7 replies to this topic

[OctaneNation]

'91 Miata. OEM pressure plate. ACT puck disc. Both new ~10 hours ago (~18 months).

 

Last race I noticed grinding when downshifting from 4th to 3rd. And occasional problems finding second. I assumed the trans was on its way out but then remembered the clutch felt a bit funny.

 

I decided to take a better look at things and noticed the pedal was about an inch low (~160mm height as measured per FSM, should be 175-185mm). A helper saw the slave move whenever the pedal moved ('no' air in the system). Pressing the pedal by hand, it just felt mushy. No detectable free play.

 

I can shift between 1st and reverse with the pedal down and there's no crunching, which indicates to me the clutch isn't dragging, so maybe the trans does in fact need changing.

 

Finally, while under the dash trying to figure out the short pedal and lack a free play, I pulled up on the pedal. Height is back. Free play is back. Feels normal. etc. I pressed on the clutch many times and it always came back to the top and freeplay remained. I didn't have a chance to road test it though.

 

What could cause this? Slave cyl? Master cyl? Bad pressure plate? Weak clutch pedal spring? etc? Maybe the trans does in fact need to be changed but I'd like to eliminate any clutch issues first.

 

Any BTDT?

 

Cheers,

Matt

[FTodaro]

Matt you have identified the parts you just have to start replacing them. Sounds like its either the slave or master, start whit the slave, but they are not that expensive to do both. Just because the slave is moving does not mean its moving enough. If neither the slave or master do the trick then you will need to pull the trans to inspect the clutch assembly.

[callumhay]

Hi Matt. If I remember correctly, the clutch slave works by return of a small amount of fluid into the master reservoir. If it does not happen, it will cause the pedal to not work properly. Adjusting pedal height and Freeplay per the factory manual is the way to get the fluid to flow properly. If you do a google search there are some videos showing what the movement at the slave should look like and how to do it. Adjusting the height and Freeplay definitely is a pia with a roll cage in the way (meaning u are on your head under the dash) and to do it right you need to go look at the slave push rod too....and go back and forth from under the car to the pedals. Changing the slave and master is easy (as you probably know) and may be the best way to start. In my own car, the backing nut fell off the adjuster for the clutch and gave me shifting issues. Also at the track I have seen guys chase soft pedal issues by replacing the master and slave and still have the same problem. Definitely the factory manual has a procedure for setting height and Freeplay and there are videos of how people put that in to practice. It's worth checking for sure before doing anything major. I'm sure there are others on here who know the specifics and accuracy of what I'm saying and they may want to chime in.
Cal

[Michael Novak]

PUT both in...  Master and Slave...   We have had two cars fail out of the blue with same kind of symptoms. We had been thinking transmission and it turn out to be clutch... 

[B(Kuch)Kucera45]

I had that the same thing going on with my car. I put a slave cylinder in it and it kept doing the same thing. The car seems to be fine when it was cold but during a race it eventually kept grinding going into gears. What was happening is the fluid was heating up in the master and getting past the cups not allowing the slave cylinder to move the full length. Replace the master and the problem was solved.

PS the master cylinder is a piece of cake to replace. You don't have to take anything apart on the inside of the car just too 12 millimeter bolts and one line from underneath the hood.

[chris haldeman]

advanced Autosports sell a very nice kit with clutch master cylinder,slave cylinder,and replacement braided steel line for around 100$

[davew]

You are describing 2 separate issues, that may or may not be related.

 

The pedal height issue sounds like a mechanical problem. Pivot bushing, wires stuck around the pedal or something like that.

 

The crunch going into 3rd gear sounds like a cracked syncro. If it is only the one gear, just drive the car slowly and shift like a grandmother. If it crunches you have broken a syncro and the box needs to come out and apart.

 

The 2nd gear issue could be a slider whose teeth are worn. Again; out and apart.

 

Clutch slaves and masters are cheap on these cars. I have started putting new ones in every build just because. I usually replace them as a set. What Kuch said above is fairly common on race cars with 20+ year old clutch parts

[OctaneNation]

A buddy checked my car after sitting for a couple days and said the free play is gone. Sounds like hydraulics...so I'll start there and then see how the trans behaves. Here's the issue getting into 2nd: https://youtu.be/NG01IKecK6Y?t=16m It cost me a race win The crunchy 4-3 shift isn't picked up on the video but I can feel it. Cheers everyone.