I recently had to disconnect the battery to remove the alternator from the car.

I remember vaguely reading that the ECU "adapts" based on (what?) and that disconnecting the battery "clears" that memory. I am curious what "it" is that is learned by the ECU.

I am not particulary concerned about losing the "learned" info since my driving style is relatively constant and if whatever it was that the ECU gleaned from my driving in the past it will surely learn again.

As a corrollary, how long does the battery have to be disconnected for this memory to get cleared? I'd image some sort of battery or perhaps even capacitive energy source to retain this info for brief periods (like a battery swap).

TIA

 

I dont have much to add to above except a personal experience.....I have a light fly wheel and RS clutch, the computer catches the revs as they drop and does not allow the car to stall.....if I disconnect the battery (for any length of time) and then drive the car and allow the revs to drop normally the car will stall.....this happens for about the first 5-10 minutes of driving the car and I have learned to catch the revs myself during this time......after that everything is back to normal and the car NEVER stalls.....so I assume the ECU when disconnected reverts back to std clutch, but after a few sessions learns that it has something different

 

There's a lot of "adaptive" software in the car. It will focus on environmental factors, how aggresive you are on applying/releasing the throttle, what RPM levels you typically run at what speeds, your max speed driven in the car, fuel quality, any error messages created in the system, and your number and type of over-revs.

 

Alex:

The ECU reads the range of values from the input sensors (Barometric Pressure, O2 sensor, etc.) and sets "mid-points" for the fuel and ignition trims. I believe that for the OBD1 cars, the adaptation occurs over a fixed number of cold starts, 2 I beleive...it's been a while since I read this.

For the OBD2 cars, some of the adaptation comes from the ECU completing the drive cycle off cold start, once the short tests have completed successfully. As you have a MY95, OBD1 car, the drive cycle is not part of the gig.

FWIW, this info is in the WSM. You are welcome to read mine the next time you are over...

 

are the little adapters (you can buy or make yourself) that let you plug a 9V battery into the cig lighter effective in retaining the learned data during battery removal?

 

When I disconnect the battery, my car hasnt forgot anything. It still reaches speeds of...............and can.................while...............as Im going down the open road doing while...........................and thats about all I got to say about that.

 

Hey Doug.

FWIW, I have used the memory "holders" when changing batteries. They are a lot easier than re-entering radio codes, station memories and worrying about readiness codes.

With regards to the car driving, "95 C4 993" has the right idea

 

Great to know. I was not confident enough to hook up my trickle-charger to the cigarette lighter because I did not know how the harness is connected. In other words I did not want to risk energizing any of the wires going to the alernator that were dangling around with the alternator removed.

Thanks for the pointer to the WSM, Jim.

/alex

 

Alex:

Please note...

Using the cig lighter to back feed the electrical system WILL produce hot circuits around the car. You need to be aware of the voltages present when handling ANY of the electrical components. If in doubt, disconnect the battery and be safe !

FWIW, the adaptation of the ODB1 cars is so painless, it really is the radio code / presets that is the worst impact !

$0.02...

 

I would not connect a charging device anywhere once the battery was disconnected - they are for charging into a storage device ( battery) - not to replace one. On most cars, a simple 9v battery connected to a cig lighter plug & plugged in will suffice to keep the codes intact when the main battery is disconnected .... but get the +ve and -ve correct

 

I was speaking with Colin Belton of 9M about my stalling issue. One of the things he suggested was to reset the base parameters of the ECU using a Porsche System Tester. So not all ECU learning goes out the window on a battery disconnect.

 

Can the ECU 'forget' - I've had a couple of unexplained stalls at lights recently which subsequently seemed to remedy themselves for a few weeks in between?? Thanks, Doug

 

Hi Doug:

If you are experiencing some stalling with your '96, you have some other issues at work here.

Failing oxy-sensors, dirty/failed Idle Stabilizer, vacuum leaks anywhere in the intake system, all can trigger such symptoms. Further, any corrosion/oxidation in electrical connectors can do the same things.

Sounds like some detailed detective work may be required.

 

I have had several dead batteries recently. 1st time I replaced the 5 year old battery...second time (yesterday), the battery went nearly dead, car wouldn't start. It had not been driven in 2 months

Open hood, press in hood latch so the light doesn't stay on, hook up trickle charger...5 hours later, battery fully charged. The ECU is happy...me too. Just back from a 2 hour get re-acquainted ride; what a great car!

I am not so certain on the cigarette lighter device. I bought mine from Porsche...but I am not that convinced it works. Perhaps a trickle charger is best for long term storage...don't really know.

 

For the the MY94-95 ECU (M2.10) the adaptation values cover: short term O2 fuel trim, knock regulation (ignition timing), throttle potentiometer idle position and ISV center position (CO regulation).