2001 GMC Safari Intermittent Upshift Failure

NOTE: I live in the UK. This is an American-version vehicle shipped privately to the UK. The mechanic's shop was busy but unusually well-ordered and tidy. The mechanic himself seemed competent. He did NOT charge me for the diagnosis which he was unable to provide via computer. He did perform a road test at highway speeds.

Diagnostic scanning was unsuccessful. Mechanic said that he suspected the problem was with one or more Shift Control Modules and/or Speed Sensors. He was perplexed that there was no diagnostic output to his instrument, but indicated that there should also have been some sort of fault code sent to the vehicle's dash disply. There was not.

Mechanic suggested two options:

1) remove pan and examine things visually and remove and test the associated electronics to find the faulty component(s). This would cost about  200, with no guarantee that it would correctly identify the problem. Fixing it would be more.

2) replace the entire transmission with a NEW one which I provide, as he fears diagnosis and replacement of faulty sensors, etc. Is going to get pricey enough to justify considering a replacement. He could get one in the UK, but indicated I could probably do much better on price with US contacts, etc. Than could he.

Can you actually GET a NEW transmission? If so, where? And for how much?

Thanks.

No GMC dealer located in London (yet). I took the Safari to another transmission shiop. Using a different diagnostic box, he also failed to get any codes from the transmission, though could get them from the engine itself. This mechanic drove me around the block to yet another mechanic who (it was obvious) works on ONLY American-built cars. Using a third diagnostic box, he also failed to get any meaningful output regarding the transmission.

Both expressed the opinion that I had two bad control modules, and suggested dropping the pan and checking them manually. The tranmision mechanic expresed optimism that the hydraulics and mechanics of the transmission are probably intact.

I asked if it made sense to replace all FOUR sensors (in two in fact prove bad) while the transmission is open -- he said he'd be happy to do so, but didn't seem to think it necessary. I thought, while this thing is open.

Anything else I should have them check/replace while it's open? They mentioned changing the fluid, the screen and the gasket. Anything else in there a "must do" while it's open?

Many thanks.

Well, it was already in the shop by the time I read your post. FINDING: Nothing. Everything inside looked fine, and all of the sensors read fine, and compared exactly with new ones which had been obtained to replace them. The old ones were retained, and the mechanic is stumped. But clearly honest. I'm out  80 for the labor and parts to drop the pan, inspect, and refill the transmission.

As to engine codes, the only one indicated a misfire on one (1) cylinder, number six (6), I believe. Over the phone, he didn't seem familiar with the request for a download to a POXXX format file. I'll check again.

He mentioned some possible issues with information from the engine throttle not being properly communicated. But, it's clearly over his head at the moment.

Thanks.