100 Series 5 speed A750F transmission won't shift into 5th (3 Viewers)

[kingkonger708]

I have the a 2006 LC 100 with the A750F 5 speed transmission, and it wasn't shifting into 5th. had the fluid changed, solenoid's cleaned, fixed the L4 switch and it still wouldn't go into 5th. There were also minor metal shavings in the fluid so thought the transmission was toast. Bought a low mileage used (not reman'd) transmission and am having the same issue. Before I return and try it again (there aren't any reman'd transmissions available) was thinking maybe it isn't the transmission since the new transmission shifts perfectly and is quiet. Anything I should try before buying/installing another 5 speed?

 

[hoser]

Long thread but for @MongooseGA it was a wiring harness issue.

Thread '5-speed A750 trans won't shift in to 5th gear.'

Oct 7, 2024

Looking to see if anyone else has experienced this, as I couldn't find much for what's going on.

2005 LX470, 345-350k miles. Trans has been perfect as long as I've owned the truck, nary a hiccup ever. I last drove the truck on Saturday this past weekend, all is great. Today, the ABS/VSC/TRAC lights came on the dash, and the transmission will no longer shift in to 5th gear. I did a key cycle and the idiot lights are now gone, didn't come back in about 50 miles of driving home. But, trans still won't shift in to 5th. All other gears, reverse included, are just fine. What's interesting is...

• MongooseGA
• Replies: 376
• Forum: 100-Series Cruisers

 

[MongooseGA]

Long thread but for @MongooseGA it was a wiring harness issue.

Thread '5-speed A750 trans won't shift in to 5th gear.'

Oct 7, 2024

Looking to see if anyone else has experienced this, as I couldn't find much for what's going on.

2005 LX470, 345-350k miles. Trans has been perfect as long as I've owned the truck, nary a hiccup ever. I last drove the truck on Saturday this past weekend, all is great. Today, the ABS/VSC/TRAC lights came on the dash, and the transmission will no longer shift in to 5th gear. I did a key cycle and the idiot lights are now gone, didn't come back in about 50 miles of driving home. But, trans still won't shift in to 5th. All other gears, reverse included, are just fine. What's interesting is...

• MongooseGA
• Replies: 376
• Forum: 100-Series Cruisers

*And/or the main ECM and a couple of sensors. Unfortunately an exact, singular culprit was not conclusively established.

By the time I got it fixed, I was willing to throw anything at it. The ECM allowed temporary reprieve, (old one was confirmed by bad swapping in to another healthy truck), but the problem came back after a week. Then replaced the main transmission wiring harness, as well as the speed sensors on the transmission.

 

[LJE]

*And/or the main ECM and a couple of sensors. Unfortunately an exact, singular culprit was not conclusively established.

By the time I got it fixed, I was willing to throw anything at it. The ECM allowed temporary reprieve, (old one was confirmed by bad swapping in to another healthy truck), but the problem came back after a week. Then replaced the main transmission wiring harness, as well as the speed sensors on the transmission.

Yup, would definitely recommend reading through that thread - tons of suggestions there.

In the case of @MongooseGA, when his ECU was bad (initial issue) his downshift from 2-1 was HARD. Like, thought I broke something in my truck the first time I braked after we swapped his ECU into mine. The computer was unable to trigger one of the solenoids that happened to control both 5th gear engagement and was responsible for the 2-1 downshift.

Of note, if you haven't scanned the truck with Techstream, there may be codes a normal scanner isn't pulling. MongooseGA also experienced that.