Hello,

I'm begining work on a rough-idle problem and I'd like to disconnect the AAM while the engine's idling (to see if it helps), but I'm worried about stressing the ECU. I know the manuals don't advise this, does anyone know for certain that this will not hurt an LH2.2 system (240, 186)?

Sure enough, the hot-valve flap was frozen to hot (fixed now via re-route).

Thanks,

TomM

Well, its probably not your AMM, but unplugging it will probably not ruin the ECU, and even if it does, it probably ain't going to break the bank witth LH 2.2 ECUs worth about $20 since failures are so uncommon. This is not high finance or rocket science.

544 has little resale, market driven, try Ebay.

Do not disconnect/connect while running. You will most likely toast the computer or something/everything connected to it. Not worth the risk.
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-------Robert, '93 940t, '90 240 wagon, '84 240 diesel (she's sick) , '80 245 diesel, '86 740 GLE turbo diesel, '82 Mercedes 300SD for sale

With all the advice otherwise, why would you want to risk disconnecting it while running? Why not just disconnect before starting the engine to test?

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David Armstrong - '86 240(350k km?), '93 940T(270k km), '89 240(parts source for others) near Toronto

Well, some of the advice implied disconnecting it while running. My main reason was that it'd be easier to see a differnece, rough-idle, disconnect, smoother.

The advice (from manuals) is to not diconnect it at all, but perform other tests. I'm a little concerned about disconnecting it and then starting ("cleaning" current and all), but less so.

Thanks for the reply.

TomM

The advice that implies disconnecting it while running usually also stresses NOT reconnecting it without shutting the car off first. In my limited experience it's harmless to unplug while running but do not plug it back in while running - that's still no guarantee you won't hurt anything but the odds are definitely in your favor.
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Chris, Dartmouth NS Canada 70 M-B 280SE, 83 245DL, 84 244 turbo, 90 780 turbo, 92 VW Golf, 90 740 Rex/Regina

Hi Chris,

I've looked at those circuits with that advice, to connect only with the ignition off, in mind. The ECU is generally pretty well protected, but there is one resistor between chassis ground and AMM reference ground return - a 0.47 ohm 1/2 watt if memory serves - that I can imagine might be stressed if somehow the power pin doesn't release before the AMM power ground pin does. Seems unlikely, but I've replaced two of those resistors in the same LH2.0 equivalent spot.

Even with the danger unlikely I still go through the tedium of turning off the ignition before disconnecting either AMM or ECU, every time. It is too easy. The effort doesn't compare with halting a server with 2K users to avoid hot-swapping a scsi drive, not that the interface really compares, apples to apples.
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Art Benstein near Baltimore