Hey guys I'm new to the yotatech forums and was planning on doing a 3.4 swap on my 1992 toyota 4runner 3vze 3.0 with the automatic transmission and thought to ask to the toyota wizards here. I'm planning on buying a 3,4 engine with the ecu and wiring hardness and I was wondering why I couldn't switch the motors and use the 3.4 ECU and it's wiring hardness to make it run, and if I can would I have to get a 3rd gen 4runner transmission? I know that you can buy a ORS wiring hardness, but I'm trying to do this as low-cost as possible.

 

If you do enough surfing through the forums here it will answer all your questions.
first off, if you keep your automatic trans you'll need a t-100 ecu or you'll end up with shifting issues and/or check engine light from what I've read.
second, if you use the wiring harness from the donor vehicle it won't connect to your transmission without some work and it will need to be re-pinned to connect to the vehicle so your gauges A/C ABS and such will work.
third, I'm just finishing my swap using the entire engine/trans/t-case from a T-100 keeping the automatic. it's no cake walk. check out my build thread.
My opinion is that the best option is to buy a complete 3.4-liter T-100 for a donor so you will have the correct oil pan/dipstick/ecu and if you find one that runs you know for sure that the electronics are OK.
Re-pinning my harness myself saved me a bunch of coins but cost a lot of time and I spent days pouring over dozens of wiring diagrams.
The only items that I bought pre-made were an exhaust cross-over, battery extension cable for the underhood fuse box, and power steering hose.
I have a ton of other brand new parts that I threw at it and I had the transmission, short block and heads completely rebuilt, but those aren't required costs if you just use the parts from your donor.

 

Good questions. Checkout the 3.4 swap sub forum