Hyundai Repair: Sonata Automatic Transmission problems, automatic transmission problems, technical service bulletins

Question
Car is a 2003 Sonata 2.7 V6 automatic in the UK.

I have a problem with the Autobox behaving oddly, not holding in third gear and the MIL light coming on and off. I read the codes and I also noticed that the battery got low standing for a couple of days so changed the battery for another after reading the codes.

The codes I initially got were ...

P1529

P0765
P0760
P0755
P0750

After changing the battery clearing the codes I tried it again.

After a few seconds in third gear it threw up a couple of errors. They were two of the originals P07xx but stupidly forgot to write down which two!

According to my reader the Trans codes refer to Shift Solenoid A, B, C, D

I also narrowed down the behaviour a bit better.

Starting off in 'D' the car runs fine in first and changes to second, fine in second then changes to third, at this point it seems to struggle to engage gear then seems to go into neutral revving freely, then while slowing down the car eventually engages first gear but the dashboard display stays showing third. It then just stays in first. Stopping and switching the ignition off and back on again and its back to working until third gear.

Starting off using the 'sport' selection (+ & -) the car runs fine so long as I only use first and second but up to third and it won't engage and goes to neutral as before.

Any thoughts on a solution?

Answer
Here's what the codes indicate:

P0750: open/short in low/reverse solenoid
P0755: open/short in underdrive soleniod
P0760: open/short in 2nd solenoid
P0765: open/short in overdrive solenoid
P1529: transmission control module (TCM) requested engine control module (ECM) to turn on check engine lamp.

Hyundai has several technical service bulletins (TSBs) relating to these diagnostic trouble codes (DTCs), all pretty much sending you in the same direction.  If you'd like to peruse them, simply go to www.hmaservice.com and sign up for a free account.

Basically, the TSBs indicate that you should replace the wiring harness inside the transmission and the four solenoid valves in question.  I, on the other hand, am going to say you shouldn't do this, and that's because I think you also have other problems.  

Note that these codes indicate purely electrical issues.  Normally, when the TCM sees a critical issue such as this, it will place the transmission in 3rd gear and leave it there until the ignition is turned off.  In fact, if all electrical connections are removed from the transmission, it should still operate in 3rd gear.  It's purposely designed this way so that if there's a significant electrical issue, the car can still be driven to a repair facility.

The part that bothers me is that 3rd gear isn't operating properly.  I'm going to say that this indicates you have a serious problem inside the transmission that will require overhaul or replacement and that replacing the harness and solenoids will only be a waste of time; it will resolve the electrical issues, but still won't make the transmission operate properly.  

It'd be my recommendation that if you're planning on spending the money to fix this car, you replace the transmission with a factory remanufactured transmission.  Because of the multiple electrical and mechanical problems, the likelihood of someone overhauling the transmission properly is minimal.  In addition, Hyundai prices its transmissions very competitively, so you may well be able to get this work done at a dealer as inexpensively as anywhere else.