Just a Thread That may save someone money after a battery disconnection (computer chip cars)

Ok starter motor went on my daughters 2000 mitsubishi magna wagon.
Guy came out replaced starter motor.
He disconnected battery during the process.
After car is fixed
Car then kept stalling at low revs
Very dangerous.
Time to take car to mechanic and part with hundreds or even thousands of dollars?
No
Simply rev the vehicle after starting at high revs for approx 2 to 3 minutes. (whilst car is a t standstill)
The computer chip is reprogrammed.
Car does not stall anymore .
Then you turn air/con on and car stalls.
Do the same rev car for two to three minutes with air con on ie reving stops car stalling when at a standstill.
Car computer is reprogrammed.
Car fixed.
Found solution on internet Australian magna forum.
The point is computer chips do not necessarily have to cost you a fortune when they fail.
This solution works.
How much would your local mechanic have charged?

reply to post by Dr Expired


Dear Dr Expired,

S&F for one of the most useful threads I have yet to see on ATS. Thanks.

also, this symptom can be a sign of a bad stepper idle controller

but usually you wont have done anything.

so if you didnt mess with battery etc, and you have stalls at low revs, get your stepper unit checked.

reply to post by Dr Expired


This makes sense to a degree but it may not work on every car,it depends on the age of your car and management system you have.

Stored in the ECUs memory will be particular maps which have settings for fueling and ignition advance/timing for the full rev range of your engine.These settings will have been set at the factory and dependant on things like engine load,coolant temperature etc but as a car gets older and the engine wears these settings aren't optimal anymore.

The management system has an adaptive learning capability which overtime learns from the sensor inputs of your engine and can slowly modify the the settings to suit your current parameters.These settings are stored in the RAM memory as opposed to the ROM.

In a lot of systems when you disconnect the battery for a certain length of time,some cars it's 10 minutes others an hour,the RAM memory is wiped and the ECU goes back to using the original settings for fueling and ignition advance/timing which may no longer suit your engine's present condition.

That's why revving or even a certain amount of normal running will allow the ECU to 'relearn' the optimal settings for the engines present condition.

reply to post by fastbob72

Thanks for the input

What you say makes perfect sense ..and as you can appreciate the car was stalling when driven normally first with air con off then with air con on.
The at rest high revs on the accelerator cured the car of the stalling.
Some unscrupulous mechanics could have charged an unsuspecting person a lot of money to replace the computer air con compressor whatever most people espescially young women have no idea about troubleshooting mechanical breakdowns.
The starter motor installer certainly didn't mention it.
Yes it may not work on all cars with computer management nut it will work on a fair percentage.

reply to post by fastbob72

Thanks for the input

What you say makes perfect sense ..and as you can appreciate the car was stalling when driven normally first with air con off then with air con on.
The at rest high revs on the accelerator cured the car of the stalling.
Some unscrupulous mechanics could have charged an unsuspecting person a lot of money to replace the computer air con compressor whatever most people espescially young women have no idea about troubleshooting mechanical breakdowns.
The starter motor installer certainly didn't mention it.
Yes it may not work on all cars with computer management nut it will work on a fair percentage.

reply to post by Dr Expired


Nice one I'm glad I was able to help you make sense of it somewhat.

Yeah the ECU would have required some revving of the engine to store enough actual output from the various sensors to calculate the right idle settings to keep the engine ticking over.

See the management system operates in two distinct 'modes'.Open loop and Closed loop.

Now under normal driving conditions when the engines at operating temperature it being managed in the Closed loop which means the ECU is basically determing air/fuel ratio and ignition advance (fueling and timing) from the feedback it's getting from the oxygen sensor/lambda sensor and engine load (from the MAF or MAP sensor).

By measuring the oxygen in the exhaust system the ECU can sense how lean or rich the engines running and modify the fuel/air mix accordingly so the engines running under a kind of 'feedback' or closed loop condition

That's ok under normal steady or mild part throttle conditions but no good when the engines cold or under full throttle or heavy load conditions when the the engine requires a much richer fuel/air mix.

So until your engines up to operating temp or when you start accelerating harder it goes into Open loop and ignores the oxygen sensor and uses the raw output of the coolant temp,throttle pos sensor etc then calculates from the factory data maps that have been modified over time by the 'learnt' data in it's RAM.

That's why when you disconnect the battery,wipe the RAM data it stalls and idles rough because its cold and in Open Loop and relies on that learnt data.

When you rev the engine and it gets enough real time data from the sensors it can start filling in the gaps left by the data it had lost.

Also when you turn on the aircon or use the power steering that initial mechanical and electrical power draw will soak up engine power and the ECU will increase the revs if it's idleing incase the drop in engine speed makes the engine falter.

Try switching it on or turning the steering wheel when the engines ticking over.The engine speed should increase by 150 or 250 revs then drop back to normal.Thats the ECU speeding the engine so it doesnt miss or falter from the power draw.

You're absolutely right,many a garage or mechanic may charge an unsuspecting victim a whack for next to nothing.

When I worked at a Ford dealers I seen mechanics remove components,clean them with a power washer,refit them and charge for a new part + labour !!!