New technical note: Refilling the GM400 XJS gearbox

In many V12 HE cars (all of which up to 1993 are fitted with the GM400 box) a common issue we’ve encountered, and one that can worry its owners, is for large clouds of white smoke to emanate from the back of the vehicle, particularly on a hot run and after a service.

This is usually a result of the gearbox being overfilled, with the excess being blown out of the breather, which is invisibly placed on top of the box. The oil drips down, hits the hot exhausts and produces an impressive cloud of smoke. Equally, it is easy to under-fill, as the procedure is not widely understood, and dipstick readings can be misleading when oil accumulates in the bottom of the dip tube.

In order to counter such gearbox issues, here is our tried and tested formula:

• Get your engine and transmission thoroughly hot – at least a 20-minute drive is necessary. When stationary, and with your foot on the brake, keep your engine running and move the gear lever through all positions twice, leaving the gearbox in park.

• Remove the dipstick and wipe off (the initial readings are not important). Repeat this at least five times until you have a clear indication of oil levels, equal on both sides of the stick. The bottom of the dipstick tube starts full of oil, even though the actual level in the sump may be lower. Repeatedly withdrawing and wiping the stick empties this ‘reservoir’ and then you get an accurate reading.

• If the level is too high on the side of the stick marked ‘HOT’, then remove oil by means of a vacuum pump or syringe (easily obtainable online) inserted into the filler tube. We suggest taking out half a litre at a time. If you have taken more than a few minutes over this before moving the gear lever, repeat the cycling through the gears process. This ensures that both the torque converter and hydraulic passages and chambers are kept full of oil.

• If the level is too low, top up with automatic transmission fluid (ATF) – half a litre at a time. You will need to repeat the dipping and wiping process, as while filling, the dipstick tube will fill up again, giving an inaccurately high reading. Do not overfill – it can damage the gearbox, and will result in excess oil being blown out of the breather pipe on top of the gearbox.

Speak to the experts

If you’d like expert advice on how to ensure the smooth running of your classic car’s engine, get in touch with KWE Cars. Call us on 01635 30030, like us on Facebook or follow us on Twitter.