Can a rough idle cause an autobox to behave strangely?

[Alias no.9]

Hi Folks,

I'm new to the club and to merc ownership. I picked up a cheap 1996 C200T auto recently. It was idling a little rough with a slight misfire so I changed the plugs and it's running sweet now.
When it was idling rough and stopped in traffic, gearbox in D and foot on the brake, the gearbox seemed to be trying to engage gear intermittantly. This seems to have disappeared now that the the idle is sorted. I'm new to autos with the exception of rental cars, so the best I can work out is that the gearbox must have been responding to the slight fluctuation in the revs. Does this make sense?

Thanks for your help.

Alias

 

[280gee]

Were you able to see the rpm indicated at the time?

 

[Alias no.9]

The engine would have been idling at somewhere between 600 and 700 rpm based on the rev counter. As I said the idle was a little rough with a slight misfire, the needle wasn't doing any major fluctuation but you could hear the misfire. The gearbox seems to be behaving properly now but I'm just curious as to how the two issues might have been related.
It seemed to be trying to engage or disengage a gear but when I moved the gear selector from drive into neutral, it was fine. This combined with the fact that the problem disappeared with the misfire leads me to believe it was responding to the engine rather than an intrinsic gearbox problem. Does an autobox enter a neutral setting when stopped or does it just slip with first gear engaged?

 

[280gee]

Idle speed seems slightly low, the spec is 800 +/- 50 rpm if I remember correctly. Certainly if the idle speed was erratic I'd expect the drive to be taken up in a similar way, i.e. jerky.

Assuming that shift lever was in 'D' and the box was operating correctly, the the lowest gear would have been selected and a lumpy idle could produce an effect of a slight surge. does that describe it well?

Idle speed and regularity could be said to be more important with an auto box, in order to avoid problems like the one you describe.