I have a 2001 Jimmy, got a P0756 shift solenoid code, had to drive it home about ten miles, I think it went into limp mode (don't even know if it has one).
It wouldn't go above 15-20 mph and wouldn't go higher than 1500 rpm.

Questions.
1 - does the second-gen have a limp mode?
2- if it does, will replacing the solenoid take it out of limp mode, because clearing the code didn't
3- if it doesn't have a limp mode, why the hell won't the truck go above 1500rpm?

ANY help would be greatly appreciated

 

Yes I believe that our trucks have a limp mode, so to speak at least in the form of gear limitations. What gears will it shift into? Is it the same rpm limit in all gears? Did "limp mode" arrive with the CEL and code?

There can be solenoid electromechanical problems but also wiring, connector or power problems that cause problems like this. Check fuse Clstr fuse 11, 10a in the IP fuse block This supplies the power to all of the solenoids. Also check the connectors at the transmission.

George

 

I have time for a more complete answer today. Your DTC is for a fault detected in shift solenoid B circuit/function. This can be a faulty shift solenoid but it can also be wiring/power and most importantly it can be an internal blockage or restriction in the valve body, a bigger possibility in an older transmission When this condition is set, the PCM increases fluid pressure, bypasses 1st and 2nd gear, starts the vehicle in third gear and may inhibit 4th gear. It also inhibits TCC, the torque converter lock up function that occurs at cruising speed. I don't believe that the PCM limits rpm but I am not positive on that. Clearing the code should clear the condition and the functional limits but if the problem is caused by an internal failure then the default of 3rd gear may be a better option to get home. I would first test and eliminate any power or wiring problems then drop the pan and inspect. A solenoid replacement might do the trick but then again maybe not. dropping and cleaning/inspecting the valve body can be a DIY project if you work clean, have patience and like detailed work. There are you tube videos. Let me know if you need any further diagnostic help.

Maybe someone else knows if an rpm limitation is involved in this problem.


George

 

Thank you George.
I checked all the connections and they seem to be good, just got two solenoids delivered, A & B, and the filter/gasket gets here tomorrow, but I won't be able to drop the pan for a few days.
When I did drive it home, I could shift manually through the gears, so I'm pretty sure that's the culprit; what I'm really concerned with is the RPMs.
Even sitting in park in the driveway, it idles fine, runs nice and smooth until it hits 1500-1600 RPM, then it just bogs down, I've never seen anything like it (granted most of my experience is older small/big blocks).

I don't know if it will clear up once the tranny solenoids are replaced, didn't when I cleared the codes, or if I have a deeper issue, never had a vehicle go into limp mode so it's all new territory for me.

 

Here's the shop manual 411, maybe this will help:







George

 

So, changed out the solenoids, that problem is gone.
Still running rough, smooth up till about 1200 RPM then it gets really rough and won't go any higher than 1300-1500 RPM, took some shots of the reader while it was running.
I'm going to start tracking down all the vacuum lines tomorrow since the vacuum dropped off dramatically, really at a loss here.

 

Its fuel, spark or air/vacuum.

Vacuum is a possibility and worth checking out. Fuel trims can help you confirm a vacuum leak because its unmetered air and will show as a rich (+) fuel command in fuel trims at idle. It would also be good info to see what fuel trims are doing when it starts running crappy. That can help determine if its fuel delivery related. Look at my article in my signature,

Have you performed the fuel pressure tests in the sticky?

Can you read CMP retard?

Do you have a misfire counter by cylinder so you can watch that while its rev limited? That can help determine if its spark related. See my misfire article.


George

 

Thank you George, gives me a few more things to look at tomorrow

 

Here are the fuel trims, I'll be honest, I don't know how to interpret them though.

 

You can read the fuel trim article in my signature to get started, We need LTFT1, STFT1, LTFT2 and STFT2 for the complete picture and we only have both values for bank 1, the drivers side bank. I do not know what the F/T A x B parameters are. You always add LT and ST together so your bank net numbers are -0.8 at idle and -1.6 at 1300 rpm. This means that the computer is reducing fuel delivery very slightly at idle to compensate for a perceived rich condition at the O2 sensors and at 1300 the same compensation is slightly higher. These are not big numbers and tell me that an engine completely rev limited at 1300 rpm probably has an ignition problem and probably not a fuel delivery problem. Now this is a first order conclusion because faulty sensors for instance could be in play but for now lets keep it simple.

So now what?

Get your bank 2 fuel trims just to check but lets assume similar numbers.

Look at the 6 misfire counters at idle and 1300

Mist around all the ignition components in the dark with a water sprayer and look for arcing.

Pull a plug wire and look for 1" of strong blue spark with a smooth cadence at idle and watch what it does at 1300 rpm.

Look for carbon traces under the distr cap

Whats your CMP retard?

Fuel pressure would not hurt but your fuel trims suggest ignition.

Checking for vacuum leaks wont hurt but this is probably not significant enough to keep an engine from exceeding 1300 rpm's


George