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Discussion Starter #1
2012 Smart Fortwo 52,000 miles mainly highway travel.

Yesterday out of the blue a hard knocking started. I reversed onto some high ramps and laid under the rear end and can narrow knocking down to the bell housing. There is a loud knocking and light scraping sounds that seem to be coming from inside the bell housing.

If you give it just a touch of throttle while in neutral it quiets down and basically goes away. The minute you are back id idle it's knocking and scraping again. If you do the same action in gear, it goes away.

I can drive it just fine, it auto-shifts the same as it always has. I can manually shift it without issues.

Laying directly under the bell housing it really does sounds like it is coming from the clutch pack. What on earth would do that within the bell housing but present no issues shifting or driving? If it wasn't for the loud knocking you wouldn't know anything was wrong at all.

Could it be something else that just really sounds like it is coming from the bell housing? Motor mounts seemed solid, Muffler seemed solid. I don't recall seeing the exhaust hangers or checking them... are they close enough to the gearbox and bell housing to knock and scrape? What could be close enough to do this and make it seem as it comes from inside the housing?

I mean it drives like brand new and it's so clean underneath you would think its a new car. I have a full time job and a 2 yr old kiddo to take care of. I really cannot afford to have a mechanic tear into it and we don't have any service centers around here either. So if it really is inside the bell housing I'll have no choice but to do the work myself somehow.

Any ideas or tips are welcome I don't want to tear it apart if I don't have to.
 

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clutch knocking

As you stated, it really is a puzzler. I have had mine apart and the clutch and actuator lever are standard run-of-the mill manual clutch setups. My suggestion would be to check out the clutch actuator adjustment and readjust it and see if that changes anything before you go any further. If you have the facilities to pull the engine (basically some kind of hoist to get the rear axle 23 inches off the floor and a trolley jack and transmission jack, getting the bell housing separated from the transmission is just a matter of removing the mounting bolts and pulling the transmission and bell housing away from the engine. At that point you should be able to see what the problem is. There are two high mileage smarts in my family and I have done several engine and transmission swaps and while they're not a one-day job, a person with average mechanical car-savvy can do it. I did a write-up last time I did it, so if you have questions on the procedure, let me know.
 

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Hi, it;s me again from the answer earlier.....I re--read your post and you mentioned motor mounts. I doubt the upper motor mounts that the engine hangs from could be the problem, but do check the lower one...its the "dog bone" type that keeps the motor and transmission from rocking fore and aft.
 

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Discussion Starter #4 (Edited)
Thanks for the info.

The dog bone is the front motor mount right? It seems stable and does't appear the be knocking while running. Could it still knock and sound like it comes right from the bell housing? I mean the sound seemed to be right inside the housing.

Since you have been inside the bell housing before do you think the actuator could cause the knocking somehow? I thought the actuator arm just engaged neutral by pushing inward against the release right? I am not 100% familiar with how the clutch pack works. I have done plenty of belts, alternators, oil, battery, tires, wheel bearings, ball joints etc. Never been in the bell housing but I do have a rough idea of how the clutch pack works.

The knock is quite literally the definition of a knock. Totally rhythmic like someone just knocking on a metal desk or door. Knock.Knock.Knock except of course faster. Not quite as fast the idle rpm but there is about .5 to .8 seconds between knocks and they are cyclic.

So in neutral it knocks and in gear it knocks. If you just increase the rpms slightly in either state its gone. Even laying under the bell housing with slight increase in RPM it seems to be totally gone. I just can't picture in my head what inside the bell housing would knock until rpm increases then vanish. I will try pulling the actuator assembly out and see if it stops. I'll also try to take a peek through the opening see if anything stands out. I gotta get a set of torx sockets. I just don't own any yet. Wife has car until Monday so I am on hold lol.

Can the bell housing be opened in car or does it require pulling the whole gearbox out?
Can anyone think of anything outside the bell housing that can knock and resonate like it's inside the bell housing? Maybe i'm missing something to look at.

Thanks for the help.
 

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A bolt may have loosened on the clutch cover & is bouncing around inside the clutch housing. If you have access to an endoscope & can insert the camera into the clutch housing, you may be able to discover what is the cause of the knock you are hearing.
 

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Discussion Starter #6
I pulled the actuator out and inspected what I could see but saw nothing torn up. I do gotta ask is it normal for the release arm to be loose? I couldn't push it inwards towards the clutch but I could flap it back and fourth as in it was loose and had play in it. I thought the release arm would be firm and solid?

Trying to better explain just for clarification. If you take your finger and push in one the release arm just like the actuator does, I cannot budge it. If I hook my finger behind the release arm and pull it towards me away from the clutch and towards the opening it moves and is loose enough I can wiggle it back and fourth.
Hope that clearly explains what I mean by loose.

If the noise isn't from inside the clutch housing and is in the bottom of the engine. I don't know what it would be. It's low miles, never ran out of oil etc. The noise goes away with throttle so I don't think a rod is knocking. I know I simply cannot afford a new engine so if it turns out to be a motor this thing is done. We still owe 4 grand on it though. 4 grand left, 2 grand plus to replace motor... screwed!
 

