Refilling the GM400 gearbox
A common fault is for big clouds of white smoke to emanate from the back of the V12 HE cars (all of which were fitted with the GM400 box) on a hot run and after a service. This is usually because the box has been overfilled and the excess is being blown out of the breather invisibly placed on the 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 the box because the procedure is not widely understood, and the dipstick reading is highly misleading because at the bottom oil accumulates and gives a false high reading. Here is our tested and successful procedure:
- Get engine and transmission thoroughly hot – at least a 20 minute drive (not just idling), and when stationary, foot on brake, engine running, move the gear lever through all positions twice, leaving it in Park.
- Remove dipstick and wipe off. (The initial readings mean nothing). Repeat this at least 5 times until you have a clear indication of oil level 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 much lower; repeatedly withdrawing and wiping the stick empties this ‘reservoir‘ and then you get a true 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 gearlever, repeat the cycling through the gears. This ensures that the torque converter is full and the various 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. Check level each time and you will need to repeat the dipping and wiping because while filling, the dipstick tube will fill up again giving a falsely 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.
If you found this article helpful please hit the like button below to let us and others know.
