940 Wagon Drive Line Vibration - RPM Related

FreshAirInspector · Post by **FreshAirInspector** » 11 Oct 2018, 15:40

Hello,

Need your assistance!

I have a mild but noticeable vibration that starts around 2000 rpm, peaks at around 2250 and disappears at 2500. It does this in every gear (automatic) including 5th (lockup) although in that gear it appears to start around 1800 and end before 2500. Note the word appears. I also feel the vibration upon deceleration in 5th from ~80 kph (50 mph) and ~60 kph (40 mph).

Also, if I have a load in the car (i.e. 3 passengers, a bunch of patio stones) the vibration is more noticeable and over a broader RPM range. I was too busy to make notes.

Last but not least, I had the centre driveshaft support bearing replaced about 10K km (6K mi), ago. This was just after I purchased the car - the vibration was worse then.

Part of me leans in the direction of driveshaft vibration but the vibration would be speed related and not RPM. Having said that, the increased vibration with a load in the car would suggest playing with the height of the centre support bearing.

One other point. When I bought the car the exhaust system was new, with all joints welded except the flange on the catalytic converter joint. I don't know if the system is Volvo or aftermarket? It is properly hung on the rubber ring hangers. The mounting bracket between the exhaust header and the bell housing does not have the proper Volvo clamp but a metal strap connecting the pipe to the bracket. Probably not a sturdy as the OEM Volvo setup but it is relatively firm.

As you can tell, I'm going around in circles.

Thanks for reading and looking forward to your feedback.

lummert · Post by **lummert** » 11 Oct 2018, 18:45

The 2 sections of the driveshaft may have been installed out of phase throwing it out of balance.

FreshAirInspector · Post by **FreshAirInspector** » 13 Oct 2018, 17:27

Hello Lummert,

Many thanks for the quick reply.

I had considered this possibility but an out of balance driveshaft would result in a speed related vibration? Correct me if I'm wrong?

For the rest of the forum - I replaced the motor mounts, transmission mount, filters (air, fuel, oil), spark plugs and flushed the injectors when I bought the car so I eliminated these as possibilities.

lummert · Post by **lummert** » 13 Oct 2018, 18:36

Research second order driveshaft vibration.

admasters · Post by **admasters** » 01 Jan 2019, 19:59

You should check the u-joints while you're at it.

dj_v70 · Post by **dj_v70** » 02 Jan 2019, 06:14

Stomp on brake and put car in drive, rev engine up to offending rpms. Do you feel vibration? If so, it is not drive line, else inconclusive. Please report back.

Post by **jimmy57** » 02 Jan 2019, 12:36

agree. driveshaft/u joint is speed dependent and load dependent but wouldn't be repetitive at that RPM in multiple gears. Engine vibration transmitted through body gets worse with more load and stays in the high resonant frequency speed range for the engine.
Collapsed motor mounts on that car with hydraulic oil filled mounts is very common.

FreshAirInspector · Post by **FreshAirInspector** » 02 Jan 2019, 13:33

Hello All,
Appreciate the feedback. I haven't had a chance to run any definitive tests due to conflicting priorities. As soon as I have any feedback, I will post the results.

jimmy57 - I replaced the motor as well as the transmission mounts less then 10K km (6K mi), ago.