The job that couldn't be done - Converting a 4.0 ltr with AX-15 manual to a GM TH-700R4 automatic

Having solved the problem of getting in and out of the Jeep it was time to work on the conversion from manual to automatic.  To Americans who grew up with autos this may seem simple but most Europeans still live in fear of automatics (something about your manhood!) and as Jeep had only imported a limited number they offered no help with this project infact they said in could not be done.  

My starting point was therefore the so called Jeep specialists.  These are the guys you see at events all over the country showing off their so-called expert knowledge.  Well let’s get it straight from the beginning, they may think they are experts but they know sod all about manual to auto conversions! 

I'll start by confirming that I'm now driving an automatic 4.0ltr TJ and the entire conversion cost less than £2,000 plus I've ended up with an ultra strong 4 speed auto with a low first gear and overdrive!   First off, I did not need to buy an automatic TJ as a donor vehicle; I didn't need a new computer or special drive shafts, I've used the stock transfer case not an Atlas, the floor plan and body tub are not different and more importantly I've not had to sell my sole to pay for it. 

Ok, negative stuff out of the way, my advise to anyone embarking on a similar project is 'Search the Net'. I found a wealth of information including an excellent set of articles by Ron Hollatz www.outdoorwire.com/4x4/ who did the same conversion on his '94 rock crawling Wrangler.  Rons articles are so detailed (I've included a link) I will not be going over all the work again but will deal with a number of issues that apply to the UK. 

Let's look at what's involved and your options if you’re interested in doing this conversion on your 4.0ltr manual TJ.  (I’m told the same will apply to earlier YJ’s so long as they use the NP231 transfer box)  First off, you can install a Torqueflite 999 from any auto YJ/TJ from '88 up.  It's a bolt in job - done! You'll have things to sort out like a gear shifter, oil cooler (if yours is an export model you'll probably find your radiator already has a built in cooler from the factory)  and your pedals (most people just weld the clutch and brake together), all items that can be sourced from none Jeep vehicles at a fraction of the cost, but regardless of what anyone says it's a straight swap.  Ok, there is one major problem, you just can't find a Torqueflite 999 here in the UK and if you do then it's going to have that magical Jeep price tag.  I was quoted £2000 by one Jeep specialist for a reconditioned transmission!  But hay, it's not that good anyway and there are better and stronger options out there.  How do I know this I hear you ask, well I spoke to the guy's who do this every day, Advance Adapters Inc http://www.advanceadapters.com or Tel No. 00 1 805-238-7000 in the States.  Their help desk is what a help desk should be! After discussing my requirements they gave me a range of alternatives.

If you follow the trends in the States the top automatic transmission used by the real hardcore rock crawlers and mud pluggers is the GM TH700R4 that's been around for years.  It's strong enough that you'll find it sitting behind 600hp plus hot rods, has a really nice 3.06 low first gear and an overdrive to counter my 4.56 low gearing when cruising.  More importantly, it's widely available over here either from breakers yards or already recondition.  Advance Adapters sell a complete kit including transmission to engine mounting plate, flex plate so that the original torque converter can be used and an adapter to the NP231 transfer box used by Jeep which is more than capable.  It's not a cheap kit at just over £470.00 excluding shipping, but if you buy direct, all you need is a credit card, you'll save thousands on the prices I was quoted over here and as you can pick up the box for as little as £95 from a junk yard it'll still work out cheaper than a second hand Torqueflite in Jeep circles.

Because this transmission has been around for years there are a number of different variations out there.  The early ones had a few problems but these will have been cured by now and shouldn’t be of any concern.  It’s also worth remembering that the same box was used in high-powered sports cars like the Corvette and had a different shift pattern to the more relaxed 4x4’s and cruisers.  The only major difference from the two and four-wheel drive transmission is the tail peace that you'll have to remove anyway to mount the AA adapter plate. 

Similarly, whichever vehicle the donor transmission comes from you'll have to change the output shaft to the one supplied with the AA kit.  It’s definitely worth remembering this when you’re looking for a donor transmission.  I wish I'd known before I purchased mine as I could have paid less and gone for one of the many junkyard specials and had it rebuilt.  Mine had already been done and had to be re-stripped to install the new shaft – it’s the first thing that goes in!  As they had to strip the box again I had them install a high performance shift improver kit that gives an even firmer shift pattern and allows you to hold each gear.

My box had come from a wrecked '95 Corvette and was already installed with the performance shift pattern but was now rated at 750bhp!  Including the second rebuild it cost £550 from a Corvette importer/dealer.  It has to be said that I never cease to be amazed at how much cheaper Chevy parts are compared to similar, and seldom better, Jeep items.

In fact it was really hard doing business with them as he was originally selling the gearbox with a complete 450bhp 350ci fuel injected Chevy engine from a ’95 Corvette.  If I deducted the cost of the adapter to mate up with the Jeep engine I could have had a Chevy powered V8 for another £500.  Believe me, I was very, very tempted but at the end of the day my marriage came first.  Who knows, maybe one day.

As for the actual installation process, well I had the work carried out by Autodesign of Ranmore Common.  They're not Jeep specialists and had never work on one before but according to them, and judging by the bill, it was fairly straight forward.  Thanks to the previously installed 1” body lift they had no problems working around the Borla headers and exhaust and they only had to shorten one drive shaft that cost an extra £38.00 (how the Jeep specialists can justify £400 for a drive shaft I’ll never know.  I’ll probably go for a CV joint type set up for the drive shafts, to prevent any unwanted vibrations, at some time in the future or if I decided to lift it higher.

 I got lazy with the gear selector and purchased a kit from the States manufactured by Lokar.  This is a direct link version and appeared better than the cable driven affairs on sale in the UK.  It’s fully compatible with the gearbox and has a range of mounting positions so that it can fit into the original gear levers location.  We did have a few problems with the shift linkage fouling the transfer box shifter which had to be re-routed. They also have a LED light indicator which tells you what gear your in but I didn’t realise that this was sold as a separate item and forgot to order it.  Ops!  Once this arrives it should be a simple task to fit.  Having said that, so far I’ve had no problem telling which gear I’m in, although I'm not impressed with the reverse gear lockout.  The gear knob has a release button in the middle which has to be depressed to shift between gears.  Sounds easy but it's very fiddly and is a real nail breaker for the ladies!  I'll be changing it for a standard 'T' shifter.  

 Well they say the proof is in the driving and so far it’s all been on the road.  To say I’m please would be an understatement; she’ll leave most Gti’s standing with the 33” BFG mud tyres smoking away and she’ll cruise on the motorway at a steady 100mph (only where its legal of course!). 

Once again thanks go to Paul xxxxx at Autodesign and to Ron Hollatz for allowing me to use his photo's.