Unlikely source of knock over bumps

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Phil Bate
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Unlikely source of knock over bumps

Post by Phil Bate »

I thought I'd share this as it has taken a long time to isolate. The symptoms were an occasional knock, heard in the cabin and felt through the body of the car. I originally suspected something on the drivetrain as I thought I could feel it through the gear stick, however I was in fact picking it up through my arm resting on the console - I also eventually felt it through the floor when manoeuvring the car with flip flops :lol:

The knock would occur with chassis movement, over irregular bumps or mounting an uneven surface/speed bump at the front end causing a diagonal load on the suspension - exactly the sort of result you would expect from a failed drop link, and sometimes infuriatingly hard to reproduce.


In my case, it turned out to be the battery tray mounting bracket contacting the PCM housing. I noticed it when jiggling the positive battery terminal to check for tightness - if I rocked the battery in a diagonal motion the tray bracket momentarily dipped at the back left corner and then knocked on the underside of the plastic PCM housing on the way back up:





You can see the housing has a foot that rests on the bracket, here:


20210824_173240765_iOS (Large).jpg


To confirm, I levered the battery tray down from the PCM housing and slipped a piece of high density foam between the two like so:


20210824_173604676_iOS (Large).jpg

I need to investigate why the bracket is able to flex like this, but the foam has solved the problem for now.
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Unlikely source of knock over bumps

Post by meaningking »

I have a similar issue with a knocking sound over bumps etc. I took out my battery to see if I have the same situation with the bracket.

My tray is making a knocking sound. However it appears the bolt that secures it head is snapped off. This is the cause of the sound.






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Last edited by warpc0il on Thu Aug 26, 2021 3:31 pm, edited 4 times in total.
Reason: Fixed Youtube link
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Re: Unlikely source of knock over bumps

Post by meaningking »

With tray removed. I will drill it out and replace the bolt
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Last edited by meaningking on Thu Aug 26, 2021 7:36 am, edited 1 time in total.
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Re: Unlikely source of knock over bumps

Post by warpc0il »

I'd try cleaning the surface and using some 3M double-sided tape...
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Re: Unlikely source of knock over bumps

Post by ChrisHolmes »

There is an old motif trade story about a new Ford car that had a mysterious knock and extensive investigations by ever increasing highly qualifies engineers failed to find the source. Then an apprentice suggested they shut him in the boot and took the car out for a drive. When they opened the boot he had his hand on a panel and said “It’s coming from here” “Rubbish” the engineers “That is an empty box section”
So they decided eventually to cut open the box section and inside found a nut suspended on the end of a piece of welding rod plus a small envelope. The note inside the envelope read “Congratulations, you found me”
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Re: Unlikely source of knock over bumps

Post by meaningking »

That’s a good one. If I find any letter in my car I won’t be laughing I tell ya.


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Unlikely source of knock over bumps

Post by meaningking »

For now I decided to put a self tapping screw in it. The broken piece was not budging at all.

I also noticed the nut at the front left was missing, so secured that also.

Have yet to drive the car to see if it made any difference to the noise over bumps. Stopped it rattling by hand anyway as you can see in the video.
IMG_3310.jpg

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Last edited by warpc0il on Thu Aug 26, 2021 3:30 pm, edited 1 time in total.
Reason: Fixed Youtube link
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