Idle too High...

[Julesprivate]

I have a minor issue with my '96 400.

The idle speed seems to "stick" at ~1400 for a few seconds before dropping to 900, and actually sometimes it doesn't drop at all.

I know the usual culprit is the stepper motor (especially as this does "feel" like something sticking) so I will pop that off ASAP and clean it (and I'll do the TB/Butterfly at the same time).

I guess I'll set the base idle speed whilst I am there and see if between them that resolves the issue.

My reason for posting is this - my speedo has started to play up a little too, it doesn't display any speed until around 10mph then suddenly springs to life. I know it's likely to be the speed sensor on the diff (so I will clean and adjust that too) BUT - because BOTH of these things started at the same time I am a bit concerned that the speed signal convertor in the dash might be at fault, mainly because if that is sending the wave output to the stepper incorrectly then the stepper is just doing its job and holding the idle high for the overrun..

Just out of interest does anyone know the correct name for that component and/or the part number?

Actually, the more I think about it the more it's likely to be an incorrect output from that unit, the stepper thinks the car is still moving and adjusts the idle accordingly.

I know I can just disconnect the stepper (engine warm, pull electrical the plug off) but I'd rather not do that as it helps in cold start situations (and mine's bad enough at that already LOL).

Any thought s on what else it could be? I know it's a bit of a minefield (idling issues) as there are a few things that impact it but I assume just cleaning the stepper motor and the butterfly is a good start as is setting the base idle.

I'll check the operation of the stepper motor when cleaning it too but I am starting to think it is that box in the dash and I'd love to find out how much it might be to replace should that prove the case

Thanks

Jules