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Civilizedape

Car surges forward when starting in N

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I am finally getting my 73' 240z (L28 S/U's 5 speed) running after many many modifications and reassembly. The problem I have is that the car surges forward when cranking even when the transmission is in neutral. With the clutch pushed in it does not do this. The transmission was rebuilt by a shady backyard mechanic, me. Could it be clutch adjustment?

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check the oil level in the gear box. did you replace the brass balk rings?

Edited by Dave WM

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I'm leaning toward the trans, input shaft binding in the pilot bearing (bushing) in the flywheel. The fact that you can defeat the surge with the clutch would tend to rule out a clutch adjustment.

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just to clarify, car in N, sitting on level ground and you crank, it will start rolling?  so when it starts and the engine in running the car will move forward again in neutral? I don't like terms like "surge"

the 5 speed is a sychromesh transmission, so all the gears on the counter shaft and output shaft are always meshed. The output shaft is locked to ONE set of gears at a time by the hubs. there is always some fluid drag that will tend to turn the output shaft since all the gears are spinning on it. Now if you have some kind of binding between the output shaft and the gear needle bearings, or if the brass baulk rings are not properly fitted, there could be some drag that is excessive. turning the input shaft by hand will turn the output shaft just by the slight drag of the oil.

did you pay attention the correct orientation of the 3/4 sychro hub? I don't know what would happen of even if you could install it backwards, but it does have a correct direction.

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Thanks for the replies. I have not gotten the car running yet. If the car is in neutral and I try to start it w/o the clutch pressed it will try to roll forward. Never heard of the balk rings? Is that the brass?? rings that sync the gears? If so yes I replaced them. 

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3 hours ago, Civilizedape said:

Thanks for the replies. I have not gotten the car running yet. If the car is in neutral and I try to start it w/o the clutch pressed it will try to roll forward. Never heard of the balk rings? Is that the brass?? rings that sync the gears? If so yes I replaced them. 

tries to roll forward? or bucks forward like it's in gear? or sort of like an automatic transmission sort of creep forward?

 

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I let about a 1/4c of fluid drain out from the fill plug. Tries to roll just a bit and cranks slow. I am hoping once I get it to idle it will free something up.

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On 7/20/2019 at 1:50 PM, Mark Maras said:

I'm leaning toward the trans, input shaft binding in the pilot bearing (bushing) in the flywheel. The fact that you can defeat the surge with the clutch would tend to rule out a clutch adjustment.

What would you suggest? Pull trans and look for what? 

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Do the clutch slave cylinder adjustment and the correct distance from floor to pedal.

You can google "240z slave cylinder adjustment classiczcars.com" and find the info.

Good luck, don't want to drop the transmission if you don't have to.

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On 7/20/2019 at 2:14 PM, Dave WM said:

jNow if you have some kind of binding between the output shaft and the gear needle bearings, or if the brass baulk rings are not properly fitted, there could be some drag that is excessive. turning the input shaft by hand will turn the output shaft just by the slight drag of the oil.

If he's getting actual movement of the weight of the car he has some serious binding inside.  Sounds like the shady mechanic.  You can turn the input and output shafts by hand, easily, in neutral, with the transmission out and on the ground.  There's no way it should move the car at all, unless there's a serious bind-up.

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59 minutes ago, siteunseen said:

Do the clutch slave cylinder adjustment and the correct distance from floor to pedal.

You can google "240z slave cylinder adjustment classiczcars.com" and find the info.

Good luck, don't want to drop the transmission if you don't have to.

 I agree. Rule out all other possibilities before pulling the trans. The pilot bushing lies in the center of the flywheel. The trans. input shaft nose fits into the pilot bushing. If there isn't sufficient clearance between the two the bushing will grab the trans input shaft and turn it even when the clutch pedal is depressed. Lack of lubrication on a new tight bushing could cause it to grab.

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would that account for why even when in neutral he has problems? If it is the trans, I guess the take away is to always spin the input shaft while in N and see if you can hold the output shaft still. I assumed this would have been done, as a check out of the rebuild (shift thru all gears and confirm nothing it binding and gears are all working). Does the OP recall doing that?

Edited by Dave WM

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oh and one more thing to the OP, I always start my car with the clutch press in and hand brake on, why add the drag of the transmission (even in N) to the starter, and the brake is a safety thing. That being said I guess I will have to go out and start it with brake off and clutch engaged just to see if I can get the car to move while starting.

Edited by Dave WM

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Ok clutch pedal out, car in N, hand brake off, start and  no movement.

Next did you see my post about the 4/3 sychro hub?

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48 minutes ago, Dave WM said:

Ok clutch pedal out, car in N, hand brake off, start and  no movement.

Next did you see my post about the 4/3 sychro hub?

I did. This was my first time rebuilding a transmission and its within reason that I F'd something up. 

