9

This question got me thinking about a possible test that anyone driving a manual transmission could perform to diagnose severe clutch plate slippage:

  • Engage handbrake, make sure there is nothing in front of the vehicle
  • Try to shift to the highest possible gear without depressing the clutch pedal
  • If the gearbox doesn't make grinding noises, this would indicate clutch slippage. Depending on the severity of the slippage, it could be that some gears would grind and some wouldn't.

Would this test work?

  • Why not use the clutch? If the clutch isn't sliding (and assuming your handbrake is good), when you release the clutch the car should stall. – anonymous2 Sep 16 '16 at 17:01
  • "If the gearbox doesn't make grinding noises, this would indicate clutch slippage." If it does, didn't you just damage it? Doesn't seem wise. – Lightness Races in Orbit Sep 18 '16 at 1:00
20

It could possibly work, but the better question is why would you want to test it this way? You'd be "grinding the gears" for no reason (even though it's the synchros you'd actually be grinding). This also would not provide a definitive "yes, my clutch is slipping" response. The reason for this is, even with a slipping clutch, in most cases the clutch would still have some grip to it, but not enough to provide full holding power. This means, up to the point where the friction disk breaks free from the pressure plate and flywheel, there is still some holding power being utilized. You would be forcing your synchros to try and cause the friction disk to break free, which would cause them an extraordinary amount of wear. The only way you'd be able to tell if the clutch was slipping would be if the synchros could hold everything until the clutch starts slipping, which really isn't going to happen. The amount of surface area the clutch has over the synchros is probably 1000:1 ... even with a slipping clutch, you won't overcome this disparity, so slippage most likely would never occur (in the vast majority of the cases*).

A much better way to test for clutch slippage is to:

  • Have the car moving at a lower speed, along the lines of 20mph (32kph)
  • Put the transmission in a higher gear than what you'd normally be driving in at this speed (probably 3rd or 4th)
  • Give the accelerator pedal a hard go, giving the engine much more gas than you'd normally give it
  • If the engine revs up without the car speeding up accordingly, the clutch is slipping
  • If the engine speed stays linear with the vehicle speed, your clutch is not slipping

Remember, in any given gear, the engine speed to vehicle speed will stay relative and linear. As the engine speeds up, so should the vehicle. If the engine is revving higher than it normally would or if the engine gains speed faster than it should, the clutch is probably slipping.

Also, the reason you want to check for slippage this way with the car moving is an attempt to keep from lugging the engine, which is bad for the entire drivetrain.

/* As an aside: I had replaced a clutch in a Ford Escape where all the friction material was completely gone from the disk. You could use this test in this case with no fear of damaging the synchros, but I think you'd not need to do the test because there was absolutely no vehicle movement with the clutch engaged anyway./

  • 20 mph is 32 kph – njzk2 Sep 16 '16 at 14:17
  • @njzk2 - Thanks. It was too early in the morning and my caffeine system had too much blood in it when I wrote this. :o) Corrected. – Pᴀᴜʟsᴛᴇʀ2 Sep 16 '16 at 14:26
9

I would not recommend it: Most likely something will experience excessive wear or damage.

There are some other advices on various "tests" around, mostly they test if the engine stalls while pushing in some gear and breaking at the same time.

I would advise the following test: Drive the vehicle on some long stretched inclining slope. Drive some high gear, but minimum speed. While accelerating (thereby producing maximum load on the clutch) press slightly the clutch until the clutch slips (rpm raise without speed increase). Now suddenly let loose the clutch pedal:

  • If you experience a sudden push (good traction) the clutch is still doing well.
  • If the clutch slips it is consumed.
  • If the clutch needs some time to produce traction the clutch pressure is not as strong as it should.

Please note that also this test puts high wear on the clutch.

5

It would make a horrible noise and probably wouldn't actually let you engage the gear due to the actions of the syncros.

A better test would be to engage a gear with the clutch depressed, handbrake engaged and brakes on. Release the clutch and the car should instantly stall.

  • I think the clutch would have to be slipping very badly to not instantly stall. Unless you're revving the engine pretty high, which seems kind of bad for the rest of the drivetrain. – MichaelS Sep 17 '16 at 9:12
4

Like @Pᴀᴜʟsᴛᴇʀ2 said, it might work. However, shifting in without depressing the clutch pedal is decidedly not good for the transmission. If done too much, your car could have trouble staying in gear. You could also mess up your shifter bushings. Of course, everything is reparable with enough time, skill, and money... but in this case, I would avoid the test mentioned for fear of an over-tax on the former and latter.

3

IIRC, the Car Talk recommendation for checking this scenario is as follows:

  1. Put the nose of the car up against a tree, telephone pole, wall, or other immovable object. Absent the presence of any of these, engage the handbrake and brake pedal.
  2. Push in the clutch, put the car in the highest gear
  3. Release clutch.

If the car instantly stalls, your clutch is fine. If the car keeps running, the clutch is slipping and needs to be replaced.

3

and if clutch is NOT slipping you can grind gears or break a clog tooth.

No, rev engine and pop clutch, if car sits there it is slipping, if car goes flying down the street, your good. use steep hill if you like, going up of course. A HEAVY trailer pull also would let you know. I was doing 60 in a 25 zone when I spotted cop with gun tagging cars, shifted to first and popped clutch, engine screamed like a mother, but cop spun around to me doing 25. So GOOD clutch.

protected by Community Feb 16 '17 at 11:45

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