Strange Vibration

[tim944]

OK,

My 944 86 NA died about a block away from home today. I pushed it back and replaced the reference sensors that I have been carrying around with me for the past year. I took it out for a hundred mile drive, and on the way back I noticed a strange kind of surging vibration. It almost sounds like you are driving on a really light rumble strip. It will vibrate foe about a second and then stop for the same amount of time. It goes away when the clutch is in.

Thanks in advance for any advise on where to start looking.

I am going to crawl under tomorrow and take a look. It's time for a belt tensioning any way.

[John_AZ]

Originally Posted by tim944

OK,

My 944 86 NA died about a block away from home today. I pushed it back and replaced the reference sensors that I have been carrying around with me for the past year. I took it out for a hundred mile drive, and on the way back I noticed a strange kind of surging vibration. It almost sounds like you are driving on a really light rumble strip. It will vibrate foe about a second and then stop for the same amount of time. It goes away when the clutch is in.

Thanks in advance for any advise on where to start looking.

I am going to crawl under tomorrow and take a look. It's time for a belt tensioning any way.

You are a brave man to replace sensors and take the car for a 100 mile drive.

At what RPM do you notice this temporary "surging vibration". If it is between 3000-3300 my guess would be the rubber clutch disc is breaking up.

While you are checking the belts, check the rollers, alternator, PS pump and bracket, AC bracket and the crank pully assembly.

The new sensors would not cause the "surging vibration". Recheck the sensor gap and the cable connection anyway when you do the belts.

You are fortunate to have had the sensors handy, were they OEM new, used, or one of the interchange numbers?

GL
John_AZ
1988 924S + 1987 924S

[tim944]

Not so much brave, but no choice. The sensors had started acting up about a year ago, so I picked up a set of used ones to keep with me in the car. I didn't think there could be a connection to this vibration.

I will double check the RPM range. Is this rubber center hard to replace? I know the clutch was replaced @ 125000 miles (It has 225000 now) is this part normally replaced with the clutch?

I saw another post that showed similar symptoms that ended up being a bad balancer belt. I think I better start there.

Thanks for the info.

[John_AZ]

Good point about the balance belt. Make sure you use the www.Clarks-Garage.com instructions for belts:

http://www.clarks-garage.com/shop-manual/eng-06.htm

If the BB has stretched, the balance belt pullys may be off 1/2 tooth and cause an intermitant vibration.

Your comment about the vibration not present when the clutch is pressed in, for me, eliminated the BB.

GL
John_AZ

[mattdavis11]

Is it under load when the rumble occurs? The coil wire might be shorting out. By that I mean coil to distributor.

[John_AZ]

Originally Posted by tim944

I will double check the RPM range. Is this rubber center hard to replace? I know the clutch was replaced @ 125000 miles (It has 225000 now) is this part normally replaced with the clutch?

1986 944 ===25 years driven-great!
225,000====about 10,000 per year miles driven
Clutch replaced at 125,000 miles==Guessing about 10-13 years ago
Rubber clutch discs still available (Performance Products closeout price clutch kit)
My crystal/magic 8 ball says that a rubber clutch disc may have been installed instead of the spring clutch disc.

When you are under the car, look in the bell housing inspection hole. You can measure the disc using Clarks procedure:
http://www.clarks-garage.com/shop-manual/clutch-03.htm

TIP: Instead of a ruler to measure, poke a long socket that fits tight between the marks shown and then measure the socket.

Try to see if you have a rubber clutch disc or a spring loaded disc. If it appears to be a rubber disc, look for small pieces of rubber-1/4 inch diameter or bigger.

It would be real nice to find you have a spring disc!

GL
John_AZ

[tim944]

Thanks again for the info. This gives me several things to check this morning.

As far the BB and being under load, with the clutch in the load would be gone, wouldn't this point back to the BB?

I am really curious about the rubber disk. Was this OEM or an aftermarket mod?

[mattdavis11]

Rubber is OEM. My clutch was changed in 1993, Sachs spring. Near 200K now, the original lasted 80K, but I was in high school then and knew where the keys were! Needless to say that's why it got a new clutch. When you let off the gas in gear, it bucked to a slow.

[tim944]

Yep, the BB has some nice smooth spots where there used to be ribs!

Time for new belts and that water pump I decided to wait on the last time.

[mattdavis11]

Don't crank the motor until they are changed. The BB can take out the timing belt if it snaps.