Do you need to flush a Torque converter when replacing a slipping/failed auto trans?

Those more familiar with witchcraft than me. When do you need to flush a torque convertor? I'm pulling the auto trans in my Boxster as it's slipping and making a horrible noise.  Sometimes it will sit and rev as if it’s not in gear and or it will do that then suddenly slam engagement. Will I need to remove, flush or replace the torque convertor while at it?

The reason I ask is dropping the transaxle is non trivial on this car. As the TC is bolted to the flex plate, naturally it's going to stay with the engine. I think that's fine and it would make dis and re assembly easier. But as something has gone wrong with the trans does this mean I will have contaminated ATF in the TC that I need to flush/drain/change?

Note this is a ZF trans that doesn’t use regular ATF.  It’s supposedly filled for life and uses Porsche witches brew or Esso LT 71141 /  Pentosin ATF-1.  I’ve done the recommended drain and fill fluid change twice before, but that only replaces 1/3 of the fluid at one time.  I even checked with the local dealer and they don’t flush if they need to change, they just do the 1/3 drain and fill too.

IS my TC full of contaminated fluid that will wreak havoc with me new (used) trans?

Look at it this way, a TC is a giant centrifugal filter. Flushing a convertor just changes the fluid it does not remove much if any of the debris that may be trapped in it.

If there is debris in the pan there is a good chance of lots more in the convertor.

If you have any doubts or if the trans has considerable debris, REPLACE the convertor. Times 3 if it a newer lock up type.

Adrian_Thompson said:

The reason I ask is dropping the transaxle is non trivial on this car. As the TC is bolted to the flex plate, naturally it's going to stay with the engine. I think that's fine and it would make dis and re assembly easier.

NOOOO! (to the bolded part above)

unless there's something drastically different with this trans versus every other automatic i've ever heard of, the torque converter is installed to the transmission and spun by hand to engage the many (= more than 1) engagements that the TC must engage to the transmission, then the trans-plus-converter is mated to the engine, then the TC is rotated to align bolt holes with flexplate, then TC is bolted to flexplate, then rotate and bolt and rotate and bolt until TC is fully secured to flexplate.

While not in a P-car, the last time I replaced a failing trans with a rebuilt one, I took the approach Bentwrench recommended. 

I replaced the torque converter to avoid circulating any contamination through the fresh rebuild.  I also flushed the trans cooler to remove any contamination that might be introduced from that as well. 

Change the torque convertor, it's full of trash that will end up in the new transmission.

Excellent thx all.  So I need to buy a used box with TC included.  Checking the ebay listing for the one I have my eye on, it is included.

And make sure the converter is engaged in the pump properly.  Measure how far out from the engine block the flex plate is, and make sure the converter is that far plus a quarter inch in the trans.  If the converter touched the flex plate before you have the trans seated around the bellhousing, you will destroy the pump.

What sort of cooler do they use?  It will be full of junk, too, which you won't be able to get out without a magic pulsing cleaner.  If there is a lot of trash circulating, you might cause expensive trouble if you don't replace the cooler, which might translate into a radiator.

Or bypassing that and installing an external cooler.

No. Just replace the TC since the Trans is out anyway.

Change of plan.  I'm going to drop the trans and TC off and get them rebuilt at a local specialist shop instead.

Replace the TC for sure.  Flush all the lines and flush them again.  Then replace the cooler.  Then flush the whole mess again.  

The cooler should be mounted to the trans itself in this case. Check with the builder and see what he says to do.

Agreed.  Always swap out for a reman converter.  A trans parts supplier may do it in-house, ship it out for a reman, or just do an off-the-shelf reman and send your core back.  It gets cut apart, cleaned, fixed, new bearings, etc and re-welded/balanced.  Good as new.