Bumpy road and Limp mode

[VSV8]

Hi all
Got a bit of a curly one here. I have a VS v8 Auto. I was driving home last night and drove over a railway line and which was proceeded by several bumps in the road. All of a sudden my check engine light came on and the car seemed to drop back to 3rd gear and be stuck in 3rd gear. Trying a manual shift I found I only had 2nd & 3rd gear, no 1st or 4th. Taking off at the lights the car took off in 2nd and then changed to 3rd but thats it, no other gear. It stayed this way for a while and after around 15kms and as I was pulling up at a set of lights the car all of a sudden went back to normal and the engine light turned off. It was fine for the remaining 15kms home.

Could this be a TPS or ECU or both problem? Certainly seems like an electrical issue rather than trans issue. Generally the trans runs perfect, it has a stage 2 kit in it and always is as tight as a nuns ****.

Cheers

[VSV8]

Oh and I should also mention that just of late the idle seems to be almost at stalling point at lights. Drops down to around 300 or 400rpm idling in gear at lights. This isnt always the case and is sometimes idling perfectly.

[BlackVXGTS]

Check out the description of limp-home mode below. Idle problem could be related to the TPS but in my experience you would also be getting harsh and high rev shifts especially the 1 -> 2 shift. Note that auto have a PCM (Powertrain Control Module) instead of an ECU/ECM.

What is "limp-home mode" ? This is a fault mode in which the transmission either loses electrical power or the PCM deactivates all of the electronics. The transmission will default to max line pressure. The driver will have Reverse and 3rd, with manual 2nd available by putting the gearshift in 2 or 1 (doesn't matter). 1st, 4th, and TCC lockup will not be available. It will not hurt the car to drive it like this for short periods, but use manual 2nd to get the car moving, and be aware that the trans will generate more heat than normal while operating in limp-home mode due to increased torque converter slippage during 2nd gear starts and no lockup in 3rd gear.

[VSV8]

That sounds exactly like what has happend, limp mode is kicking in.

Where is the PCM located? Is that a part of the ECU or is it a seperate component?

[hako]

First do a fault code check as any transmission related fault will leave a code - this should tell you what caused it to go into LHM. My guess would be one of the vacuum hoses to the throttle body - these can slip off easily in older cars and will cause the idle problem. The LHM may have been caused by something as simple as a loose connector to the transmission or the VSS (vehicle speed sensor)....but do the code check as it only takes 5 mins.
Checking error codes VR/VS (Revised)

[VSV8]

I did check for fault codes. It has logged nothing. The light was on for a good 20 mins during limp mode but no faults logged. I am thinking this is also a sign the ecu is stuffed. The plot thickens!!!

[crumpledoor#1]

Its a bit strange that your ECU would not log any fault codes. Im not sure what that means but it sounds like your ECU may have issues.

[BlackVXGTS]

Where is the PCM located? Is that a part of the ECU or is it a seperate component?

Maybe I didn't make myself clear. Manuals have ECUs - Electronic Control Modules (although Holden call them ECMs - Electronic Control Modules) to control the engine. Autos have a PCM - powertrain control module to control both the engine and transmission. It's in the same position as the ECU/ECM in a manual, inside the passenger side footwell.

[dinnzyvrv8]

if you check your codes it will tell 81 shift soloniod there is 2 of them change them both 20 dollars each. i have only just rebuilt my box and also clean your wiring loom with electrical cleaner spray and do get a oil filter and gasket set this will fix this problem dont forget to disconnect the batt. as it will still be in limp mode. while you are working on it.

[VSV8]

Ok, it's not stuck in limp mode and it's logged no error codes. The only code it shows is 12. I changed the TPS today, just in case (cheapest option). Car seems ok, running fine again but only more driving will tell. Just bizarre that it all happened from going over bumps. Where do you buy solenoids from for $20? I have not seen them that cheap anywhere. I will change them, plus the filter as well. Failing that I will change the ECM or PCM. All cheaper options than a full trans rebuild. I really don't think the trans is stuffed, it runs and had run fine up until the bumpy road.

[hako]

Maybe I didn't make myself clear. Manuals have ECUs - Electronic Control Modules (although Holden call them ECMs - Electronic Control Modules) to control the engine. Autos have a PCM - powertrain control module to control both the engine and transmission. It's in the same position as the ECU/ECM in a manual, inside the passenger side footwell.

With due respect, most if not all DIY workshop manuals - Haynes, Ellerys and the most popular Gregorys only mention the ECU and do not mention PCM, so whilst I'll agree you are probably correct, most people reading posts on this forum will understand ECU but not PCM. Please do not take offence by this post, I've found all your posts to be factual and informative.

[VSV8]

Seems the problem is fixed. Cleaned every plug I could find and disconnect. Also replaced the TPS and cleaned the throttle body. Also found a loose vacuum hose on the canister. Finally found the rocker breather hose disconnected. So all in all the car runs better than ever and no probs with the limp mode again. Fingers crossed it's fixed the issue, only time will tell. Still bizarre about the ecu not logging any error codes, so I will probably replace it just in case.