Worn diff or what?

[AndersE]

If i turn the propellershaft clock and anticlockwise and look at the driveshafts, only left shaft moves right away, there is a delay or slack in the right one (about a 1/20 revolution).


The symptoms is when i drive slow in traffic or over speedbumps, it is hard to keep steady pace without the car wanting to start "oscillating" (because my foot on the pedal). If i enginebrake there is a delay before catching on, same symptom when accellerating after a enginebrake. Difficult to keep steady foot on the acc-pedal.

[DR SCHNITZEL]

If the car is not under any load i wouldnt be suprised if only one wheel turned and the other stayed still.
 
Apart from that i dont really understand whats happening with your car. There will be slack in the diff but when it catches the cogs the car should run normal i e straight.

[David Nock (RIP)]

Agree, worn diff will show itself as whining noise or \'clonking\' noises from the rear. One shaft turning slightly and not the other is normal - the free play in the diff is taking itself up that\'s all.
Sounds more like engine control to me - I don\'t know anything about ECU\'s but you could check all mechanical parts in the accelerator system?

[jackaleks]

mine does this too at very low engine and road speeds, below 15mph and around 1000-1500rpm. it\'s only slight, but you can feel the car \'pulsing\' along.
 
my throttle cable is a bit loose, that\'s the only quick visible thing i can see that might be wrong. maybe there\'s enough play there to have an effect when the throttle is barely open?

[Jim_rx7]

Mine will also do it at low speeds and low revs.. it will want to jump along. Just change down a gear again to get the revs up and it\'s all fine.

It can be hard keeping very light throttle and still driving smoothly at very low revs.

My last Rx7 also done the same.

[DR SCHNITZEL]

i think this symptom is caused by the car rocking once as you move off and your foot following half a second later. if you relax your foot after accelerating in 1st you can really get it to jerk. its just pedal control. i think i know what you mean now. dont worry about it.

slip the clutch a little or be smoother on the loud pedal

[Jim_rx7]

think your right mate. It can do it very low revs in 2nd, but if you really keep the foot steady it will stop. Very light and very steady pedal action.

[jackaleks]

hmm, mine does do it even when i\'m just coasting with no throttle or clutch. it doesn\'t feel like the engine is about to stall as there\'s none of the usual feedback through the transmission...
 
i\'ll solve it by driving fast all the time!

[stacy]

When you say oscillating, this is presumably a front to rear lurch?

If you\'re saying you can feel mechanical drag of some description, have you checked the rear brakes aren\'t sticking and binding or maybe the handbrake? What happens if you park on a very gentle slope and let the car roll then stop using drag only - is anything binding?

If that\'s ok then as suggested, it\'s just the way the thing picks up I guess.

S.

[DR SCHNITZEL]

i\'ll solve it by driving fast all the time!

Amen                           :Viking

[AndersE]

It\'s nothing like what you guys describe. I KNOW there is something wrong, a car should not behave like this, it is impossible to drive properly.

If i drive on say 3rd or 4th gear and enginebrake, then accellerating hard the is a BIG KLONK sound when the diff catches up.

I had it on a lift today and a fellow rx7 (FC) owner looked at it, he said he never had seen a diff behave like mine. So much lag had he never seen.
When jiggling propellershaft there is about 10-20degree lag before any driveshaft move. The driveshafts also move in and out about 1-2 mm when jiggling propellershaft. I have NO abnormal noise from the diff.

I dont know how a diff works in an FC, but when we jiggles his diff there was no lag at all.

[David Nock (RIP)]

Do you have universal joints on the rear axle. (Mine is FB with enclosed half shafts). If so have you got a UJ falling apart? Otherwise if you\'re sure the diff\'s not right then side bearings inside the diff. maybe collapsed, but then how would it drive with no obvious noise?

[AndersE]

i don\'t know what you mean with "do you have universal joints"?

I have never touched an FD before i got this one.

I\'m taking a look at a fellow FD owners car before i say more.

[David Nock (RIP)]

OK, that\'s fine, older cars like mine have an axle at the back with shafts inside a big casing. The whole thing is attached to the car through link arms which can move up and down, and the axle goes up and down on the springs with the up and down of the wheels.
This is called a \'live axle\' and the FB (1st generation RX7) has this arrangement.

Then some makers started bolting the diff in it\'s casing to the chassis. This meant that the drive shafts were visible coming down to turn the wheels but obviously they have to be able to turn the wheels through joints which allow the wheels to bounce up and down as the car moves along. These joints were called \'universal joints\' and are similar to the joints which are located in the line of the propellor shaft and serve the same purpose.
I stress that I don\'t know what the FD has for it\'s rear drive but I would guess it\'s something like I\'ve said. That being so I\'m asking really \'has one of the UJ\'s seized up or fallen to bits inside\'. They used to do so on old Triumphs and it was like riding on pebbles when they went.

[AndersE]

ok i understand what you meant now. The FD has dual joints. One near the diff and one out near the wheel.

There is some wear in the inner ones but not much. I am tearing down the diff and driveshafts this week. Will inspect joints further then.