[FIXED] Mazda P0758

Got called out to a 2006 Mazda 3 with a 1.5 engine. The transmission bracket broke causing it to drop. After the repair, reverse was slamming in and it would only only go on 2nd gear when drive is selected. A scan indicated P0758 shift solenoid b (SSB) electrical fault, the mechanic changed the solenoid but error persists.
Findings:
I gave the new solenoid 12v through the harness and heard it clicking, so it's working.
The SSB is responsible for 2nd-3rd gear shift and is power side controlled. I found that it was receiving 12v continuously, causing it to be stuck on, this is why the transmission goes to 2nd. I cleared the code, pulled the wire from the valve body harness and switched on the car. The transmission stayed in 1st and error didn't return until I grounded the said wire.

I was thinking that may be the PCM was getting a speed signal, Tps signal but all data pids look normal. Suspecting PCM is bad but will do more tests tomorrow.

We don't have anything over here with a 1.5...
I was going to check into some diagrams but no luck.

Yeah you guys got the cool versions.

You need to check that circuit for a short to power. You said it was power side switched and it had 12v on it all the time right?

Yes. I forgot, I did get a square wave for about 2 seconds then it went 12v straight.

Caritech wrote: Yes. I forgot, I did get a square wave for about 2 seconds then it went 12v straight.

Interesting!

I cleared the code, pulled the wire from the valve body harness and switched on the car. The transmission stayed in 1st and error didn't return until I grounded the said wire.

Great test here, too. Would I be correct in thinking that there was 12V on the disconnected wire during the test drive? I ask because I'm wondering if the PCM is monitoring current flow through that high side driver, not necessarily voltage.

Just thinking out loud here... Any chance that the SSB solenoid control wire is shorted to another solenoid circuit? As in, the SSB control is shorted to the SSA control, which ends up energizing both solenoids? This is the only way I can think of to explain what you've seen, besides a bad PCM. A band/application chart would help prove or disprove this theory.

Did the same mechanic that called you do any wiring repairs before you arrived?

No repair was done to wiring. I'm going back today to scope the speed sensor. I didn't get any code or pid to be concerned about but I'm also thinking that it's a major input in shifting so a check of the signal won't hurt.

Awww, there goes that theory Oh well.

I was wondering about a possible shifter input problem, but that doesn't explain the code... Eager to hear what you find!

Went back today and found the gnd wire on the 3 pin vss was loose in the connector of the sensor, it can easily be pulled out by hand. The speed signal was a hashy >200mv one @ idling and was a 0-5V square wave on a drive.

I decided to try something. I kept the SSB disconnected and went for a drive. The car drove normally, with a few rough shifts. I thought this could be a nice temporary fix till I can get more info on the issue but the error came back, according to the call I got from the customer.

Btw. I found no evidence off short to power, the 12v is coming from the pcm.

Thanks to my cousin I was able to find a known good. He has a 2007 but it's basically the same, the pcm sends no power to the SSB @ idle and the speed sensor signal was a little lower but just as hashy. The inputs seem the same in both cars.

Caritech wrote: Thanks to my cousin I was able to find a known good. He has a 2007 but it's basically the same, the pcm sends no power to the SSB @ idle and the speed sensor signal was a little lower but just as hashy. The inputs seem the same in both cars.

Good move, sir!

Is this looking like a bad PCM to you? 'Cause I'm struggling to think of other reasons for the computer to command the SSB the way it is...

Did the PCM harness get stressed when the transmission gave up? Any physical damage to the computer itself?

VSS ground wire was loose and all inputs looked normal. No physical damage to the brain seen.

Hi there, which bracket broke? Did you take the valve body out ? The tranny was shifting good before the damage?

The bracket on the transmission broke according to the customer. SSB was changed only by mechanic. From what I was told car was normal 100%.

This sure is a weird one. I wish I had some service info for this car...
It's just hard to associate a broken trans mount to a faulty PCM. The only damage I can envision by such a fault would be a stressed harness connector (like Tyler already mentioned) or perhaps even a completely broken off ground cable. I suppose any number of things could have been smashed inside the pan. Have you had it off the trans yourself, or has the customer mentioned having the mechanic replace the pan for damage while changing out SSB?

Yes. I forgot, I did get a square wave for about 2 seconds then it went 12v straight.

This kind of makes me think the constant 12v could potentially be intentional. Like a forced limp mode operation, but I would imagine there would be a lamp illuminated for such, or a corresponding trouble code. That's just a thought though, I have no idea what if any limp mode strategy this car may employ.

New pcm solved the problem. Saw the owner finally.

Sweet! :woohoo: Thanks for the update on this one, sir. I'm still trying to imagine what happened to the PCM during the mount failure :blink: