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4L30E Gearbox swap - What a nightmare!

During the Australia Day long weekend in 2024, we took Monty (HSV SE #62) up to Coffs Harbour for a family holiday. It was the first time we’d taken the car away since buying it. We’d done several short trips around Sydney before, but nothing too ambitious. While in Coffs Harbour, we drove up a few trails, did some sightseeing, and generally had a good time, sticking to low-speed driving on hilly terrain.

We already knew the 4L30E was a weak gearbox, but we hadn’t given it much thought. The car had handled fine beforehand, including the drive up to Coffs and around town with no problems. However, on the trip back in pouring rain that didn’t ease until we reached Raymond Terrace, things went wrong. After refuelling, we hit holiday traffic and, within ten minutes, third gear started slipping. With no choice but to keep going, third gear was completely gone by the time we reached the Mooney Mooney Bridge. We stopped at the foot of Mount White to let the car cool before attempting the 13 km climb out from the Hawkesbury River Bridge into Sydney. Halfway up, second gear failed. We limped the car back to Carlingford in first gear, then had it towed the remaining 99 km home. We cancelled the registration and laid the car up on 28 January 2024 to decide our next move.

Knowing the gearbox was likely done for, we immediately looked for a replacement. We found one in a 1992 Jackaroo located on the Bridle Track near Bathurst. After getting lost, doubling back, and spending several hours wrestling with that car—trying to pull its engine and gearbox together—we gave up around 8 pm and headed home. I didn’t see it as a total loss, more of a learning experience, though it turned out that particular car and gearbox were a complete waste of time.

Over the following months, I researched the problem, hoping I could fix it. I ordered new shift solenoids, thinking that might be the issue. Over the Queen’s Birthday long weekend, I attempted a repair. Ironically, this wasn’t my first time dealing with a gearbox on the Queen’s Birthday weekend, and both attempts ended in a similar fashion. After multiple frustrating attempts at servicing the gearbox, replacing the fluid, and getting the car running, I put it into gear and felt a brief sense of relief when it shifted through all the gears. Unfortunately, a test drive showed the same issue: second and third were non-existent. The car was laid up again while I considered other options. It sat for a few more months, used only once or twice to tow another car out of our boggy backyard.

When spring arrived, we finally found a donor car: a 1997 Jackaroo SE (not the HSV type). After a brief discussion, we drove three hours to inspect and test drive it, then brought it home on a UVP. My plan was to follow a YouTube tutorial and swap the gearbox in our carport, but I didn’t realise how big a job I was taking on.

I stripped anything useful from the donor car for one of our three other vehicles: mine, my wife’s (Monty), and my daughter’s. Fortunately, I had access to a hoist. What I assumed would be a three-day job, based on YouTube videos, blew out to ten days, with the car fighting me every step of the way.

Step one was removing the engine and gearbox from the donor. It was my first time pulling a gearbox from a car, and I learned you cannot physically remove the motor and gearbox together without either taking out the diff or separating the body from the chassis. The transfer case on the back of the gearbox simply makes it too large to come out in one piece. In hindsight, leaving Bathurst without that original gearbox was a good call—it never would have come out in one piece anyway.

On 26 December, I began removing the engine and gearbox from the donor. By 2 January, it was completely dismantled and out of the workshop. Then I brought Monty in, and from that moment, more problems emerged. The gearbox in Monty was clearly not original: the inspection plate was missing, several suspension bolts looked like they’d come from Bunnings (and were the wrong size), and many bolts holding the gearbox to the motor were missing or over-tightened. This theme continued throughout the car. The worst part was discovering the torque converter held on by Bunnings bolts and washers.

Despite that, I pressed on, keeping all the good hardware from the donor car. By 4 January, Monty’s gearbox was out, and I started cleaning the replacement, which was caked in mud from its life in the donor. I ordered a complete service kit, transmission fluid, and new seals, then had to wait for everything to arrive.

On 6 January, more trouble began. I checked the rear main seal and was reinstalling the flywheel, but while torquing the bolts, one snapped off in the block. Luckily, it came out easily. Comparing the original bolts to those from the donor engine, I suspected they’d all been over-tightened and stretched. I knew I should have used new bolts, but it was early January 2025, everywhere was shut, and Holden spare parts basically laughed at my request—like J. Jonah Jameson in Spider-Man. With limited time on the hoist, I used the best bolts from the donor, torqued them carefully, then rolled the gearbox into place under the car.

Just as thunder rumbled in the distance, I began jacking the gearbox into line with the motor. A sudden storm hit, and water flooded the workshop ankle-deep. The gearbox jack slipped, and I jumped clear as the bell housing caught on the diff and the one extractor I couldn’t remove (due to a badly stripped bolt). Thankfully, that stuck extractor ended up saving the gearbox from further damage. I strapped it to the car and called it a night.

The next morning, I had help recovering the gearbox and preparing it to go back in. On 9 and 10 January, I worked to get everything back together, though it was still a challenge. I replaced a series of bolts, dealt with broken transmission line fittings, bought new exhaust bolts, sorted out wiring, repeatedly hit my head on various parts, and so on.

Once reassembled, I noticed the gearbox wiring in the engine bay was swapped around (the blue plug was connected to the black plug and vice versa). This meant the gearbox couldn’t communicate with the car. After some fault-finding, I realised there are three different looms for this gearbox across the years it was used. Although it’s supposed to be ‘plug and play,’ you still need the original body loom. I went home, removed the loom from the old gearbox, spent 45 minutes looking for a 7 mm spanner (eventually using vice grips), and then reconnected everything correctly. Finally, the gearbox and car talked to each other, no warning lights appeared, and a test drive showed smooth shifts.

On 15 January, Monty passed its roadworthy inspection—almost a year to the day after the old gearbox failed. It’s now fixed and ready for registration. It was a massive ordeal, but I’m glad I had the experience. I also have huge respect for anyone who tackles a job like this using just ramps in their driveway.