GM TH180

DaveEFI · Post by **DaveEFI** » Wed Jun 18, 2014 10:41 am

Perhaps a bit OT for this group, but hoping for some help from the experts.

Late SD1 autos used the TH180 box. According to the BL manual. there are several vacuum modulator valves, according to engine type. Rimmer no longer list any.

They are colour coded Orange, Grey, Yellow. Not sure if these are generic GM colours or BL ones.

Ages ago, mine failed, and I got a replacement from my Rover main dealer. It didn't work correctly - and had no colour identification. They were adamant it was the correct one. I eventually got a secondhand one correctly marked, and that restored the box to health. (The main dealer one delayed up shifts on a light throttle by a large margin and made it change down too easily)

Googling shows several US and OZ companies who do this valve for the TH180 - but none with the correct colour.

Several of the above list adjustable regs - but one lists three different adjustable ones, so obviously 'no one size fits all'. I contacted them, but their reply was they only sold them and couldn't offer tech advice.

Thing is this box when working correctly is about as good as a three speed gets. With the wrong modulator valve it turns into an old clunker.

Eliot · Post by **Eliot** » Wed Jun 18, 2014 12:29 pm

presumably when yours failed - it also exhibited the late and harder upshifts?

Essentially all it does is raise the line pressure when there is low/no vacuum. I would say get an adjustable one and suck it and see.

Bit of googling threw this up:
http://www.rostratransmission.com/pdf/r ... -guide.pdf
Lists three types along with the calibration details in pounds(last page has the key to the acronyms). The dual green adjustable one is middle of the road, so would start with that one.

Personally, I would probably buy all three of them - find out which one works the best and put the rest ebay.

DaveEFI · Post by **DaveEFI** » Wed Jun 18, 2014 2:09 pm

Thanks Elliot. The normal failure mode is a ruptured diaphragm - and this results in it not up shifting until it reaches the maximum speed for that gear. And if you leave the vacuum pipe connected to the engine, clouds of smoke from the exhaust as the ATF is sucked into the engine and burnt. Quite spectacular when you first experience it - you think it's a holed piston or whatever.

My EFI reads what I think to be a pretty poor vacuum at idle - according to MegaSquirt - so wonder if this gives a clue as the best one to go for?

But as you say it could be one would do - and they're much cheaper from the US.