Sevice engine soon,downshifts to second.

[93winnvectra]

Have a 1993 Winn Vectra with a 454 P37 chassis. Get a random service engine light, then shifts into 2nd gear. I have tried using manual shift(dropping to 1st to see it it will override it.) it does not drop to first. Will not shift normally until the service engine light goes out. It actually stalled yesterday on the way through houston on I-10 after the light went on. Not fun trying to stop a class A with no power steering,brakes! Have checked for diagnostic error codes and only get the normal "12" ecm code. I am thinking it must be a loose wire somewhere? Not sure where to start looking though. Any help would be greatly appreciated. thanks in advance for any help.

Rich

 

[Steve CDN]

What engine/transmission does that chasssis have?

In a Cummins C8.3 with an Allison 6 speed transmission, we have seen this behaviour resulting from a defective oil pressure sensor.

 

[93winnvectra]

As the 1st post stated it is a 454 (tonawanda)chevy engine with the 4L80-E electronic transmission w/overdrive. (4 speed) I was messing with it today and actually made it give the service engine light.  With the parking brake manually set.(It has a automatic parking brake option) While applying the brake pedal and releasing the parking brake manually, I got the service engine light and it began to idle roughly. Not sure why ,but was able to repeat the same situation several times in a row. Was reading the manual today, could it be the "powertrian control module"? thanks in advance. ???

Rich

 

[Gary RV_Wizard]

You engine is going into "limp" mode, where the ECM changes to a fixed power profile and transmission in second gear to try to guarantee that the vehicle will still run and move enough to get you to a safe place.  Does it show a code of 12 while the Service Engine light is on or is that afterwards?  There should be a stored code (different than the current run code) that says why the ECM decided to go to "limp home" mode. A good coe scanner should be able to retrieve the sored code for you - that's where I would start.

"Limp" mode is used when the ECM decides that its sensor input is unrelaible or that the engine is not responding to its control commands. A short or open in the sensor circuits that feed the ECM could cause this.  I don' t know why the auto-park brake mechanism trigger sit, but perhaps tries toi  automatically shuts down the engine when it applies? Since it seems to be related somehow, I'd start there.  Find a P3x (usually just called a P30) chassis manual for assitance in diagnosing the problem. Sometimes they are for sale on ebay at reasonable prices.

 

[93winnvectra]

The service engine soon light stays on when it shifts to 2nd and stays on without flashing until for some reason it resets itself after awhile and then shifts the trans normally. I got the "12" code only after jumping the A + B jumpers pins together on the ALDL plug. No other code is displayed. It is a OBD I system and only has 4 pins on the ALDL terminal. The A,B,G,M pins. I did look under the dash around the emergency brake assembly and found that a cotter pin was cutting into a wiring assembly. :'( Maybe this is the problem? This is like chasing a ghost. And Im 1000 miles from home.

Rich