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58 minutes ago, Dave WM said:

would that account for why even when in neutral he has problems? If it is the trans, I guess the take away is to always spin the input shaft while in N and see if you can hold the output shaft still. I assumed this would have been done, as a check out of the rebuild (shift thru all gears and confirm nothing it binding and gears are all working). Does the OP recall doing that?

I was able to shift smoothly through all gears while it was going together. Not sure about the input vs out put turning test. 

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On 7/20/2019 at 12:22 PM, Civilizedape said:

The problem I have is that the car surges forward when cranking even when the transmission is in neutral.

With the clutch pushed in it does not do this. 

I think that other types of problems might be getting blended together in the comments.  With the clutch pedal pushed in the engine is disconnected from the input shaft.  So the clutch seems to be right, it disconnects the engine from the trans, like it should.

With the transmission in neutral the input shaft is disconnected from the output shaft.  since the car moves with the lever in the neutral position, apparently, then there's a problem with the transmission.

Here's another possibility - neutral isn't really neutral.  I assume that you are assuming that the transmission is in neutral because the lever is in the middle and/or not in one of the gear slots.  Maybe the transmission controls aren't doing what they should.  Are the shifter bushings tight?   Can you get the lever in to any position that frees things up?  Maybe the detent pins/balls aren't detenting.

Here's another - a broken or loose shifting fork.  It might not be releasing the coupling sleeve.  If this was the case you might find that certain gear positions actually lock everything up since you'd be in two gears at once.

Overall though it looks like you'll have to get in to the transmission to see.

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Isn't it possible to put the rear case on but not in the neutral position?  I believe that locks up the transmission some how. It's a foggy memory...

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Yes, you can get both shift rods locking up a gear, two separate gears, before installing the case.  That's actually very likely, looking back through the posts.  But I think that the gear shift lever would be locked up, since it has to move the rods.  But, if the shifter bushings are bad it might not feel like it.

Mr. Ape threw me with his comment about the shady mechanic.  Didn't realize it was him.

What happens, what Patcon is suggesting, is that when you're installing the rear case both shift rods get moved at the same time, resulting in the trans engaging two gears at the same time.  You could probably figure which ones by looking at the diagram.  What people do to avoid that is to put the trans in to one gear before installing the rear case.  Sounding like you installed the rear case with the trans in neutral and have that problem.  It's a fairly common problem, apparently, I've done it myself even after reading about it, but noticed it before I torqued everything. 

And....you still have to remove the transmission to fix it.

 

Edit realized that it's actually the front case that is put on last.  AKA "bellhousing".  That's when the shift rods get pushed backward.  I first saw it described in the "71C" swap pages, then did the exact thing when I put mine together.

It's on page 3 - 

http://zhome.com/ZCMnL/tech/240SX5spd/transmission.htm

"When assembling the transmission, I found that having it in 3rd or 4th gear keeps all the shift rods locked so you won't bump the rods out of position and end up with 2 gears engaged and a locked up transmission. If you are in neutral, any of the rods can move. If in 1st or 2nd, the reverse-5th can move and if in 5th or reverse, the 1st-2nd can move."

 

Edited by Zed Head

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The function of a balk ring is to "lightly" couple the input and output shaft. this lets the disconnected input shaft (clutch pedal down) match the speed of the output shaft for the gear that is "about" to be engaged by the shift fork. as the speed match up the gear dogs engage the hub, you get a smooth crunch free engagement. They are sort of like a small clutch between each gear change. My guess is something is wrong and a balk ring is not allowing free spinning on one of the gears while in neutral (all the balk rings should be fully disengaged in N). I don't know how this could be done during a reassembly, I do know the only place that was mentioned to be extra careful with respect to the 3rd/4th sychro hub. I am not sure if you could be put it the incorrect way and get the gear box together, but there was a warning in the FSM about direction.

Edited by Dave WM
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OP did you keep the sycro hubs on a dowel or make some other provision for keeping the Front pointing front? on mine I put a mark on the hub to make sure it went back correctly

see step 3 in the middle of the page

sychro.pdf

Edited by Dave WM

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On 7/20/2019 at 2:58 PM, Civilizedape said:

 If the car is in neutral and I try to start it w/o the clutch pressed it will try to roll forward. 

Lots of good ideas here but they all say the transmission will probably need to be removed to find out.  It's still not even clear that the gear shift lever moves.  The OP hasn't described moving the lever.  It might not even have a lever.

Trans rebuilt, installed, key turned to Start car jumps, clutch pushed in car doesn't.  That's all we really have.  The only conclusion is "transmission stuck".

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16 hours ago, Civilizedape said:

I was able to shift smoothly through all gears while it was going together. Not sure about the input vs out put turning test. 

This is not the same as after it's together.

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I edited Post #19.  It's the front case that causes the rods to move.

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