Thread: ZF Auto: RPM bounce in D when stopped

I believe RPM is derived from the alternator, single thin wire at the back? My guess is in D the system is under load, fluid pulse(?), and may cause fluctuations in alternator speed, in N little or no system load - maybe the gearbox needs a filter change and new oil. Recall somewhere it being said fluctuation may mean the alternator is on the way to being replaced. Sometimes the wire at the back is loose. As long as my batteries don't go flat and the headlights are bright I don't worry.

As said id check your Alternator for first port of call

I can feel/hear the engine rpm shift so it is not an issue with alternator signal.

I dont fully understand what the auto is doing in D when stopped but pulse from filter restriction sounds plausible.

I will try a filter/fluid change.

What about play in the driveline/flanges could that cause the auto to pick up and drop off?

More likely the idle stepper is changing the revs slightly, I can't see how the trans would change the idle speed and not have big problems under load. I would try cleaning the stepper, the MAF and possible adjust the base idle slightly higher.

Isn't yours a tdi??
If so it is likely that someone has screwed the main fuel up slightly. This can make them hunt slightly in drive. To fix it just raise the idle adjustment up about 150rpm.
Jc

Yep TDI. Will check to see if fuel screw still has its locking collar. Idle is under 700 when in neutral, slightly lower in D.

What year is it? If you fit a later alternator to the early pre 1996 model the tach will read high by at least 200rpm. Different pulley size used in early one and the tacho in the dash reads incorrectly when smaller pulley later alternator is used

1997. Should have smaller pulley, will check. Will also double check idle rpm, may have read it wrong as there appears to be an extra marking between 0 - 1000rpm which makes the division somewhat odd.