tranny wont shift into 3rd under hard throttle

[shawn]

I have a 98 grand prix gtp and when i give it full throttle it knda pulses and shakes a lil at 50 then at 85 it starts really jerking like its hitting a rev limiter and wont shift at 95 mph i went to 100 and let off and it shifted i took it to a shop and it threw a code (p0748 pressure control solenoid electrical) is what that means i guess he reset the code and drove ok till i hit 100 again. Is this elecrical or can it be fixed by replacing the solenoid or does it need rebuilt? I really dont have the money for a rebuild so if replacing the solenoid is a possible fix id like to know... thx the car has 160000 miles on it but shifts fine when jus driving normal

 

[Trannyman95]

Hi Shawn, I did recieve your PM and email but find it best to reply in the threads so that others can join in and or obtain info as well. The code you have is not very common to see and could be as simple as wiring or could be internal in the trans on the internal wiring harness or pressure control solenoid being bad inside. Below is the code description and some info about what happens and potential causes....

DTC P0748 Pressure Control Solenoid Circuit Electrical

Circuit Description

The Pressure Control Solenoid Valve (PC Sol. Valve) regulates actuator feed fluid passing through the solenoid into torque signal pressure. The PCM uses a pulse width modulated signal in order to control the torque signal pressure. The PCM compares various inputs in order to determine the appropriate pressure for a given load. The PCM varies the current to the PC Sol. Valve from 0.1-1.1 amp. An internal current monitor within the PCM provides feedback in order to determine actual PC Sol. Valve current draw.

If the PCM detects a commanded current draw that differs from the actual current draw by more than a calibrated value, then DTC P0748 sets. DTC P0748 is a Type D DTC.

Conditions for Setting the DTC

The Pressure Control Solenoid (PC Sol.) Valve is enabled.

The system voltage is:

Greater than 11 volts at a low temperature -40°C (-40°F).

Greater than 13 volts at a high temperature 151°C (304°F).

The PCM commands the PC Sol. valve ON and the circuit voltage remains high (B+).

The PCM commands the PC Sol. valve OFF and the circuit voltage remains low (0 volts).

Action Taken When the DTC Sets

The PCM does not illuminate the Malfunction Indicator Lamp (MIL).

The PCM commands maximum line pressure.

The PCM disables shift adapts.

Conditions for Clearing the DTC

A scan tool can clear the DTC from the PCM history. The PCM clears the DTC from the PCM history if the vehicle completes 40 warm-up cycles without a failure reported.

The PCM cancels the DTC default actions when the fault no longer exists and the ignition is OFF long enough in order to power down the PCM.

Diagnostic Aids

Inspect the wiring for poor electrical connections at the PCM, the transmission 20-way connector, the PC Sol. Valve connector and all other circuit connecting points for the following conditions:

A bent terminal

A backed out terminal

A damaged terminal

Poor terminal tension

A chafed wire

A broken wire inside the insulation

Moisture intrusion

Corrosion

When diagnosing for an intermittent short or open, massage the wiring harness while watching the test equipment for a change.

Extended cranking with a low battery could set DTC P0748.


The only way to know if the solenoid is really at fault is to put a trans pressure gauge on the trans and hook up a scanner go out and drive it to see if the pressure is where it should be as per commanded pcs, or in the tech manuals there are testing proceedures to use a scan tool to control the commanded Amps from 1 down to 0 and watch the pressure on a gauge. Being that your trans has 160K on it I would highly suspect it to need rebuilt even though it seems fine during normal driving. You were given a very fair price to replace the solenoid but it needs to be verified as being the problem before replacing it or you may find that it is not the problem. A bad pcs can definately cause soft shifts all around but will typically cause a code P1811 from long shifts and once either codes throw the pcm will command max line pressure as a default or fail safe to prevent the trans from burning up, so if the code is present and it is slipping and not shifting then there are likely other problems. Were there any other codes in the pcm history? A bad or dirty maf sensor will cause low line pressure and will also make the engine run lean at WOT and can cause it to surge or mis-and if your lucky no piston damage will occur! You definately should check over the engine end of it to rule it out and make sure you dont have an ignition or fuel problem fighting you.

 

[shawn]

no there was no other codes besides p0748 and it did not shift into 3rd at all till i let off the gas i noticed this happening after installing the 3.4 pulley and when we had the hp tuner on it, the cars engine at the time of the jerking and hesitation cut back 5 degrees of timing and had 5 degrees of kr and i noticed that at the time it wasnt reaching 6000 rpms past 95 at wot in 2nd and it would not go past 93 mph its like it hit a rev limiter but there is none on the settings at all

 

[shawn]

we were trying to adjust he tranny after this happened and with the key in the on position not running, the radio went all crazy can this mayb jus b a short somewhere causing this?

 

[shawn]

and i beleive i was gettin max pressure cause the shifts were firm then soft after the code was erased

 

[DanGTP]

Hmmmm. I think I remember mentioning that we may need to clean the MAF. Next time you come over here, we'll clean it and check the connections for the PCM and trans and see if it makes a difference.

 

[kdiethrich]

mine does that also. I had mine hooke to a computer and was told it is an ignition problem. hope that helps

 

[DanGTP]

We ended up figuring out the problem. I got it to stop hesitating if we lowered the shift points a few hundred RPM. Even with low timing and a raised rev limiter, it would hesitate when trying to power over the 6000 RPM mark. He had a set of headers put on a few weeks after this thread was started and all of the problems went away. I believe there was just too much restriction in his stock manifolds. Plus it didn't help any that his trans was just about shot at the time.