RPM fluctuations while cruising
 

Hi

Recently bought a 2004 S80 D5 automatic with 250k km on it. When cruising between 55 to 85 km/h, it will repeatedly disengage and engage the lock-up clutch every five seconds or so, even though I keep the accelerator pedal fixed. This is not so noticeable when driving on flat roads but in uphills it is kind of annoying because the RPM varies by more than 500 rpm or so. It happens in both 4th and 5th gear.

However, when cruise control is activated this behavior is immediately absent.

It seems to be the same issue as discussed in this forum: https://forums.swedespeed.com/showth...RPM-fluctuates

Any idea for a new volvo owner?

I see I have managed to put this into the wrong forum, maybe an admin can move the thread?

The first suggestion most will make is to change the oil. At that mileage the fluid will be so degraded.

The transmission oil was replaced a year ago according to the service history. It was performed by a Volvo workshop. Previous owner said this was done because the transmission was starting shift hard in the low gears. For me the shifting feels OK now.

I don't know if the fluid change was thoroughly done, so I will do a few dumps and fills myself soon.

if it was a transmission problem, why it would vanish when cruise control is active? Could the reason be external to the transmission?

You are probably driving at the threshold speed , what happens if you go a little faster ? It is important to have the latest software too , both engine and gearbox , when was this last carried out ?

I have found out that it happens below 1700 rpm.

Variations in speed does not matter, it will be the same until I reach 85 km/h. Then the engine speed will be greater than 1700 rpm and the lock-up clutch is continuously engaged.

The workshop said there was no new software for the car, and therefore assumed it had the latest versions. They also said the transmission was flushed (11 liters of oil was used)

are you sure it is not actually dropping down a gear and then changing up a gear again ?

Yes. The increase in rpm is proportional to the load, where almost no increase is seen during low load. Under higher load, the torque converter slip might be so high that a lower gear in lock-up would give a lower rpm. It doesn't change gear, this I have verified by manually switching the gear selector to 4, which will make it shift.

I did the first measured fuel up today, and the fuel consumption from the trip computer has a 25 % error, which I believe is out of normal accuracy range. Could this be a connection with the gearbox?

Our two XC70 models, one D5, one D4, does something similar. But at lower revs. You have to be somewhere down below 1300 r/min or so (depending on load), when they sometimes do a smaller, hydraulic downshift by just releasing the lock-up. Either completely or at least somewhat (limited slip).

This is to avoid vibrations at certain load levels. Maybe your older model (ours are 2012 and 2016, with the six speed gear box) does this at a higher engine speed?