which all the manufacturers use) or the stuff at the auto parts store "Great Stuff" is also acceptable. Surfaces gotta be clean. Don't scratch the aluminum with razor blades. I used Mazda parts counter sourced transmission fluid -- probably more expensive, but it is guaranteed correct for the car. It was a decade ago, and as I recall it was at least 6 quarts. Change the transmission oil filter while you are there. Keep track of the bolts so you don't have one missing while you are putting this together.
 

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Discussion Starter · #9 ·
It isn't hard, but you have to be careful of a couple things. You must use a torque wrench since these bolts go into aluminum. On the FN5A, a couple springs will drop out and they are not the same and must go back exactly how they came out. If I remember correctly, there is a part that has to do with putting the car in park (sometimes called the parking pawl) that must be properly seated. The transmission oil cover is on with RTV and not a regular gasket, so you'll need to use that -- either get it from Mazda or I used AC Delco in a caulking tube stuff (both are some version made by Three Bond which all the manufacturers use) or the stuff at the auto parts store "Great Stuff" is also acceptable. Surfaces gotta be clean. Don't scratch the aluminum with razor blades. I used Mazda parts counter sourced transmission fluid -- probably more expensive, but it is guaranteed correct for the car. It was a decade ago, and as I recall it was at least 6 quarts. Change the transmission oil filter while you are there. Keep track of the bolts so you don't have one missing while you are putting this together.
Seems very doable. I wish I had a lift for this kind of situations.
I will report back then. I will be gone for 2 weeks, so it is going to take me a little.

Once again, thanks for all the information.
 

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Discussion Starter · #11 ·
Just a follow up, I pulled the code again and yes, same code, P0758.
I also did a Data log with FORSCAN and there seems to be no signal going from the PCM to solenoid B. Signal is flat, when is not into solenoid A and C. Could not test D and E because the transmission is in limp mode.
 

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Discussion Starter · #13 ·
My initial diagnosis was also good, no signal coming out of the PCM, so I replaced it and worked for like half an hour.
I just gotta find what is making the PCM to blow.
I mean, other than a short in the plug as @Akmnhg5A suggested or a intermittent faulty solenoid, there is nothing else in between the PCM and the solenoid.
 
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