A short while back I posted about a “mysterious” fast idle in our ‘89 wagon. Seems after going freeway speeds, then in town the wagon gets a fast idle like at 1500rpms. Then if shut off, turn on again, idle is fine at 800rpms.

(I had a tach put in). Same thing happens if accelerating fast in town say to 45 then slow down, idle is fast until I switch off then on again..

Reason Im posting now its been “narrowed down” to the speedometer, something to do with the fact the speedometer has been replaced with one from a newer 240-like a '92-'93 and some sensor that reads something at or by the driveshaft that has to do with (the speed) sending a signal to tell another “thing-sensor” how much gas at any given point to go into the engine . (Already replaced the idle sensor thing with a new Bosch one.)

A suggestion was that I replace the speedo with a 'true' ‘89 speedo. Seems some reading taken (by a 'professional Volvo mechanic') with an ohm meter at the speedo (and a volt meter) showed “faulty readings“. Haven’t done that (replaced the speedo) yet, but am seeking more insight along the lines of what I just wrote.

Im not sure if Im even making sense to anyone.--? :o/ Thanks.