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You might also get some engine oil samples, and transmission oil samples... if you see metal in either, that could be an indication of the problem. If you do not... best to send to a lab, for a diagnosis.
There have been several cars listed for sale as part outs... some with engine problems that may be able to be repaired. Just a thought.
 

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Discussion Starter #9
The clutch arm should not be loose.
Maybe that is the issue than. I will pull the actuator out again and take a video of what I am seeing for confirmation but I was able to push the release tab inwards until it stopped then pull it backwards towards me and essentially freely wiggle it and I could even make it knock against the clutch housing there too but not as loud as it sounds when running though. So if that clutch arm isn't supposed to be able to wiggle forwards and backwards ... maybe something in there is indeed broke and just happens to stop knocking when rpm increases (that part is what throws me that it stops knocking when rpm just barely goes up).

To be continued -- will post video soon.
 

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In the interim here is a video that shows what I am seeing from the actuator opening.

Notice how he can wiggle the arm back and fourth and knock it against the housing. Same thing on mine, I can take finger and easily push/pull the arm back and fourth. Are you saying that arm shouldn't be able to go back and fourth? Is it just supposed to be tight against the inside housing? I am confused on its operation. I thought the arm was supposed to move back and fourth to engage/disengage the clutch or is that correct and it's supposed to be a firm back and fourth action not loose and banging?

Due to budget it looks like I will be tearing this apart myself in my garage so I am really trying to get everything down before tearing into it.
 

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It is a pivot arm that contacts the fingers on the clutch pressure plate. It should not Co e in contact with the metal clutch or bell housing except where it attaches to the pivot.
 

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It is a pivot arm that contacts the fingers on the clutch pressure plate. It should not Co e in contact with the metal clutch or bell housing except where it attaches to the pivot.
Ok. Well I can definitely push/pull that actuator arm and knock it against the bell housing. So did you ever do a write up or can you recommend one for cracking the bell housing open? Can it be done in situ or do I need to lower the engine? I have 2 jack stands, 1 small floor jack, 1 large heavy duty jack. I am not even sure jack stands will give me enough height to drop the bell housing out from under the car.

So any and all tips/hints/advice here is welcome. I can start tearing it apart this week and spend a few hours each evening on it.
 

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Here's a video for a 450 but the clutch is very similar to the 451:


451 video shows everything up to removing the bell housing but that should be similar to other cars:
:)
 

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Discussion Starter #14
Ive got the back tore down and wheels loose. I just need to get it on stands and start tearing the rest apart.

Im doing all this on the ground. I see in photos from FQ that they put jack stands under the car but I cant tell exactly where.

It says in front of engine subframe. Do they mean the large round u shaped pipe that makes up the engine subframe itself? So I just put jack stands right under the subframe pipe or do they mean there are points behind the subframe front car side that can be used?

Its hard to tell. I haven't dropped the belly pan plastics and I didnt want to if not needed.

Can I get away with stands right on the engine subframe itself?
 

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Yeah the large round tube I have put jack stands on there many times to hold my back end off the ground. Just use the regular jack point in front the rear tire that will get you enough room to slide the stand under them
 

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Thanks, that's what I needed to know.

Now I can get this thing up in the air and tear the rest apart. It really didn't take long to tear the back gate and panels off and I dont expect it will take much time to tear down the rest. I just hope the issue is in the clutch and not bottom end of motor.
 

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Discussion Starter #17
Ive been taking my time tearing things down. Im at the step where i need to unbolt the gearbox. The guide says support the engine and gearbox with some jacks. Where do I jack the motor? There is nowhere on bottom of engine to jack at. Just oil pan/sump.

It even says to slightly jack the side of the motor to allow clearance to drop the gearbox. I just dont see anywhere safe to do it.

Any advice or guides please tell me. Ive been using the fq guide but the pics there dont match all the spots on mine as its a 451.
 

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Discussion Starter #18
Transmission is out and on the floor. All the guides I found were for the 450 model and the 451 transmission is a bit different on mounting bolts, brackets, jack points etc.


The dog bone and top right mount will help hold the motor. You can support the bottom of the motor right at the bellhousing split and dog bone mount. Good heavy metal there as a jack point. There is plenty of play and movement on the motor.

Then support the transmission on a transmission jack, diy platform jack, or what I used was a motorcycle platform jack.

Then you can play with the motor and transmission movement to crack it open. Pull as straight as possible towards driver side to pull it off the alignment studs. You do have to play a bit and angle the motor and tranny downwards to get it clear. If I had known how light it was, I would have just laid underneath and pulled it. I used the platform jack to help catch it since I wasn't sure of the weight

One little tip.

I for the life of me couldnt figure out how to remove the shifter linkage cable. Turns out that little sucker pops off a ball socket joint. The white plastic collar is spring loaded just push the collar up like an air chuck attachment and pull the whole assembly up and out of the u bracket it sits in.

In my case here the release bearing and arm are busted. Just like the guys video one page back. My bearing is broken off the release arm and the arm has a ton of play. Im fairly sure that is the cause of my issues.

Strange thing is, mine still shifted smooth and drove.
 
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