Replacing the Thermal Expansion Valve (TXV) on a LHD BX
As part of returning the AC system on a ’92 19D to working order, it was necessary to renew the compressor (it had a significant leak), the dryer and pressostat, and the engine-bay O-rings. The TXV was initially left alone, optimistically, but after re-gassing, when the compressor was switched on, nothing circulated round the system. This suggested that the TXV was obstructed, and this turned out to be the case.
There were warnings that replacing the valve was no fun on a LHD car, and these turned out be spot-on. The LHD footwell is nicely cluttered with pedals and the steering column, all in annoying proximity to the TXV.
This is how the valve replacement was tackled, with some tips at various stages . . .
Getting access to the valve, and removal
With the system emptied of any gas,
- Disconnect battery.
- Remove steering wheel.
- Remove lower half of the steering wheel shroud, unplugging dimmer, relay, etc. Tie the wiring aside.
- Remove the driver's seat.
- Provide some cushioning on the floor (and the ground outside), as you'll be crouching on it (through the open door) for a good while.
- Locate the black plastic casing (about postcard size) that covers the TXV on the LH side of the heater unit.
- Remove the lid on the casing (one release tab nearest to you). This allows sight of the valve.
Special tools needed?
- Bi-hex (home cut) and single-hex flare nut spanners
Sizes needed for the TXV were 16, 19 and 22mm. I resorted to cutting a slot in a couple of ordinary ring spanners to provide those bi-hex sizes.
- Unhitch the two hoses at the firewall grommet, without putting undue strain on the copper pipes that lead to the TXV and evaporator.
The TXV has 4 threaded ports…
The two at the top are the same thread size, and need the 19mm spanner to undo them.
The two on the underside are:
16mm (with two small copper pipes siamesed into the union on this car) – refrigerant feed to the TXV
22mm – refrigerant return to the compressor.
- It is necessary to unscrew the two lower unions FIRST, so that the valve remains in its upper position, allowing the two lower pipes (which go through the firewall) to be pulled out of the valve and pushed aside. When these lower ones are out, the top two 19mm unions can be undone.
Important that the stiff copper pipework is not bent by forcing it, which would make alignment and refitting very difficult. The TXV should not be allowed to twist while undoing the unions. Its c.25mm thickness can be gripped from the end nearest you with a pair of moles, either at the top or the bottom.
Spanner access
With the plastic valve cover off, the base part of the plastic casing was still in the way. This could be levered aside after removing the two visible cross-head screws. A small piece of casing broke away, but would be easily refixed with the screws and some silicone sealant for its groove.
Tight!
For the car here, I found all four of the union nuts extremely tight, and reluctant to undo. The top two benefitted from some PlusGas penetrating oil (overnight), but this is not so easy on the lower ones. I tried heating the valve body with a hot-air gun, but this didn't seem to help.
In the end, I gripped the valve body with moles, wedged with a wooden block against the steering column, and used a flare-nut spanner with another slotted into it as an extension. All of the nuts slackened with a loud 'snap', but were then were easy to unscrew.
- With the upper 19mm unions undone, the valve can be dropped down and removed.
Fitting the new valve
Valve O-rings
Some new TXVs come with new O-rings. This one didn't.
Had real difficulty in finding the correct sizes of O-ring for the valve, but they are:
int.dia. x thickness
7.65 x 1.78mm qty.1
10.82 x 1.78mm qty.2
14.00 x 1.78mm qty.1
Like the rest on the AC system, they need to be HNBR quality, and found that Amazon did a useful assortment box (of 270, 18 sizes) that contained all three:
https://www.amazon.co.uk/270Pcs-Assortm ... NrPXRydWU=
- Clean up the pipe ends and union nut threads.
The top two (19mm) need to screw in first, to keep the valve raised.
Keeping the new O-rings in the correct position (to prevent them being damaged) I found no fun at all. The seat in the valve body wasn't sufficient to fix them in place without risk of displacement, and so they needed to be attached to the pipe ends, next to their flanges. Fine, but the upper pair tended to fall off (gravity!), even when some grease was applied (PAG oil stood no chance). In the end, and reluctantly, I ended up solvent cleaning the pipe ends to dry, and applying three microscopic dots of superglue to the inside circumference of the O-ring, to hold it in place on the pipe end. That worked.
Applied some PAG oil to both the O-ring and the union thread, and both upper unions screwed into place, finger tight (to allow some wiggle of the valve body for the lower pipes).
Similar glue dots and oiling for the lower O-rings, and these screwed in – the smaller (16mm) one first, because the big one obscures it. Thread alignment was not easy, but went in on the umpteenth attempt. Oiling the threads is a must.
Wiggle-room for the lower unions comes from leaving the upper two slightly slack, and being able to move the two lower pipes around in the firewall neoprene grommet.
- Attach the two engine bay hoses, finger-tight only.
- When all six unions are in place, they can be tightened. Mole-clamp the TXV again, to prevent strain on the upper pipes, which are very much fixed into the evaporator.
With the plastic valve casing (and the lower steering shroud) still off – to be able to UV check for any leaks - I got the system pressure tested and refilled. Now working (and hopefully reliably), these items can now get reattached.
In conclusion
A fiddly and unpleasant job, mainly because of the very poor access space via a LHD footwell. Must have spent 90% of the time thinking how to do the next bit... Would be very much easier next time, heaven forbid, having worked out materials, tools and – importantly – the order in which the pipework must be detached.
Hopefully, the notes above will be of help, on this very specific job, not readily found written up in detail.
For its age, the BX system, on R134a, seems impressively powerful, tested as it was today on a first road run in 30degC Andalusian sunshine.
A big thanks to everyone who has helped with advice for this recommissioning over recent weeks.
