Change in engine revs and corresponding speed

[jaydbass]

OK guys, thinking caps on. There may be a simple explanation for this (fingers crossed). On a run to Edinburgh yesterday I decided to switch on the cruise control - only to find it wasn't working. However, i noticed 2 other concurrent factors. 70 mph has been 3000 revs for as long as i've owned the car; now it's just above 2500. In fact 80mph is now 3000 rpm?!? Generally at higher speeds the revs seem to have dropped by 500 or thereabouts concurrent with the cruise control failure. The other thing i noticed is the rear window washer (hasn't worked for ages) has started working again. Since its the end button on the opposite stalk, i was wondering if i have an electrical gremlin? What about the revs? Don't think its the rev counter as car certainly seems correspondingly quieter. Hmmm...
Any ideas?

[Peter Milnes]

Is your car manual or auto? If auto the box may have corrected a fault condition. The other faults seem to be coincidental. The ZF22HP4 box has a "lock-up" facility which results in around a 500 rpm drop in engine revs above 56-58 mph. Try a flush, oil change and filter screen cleanup of the gearbox and give it a little time to settle.

All the best, Peter.

[Volvo-in-my-blood]

Originally Posted by jaydbass

However, i noticed 2 other concurrent factors. 70 mph has been 3000 revs for as long as i've owned the car; now it's just above 2500. In fact 80mph is now 3000 rpm?!? Generally at higher speeds the revs seem to have dropped by 500 or thereabouts concurrent with the cruise control failure.

Sounds like a blessing in disguise, it has to be the lockup of the torque converter (assuming you do have an auto). Unless you have gremlins which installed a higher ratio when you haven't been looking! ;-)

[jaydbass]

Thanks guys, however, the big beastie is a 5-speed manual. Any further thoughts would be appreciated.

[S60D5NI]

If it's a true drop in revs, on a pure manual (i.e. no electric switched overdrive or similar) surely can only be due a change in gearing!

Therefore someone has changed your gearbox or diff. or has fitted much larger wheels / tyres (like off a tractor) LOL

3000 RPM @ 70 MPH implies a final ratio of 23.3 MPH / 1000 RPM which would seem low for a 5 speed manual diesel.

2500 RPM @ 70 MPH implies a final ration of 28 MPH / 1000 RPM which would be more like what you expect. As a guide, 6th on my D5 S60 is around 32 MPH / 1000 RPM.

Rev counter is electrically driven, so would stongly suspect it's a fault here (or the fault has corrected itself and it's now reading correctly).

[S60D5NI]

Did a quick check on the way home this evening.

For S60 D5 with 6 speed box, 5th equates to 29 MPH / 1000 RPM (2000 RPM = 58 MPH or 70 MPH equates to just over 2400 RPM) and 6th equates to 35 MPH / 1000 RPM (70 MPH = 2000 RPM).

Obviously these are approximate values (there will be inaccuracies in the spedo / rev counter) but should be reasonably close.

Therefore more convinced than ever that your motor should be revving around 2,500 RPM at 70 MPH in 5th (which works out at just over 2850 RPM @ 80 MPH). 'Original' value of 3000 RPM for 70 MPH in 5th must surely be erroneous. Yes / No??

[GraemeS]

I would suspect some chafing to the wiring in or around the steering column, if not a failure of one of the switch units itself. Earth leakage or additional voltage could account for the variation to the tacho readings and may account for the temporary failure of the rear wiper switch.

[jor]

On an L reg 2.4 dTic with an M46 gearbox, 3,000 revs gives 73 mph in overdrive with a reduction ratio of 0.79:1. The reduction ratio of the M90 gearbox is 0.81:1, so either the rev counter or the speedometer is not correct.
The only simple electronic thing of which I am capable is to remove the instrument panel and clean the connectors (which has not fixed my on/off temp gauge unfortunately).


jor

[jaydbass]

Thanks for all the info guys. Fixed the cruise control myself. Here goes with the technical stuff (ahem!) - the throttle cable connects to a unit on top of the engine or thereabouts which turns. therees a cable leading away to another component which has a rubber cup connected to it. a hose then runs from this to another unit at the bottom of the engine bay. It was this hose which had come off. If anyone out there could translate what i actually did (so i know!!!) i would appreciate it.