Chevy 700r4 transmission knock

[Stuball]

Ok, got a Chevy Astrovan with a 305 sbc and 700r4 transmission

Havent driven the van much since aquiring it (about 2500 miles).

Ive noticed that when first started, and the idle is a little rough, that there is a very distinct metallic knocking coming from the autobox

I though it was the engine, but put my hand on the transmisison pan and can feel it. Seems like a fairly heft knock.

Once engine is smooth and eveyrthing is warmed up it goes away.

Any thoughts chaps?

Stu

[kiwicar]

Hi Stu
I am on very new territory to me here, but have been trying to get my head around auto transmissions, as Eliot has not turned up on this one yet and I think he is your best bet, I will have a go. First surgestion is to check line pressure as this seems to be esential to all the funtioning of the box, from the reading I have done you don't look at anything until you know line pressure is Ok!
Next is a question, is the knock present all the time, or only in park, drive reverse or neutral, you don't specify and as a result I am assuming it is always the case byt worth a check.
If the knock is present all the time including in park (and line pressure is good) then as the park pawl locks everything stationary in the actual gearbox I can only assume this is a torque converter issue, or the pump, does it occour at engine speed? are all the bolts on the torque converter done up tight?
Sorry this as I say is new territory to me so I can only surgest places to look and hopfully Eliot will be along soon
Best regards
Mike

[Stuball]

Had another look today.

Line Pressure - cant check this at the mo, but will get a gauge ASAP.

The knocking is present in all gears, but ONLY when the engine runs too slow/a little bit rough. Once you pick the revs up it goes away, so hard to say if it changes in relation to engine speed.

Torque converter is nice and tight.

It still operates through all the gears as it should, and drives very smoothly. So im a bit baffled!

Will try and get a vid of it if i can.

Stu

[kiwicar]

Hi Stu
This is really getting beyond any knowladge I have, however as it is common to all gears and happen as you describe I would take a guess as to input shaft side of the box, and could be worn (out) input shaft bearing/ thrust race, but that is a guess based on it being related to engine revs in they way you describe. as the box is funtioning otherwise then I don't think it is bands or clutches. I also think you need someone who knows alot more than me!
Best regards
Mike

[DaveEFI]

Dunno this box, but on some you can get (a) valve chatter if the line pressure is low - or there is a leak somewhere. May not effect the operation of the box, though.

[Stuball]

Cheers for the replies guys.

Need to get on and check the line pressure in this box ey!!

Stu

[Eliot]

Try to get a sound clip if you can.
I trust the fluid level is correct and the fluid is a redish colour and doesnt smell burnt.

[Stuball]

Will attempt a clip tomorrow eliot.

And yeah fluid is nice and red with no burning smell and is correct level

Stu

[mgbv8]

Cracked flex plate maybe??

Had that on my old Galaxy. Similar symptoms !

[Stuball]

Well i never did get round to doing a video, but i think i found the cause.

Went and rechecked the flexplate - torque converter bolts, and although they
appeared tight with a spanner on them, I happened to find that under
one of the bolts, the flexplate had about 1mm, movement front-back. Doing this fast/hard enough it sounded exactly like the knocking noise. Now tightened the offending bolt, will have to wait until i have the wiring done to test it though

fingers crossed

and cheers for all the help guys

Stuball