Grinding first

[skelly]

So I read around what I could find about people having problems grinding first gear on 1G MTX while the car is at a stop and I just wanted to get a bit more in-depth information about it. The remainder of this post is informative with postulations in the last paragraph.

Synopsis: With the car sitting at a dead stop, putting the clutch to the floor, 9 out of 10 attempts to quickly shift from neutral into first gear will cause a skull-rattling grind. Slowly shifting however, by gently encouraging it to slip into gear will result in a quiet engagement after 1 to 2 seconds of patience.

Mechanics: Grinding is the result of the gear selector hub bashing into the side of a gear at a rate of rotation which is different from that of the gear itself. The "pins" of the hub which are meant to engage into receiving "holes" on the side of the gear are what make the grinding sound, not the teeth of the gears themselves as those who are unfamiliar with transmission internals might believe. Under normal conditions, a pair of "synchro" plates are designed to use friction between the faces of the gear and the hub to match their rates of rotation to one another before the hub finally engages the gear. When these synchros fail to match the rotational rates before the hub is forced into the gear is when we hear the signature "C--r-r-r-unch!" sound.

Causes: There are two cases in which the selection of a gear may cause a grinding sound as a result of mis-matched RPM's:

1) The vehicle is in motion and the transaxle is being shifted from neutral or another gear to a new gear setting.

2) the vehicle is stopped and the transaxle is being shifted from neutreal or another gear to a new gear setting.

In the first case, because the output shaft if permanently mechanically linked to the motion of the wheels, the vehicle being in motion causes the output shaft to be in motion. This means that the synchro must match the RPM of the input shaft, clutch disc and at least one third of the gears in the transmission to the speed of the output shaft before the gear can be engaged without a grind. The greater the difference in RPM and the more wear on the synchros, the more time it takes for a complete match-up. A shift which forces the engagement before the match-up completes will result in a grinding sound.

In the second case, the output shaft will be stationary. If the car was in neutral with the clutch out, then the engine will have been turning the input shaft at the rate of the engine's RPM. If however the car was in neutral and the clutch is in, then the input shaft should be stationary. The case that deviates from this rule is where the clutch is not allowing the disc to completely disengage even with the clutch all the way in. The resultant light friction which occurs is enough to encourage the input shaft to begin rotating and depending on the severity of the friction and the duration it is allowed to do this, it may bring the input shaft up to the full engine RPM as if the clutch were not in at all. Any attempt to forcibly select a gear in this state will result in a grinding sound because the input shaft is not matched to the output shaft RPM.

The problem with my car is definitely stemming form this second case, however I am wondering if there are any other considerations that are particular to 1G AWD DSM's. My clutch is not completely disengaging whether by adjustment or by design. I am wondering if an adjustment to the clutch pedal/actuator could be expected to make the clutch disengage better, or if clutches for these cars tend to do this no matter what. I am anticipating that an adjustment should correct the problem since it'd be quite an oversight if they are like this by design. If adjustment doesn't fix it, should I assume that the clutch itself is at the end of its life and abnormal wear is resulting in loss of function? Is this problem also symptomatic of near-dead synchros? I have read about a third party who builds up transmission internals and saw some comments about doubled up synchros - is this intended to address a premature wearing problem from the factory designed parts? This transmission only has about 100K miles on it, but I get the impression that one of the previous owners may have beaten the crap out of it.

Any additional input would be appreciated!

 

[oldman]

I am wondering if there are any other considerations that are particular to 1G AWD DSM's.

If you have already tried everything else, it's probably the clutch pedal assembly.

 

[92awddsm]

here

So I read around what I could find about people having problems grinding first gear on 1G MTX while the car is at a stop and I just wanted to get a bit more in-depth information about it. The remainder of this post is informative with postulations in the last paragraph.

Synopsis: With the car sitting at a dead stop, putting the clutch to the floor, 9 out of 10 attempts to quickly shift from neutral into first gear will cause a skull-rattling grind. Slowly shifting however, by gently encouraging it to slip into gear will result in a quiet engagement after 1 to 2 seconds of patience.

Mechanics: Grinding is the result of the gear selector hub bashing into the side of a gear at a rate of rotation which is different from that of the gear itself. The "pins" of the hub which are meant to engage into receiving "holes" on the side of the gear are what make the grinding sound, not the teeth of the gears themselves as those who are unfamiliar with transmission internals might believe. Under normal conditions, a pair of "synchro" plates are designed to use friction between the faces of the gear and the hub to match their rates of rotation to one another before the hub finally engages the gear. When these synchros fail to match the rotational rates before the hub is forced into the gear is when we hear the signature "C--r-r-r-unch!" sound.

Causes: There are two cases in which the selection of a gear may cause a grinding sound as a result of mis-matched RPM's:

1) The vehicle is in motion and the transaxle is being shifted from neutral or another gear to a new gear setting.

2) the vehicle is stopped and the transaxle is being shifted from neutreal or another gear to a new gear setting.

In the first case, because the output shaft if permanently mechanically linked to the motion of the wheels, the vehicle being in motion causes the output shaft to be in motion. This means that the synchro must match the RPM of the input shaft, clutch disc and at least one third of the gears in the transmission to the speed of the output shaft before the gear can be engaged without a grind. The greater the difference in RPM and the more wear on the synchros, the more time it takes for a complete match-up. A shift which forces the engagement before the match-up completes will result in a grinding sound.

In the second case, the output shaft will be stationary. If the car was in neutral with the clutch out, then the engine will have been turning the input shaft at the rate of the engine's RPM. If however the car was in neutral and the clutch is in, then the input shaft should be stationary. The case that deviates from this rule is where the clutch is not allowing the disc to completely disengage even with the clutch all the way in. The resultant light friction which occurs is enough to encourage the input shaft to begin rotating and depending on the severity of the friction and the duration it is allowed to do this, it may bring the input shaft up to the full engine RPM as if the clutch were not in at all. Any attempt to forcibly select a gear in this state will result in a grinding sound because the input shaft is not matched to the output shaft RPM.

The problem with my car is definitely stemming form this second case, however I am wondering if there are any other considerations that are particular to 1G AWD DSM's. My clutch is not completely disengaging whether by adjustment or by design. I am wondering if an adjustment to the clutch pedal/actuator could be expected to make the clutch disengage better, or if clutches for these cars tend to do this no matter what. I am anticipating that an adjustment should correct the problem since it'd be quite an oversight if they are like this by design. If adjustment doesn't fix it, should I assume that the clutch itself is at the end of its life and abnormal wear is resulting in loss of function? Is this problem also symptomatic of near-dead synchros? I have read about a third party who builds up transmission internals and saw some comments about doubled up synchros - is this intended to address a premature wearing problem from the factory designed parts? This transmission only has about 100K miles on it, but I get the impression that one of the previous owners may have beaten the crap out of it.

Any additional input would be appreciated!

Thats way too much information. After I read it all, I forgot what the question was. If you think it might be adjustment, look here for adjustment procedure. If its only grinding in first and no other gear, It probably has nothing to do with clutch or adjustment at all. More than likely a syncro problem. I have always had the same problem so Ive just learned to shift into second then first. No grinds and its easier and cheaper than to pull the tranny and have it built. It has actually become more instinct than habit because I find myself shifting all manuals the same way.

And the double syncro is for fast, high rpm shifting. Not for a poor, fail prone design.