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9000: ZF HP 18 automatic: Draining &, refilling the oil & changing the filter

The ZF HP 18 automatic gearbox is a complex -and heavy- beast that does not lend itself to DIY tinkering.  Even so, most gearboxes will benefit from regular changes of fluid.  The service schedule specifies that the Dexron 2 fluid should be changed at intervals of 24,000 miles and if this recommendation has been followed, gearboxes will pass 200,000 miles before needing an overhaul.

Unfortunately, the recommendation to change the fluid is often overlooked. Sometimes this has happened because technicians or owners have been misled by the nice clean colour of the fluid on the gearbox dipstick... which is NEVER representative of the filthy fluid you will find inside the box. Even when the fluid has been changed, very often only half a job has been done because what is drained only represents the contents of the gearbox itself – not the converter or oil cooler. For this reason, dealers carrying out the task used a special rig that cycled the oil until clean before changing the filter. This doesn't’t present a problem for the DIY owner on a budget because the same effect can be achieved by the following approach.

Tools required Hydraulic jack & axle stands (OR ramps)
CLEAN container for draining fluid (10L)
Funnel (to fit gearbox dipstick aperture)
Measuring jug
Socket set (with hex keys for drain plug)
Components & lubricants required Gearbox filter kit (OE part # 7575625)
Dexron 2 or Dexron 3 (8-10 litres)


Stage 1
First, run the car until the engine is warm, before raising the car on a hydraulic jack and securing it on axle stands.

Take suitable precautions to avoid the risk of scalding, as the fluid will be very hot!

Alternatively, drive the car over an inspection pit (or up a pair of ramps). Having stopped the engine, drain the fluid into a container (this may take up to 10 minutes). Measure what comes out.  By now you will have observed that what you have drained bears no resemblance whatever to the nice clean fluid that you saw on the dipstick! The container should look as if it is full of dirty tea mixed with tar - not nice clean, bright red Dex 2! This is EXACTLY why you are doing this job. Replace the drain plug and tighten. Refill the gearbox with the same quantity of Dexron2 (or 3) as was drained.  Fill the gearbox using the dipstick tube (dipstick out, of course!). After replacing the fluid and waiting 5 minutes for the fluid to settle, start the engine. Next, with your foot on the brake pedal, move the gear selector through all positions at least 5 times before holding the transmission in D. Now check the level on the dipstick.

Filter kit for the ZF HP 18 automatic gearbox

Stage 2
Involves driving the car for several miles. This is to get the fluid circulating properly. Drain the oil again and as before, measure what comes out.  The fluid WILL appear to be much cleaner than before. Replace the drain plug and tighten. Refill the gearbox with the same quantity of Dexron2 (or 3) as was drained.

Stage 3
Involves repeating stage 2 but after draining the gearbox again, remove the three bolts that retain the gearbox oil filter.The filter needs to be changed because it is of twin wall construction and CANNOT be cleaned effectively. In the photo (left), note the very fine mesh. Don't mess about - BIN the filter and replace with a new one. Do not panic if the small magnet appears to be covered with small metal filings - this is perfectly normal.

RELATED article:
9000: adjusting the brake bands on the automatic gearbox





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