5-speed A750 trans won't shift in to 5th gear.
- Thread starter MongooseGA
- Start date
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- Thread starter
- #302
If there is I would love to know about it. I've driven with TS hooked up and watched the truck recognize its shifts from 1-4. It also recognizes that the gear selector is in D in real time. If there's a manual override, I'd try it right now.
LJE's been sending me a bunch of tests to try, back-probing various wires going in to the ABS ecu and main ECM. I've gone through those this morning, nothing conclusive just yet.
What’s the nuclear option in this situation? You’ve ruled out ECU, so what’s left…replace the harness and all sensors?
I can't wait to celebrate this getting solved...think of all the good karma associated with circumnavigating this rabbit hole of a problem so others can possibly learn from it and save all the time and energy that has been required of you and everyone helping. Just know I'm in the cheering section on this one.
- Thread starter
- #307
Wait wait wait... hang on..
Aren't you literally a mechanical engineer?
does this go back to our conversation about what you call the guy who graduated last in med school?We've done more tests today. I had my console and shifter pulled apart to test the switches that interact with the lever. All is well there. Ecm is reading properly where we've tested by back probing wires specific to trans gear position.
I had my Ms help me with an adjustment on the shift linkage as it wasn't exactly perfect coming back from the shop. It's perfect now, but no dice.
We did a wiring jiggle test to see if we could get lights to pop on the dash from it. No dice.
But during my underside inspection just now while jiggling, I found this speed sensor (I think? ) connector on the driver's side of the trans smashed with the backs of the pins exposed. Doesn't look recent, but if it's the issue, this connector is a part of the harness I just ordered.
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Yes exactly. Everything can be solved with wd40 or duct tape. All this other electricities talk makes my brain hurt.Wait wait wait... hang on..
Aren't you literally a mechanical engineer?
- Thread starter
- #310
Same. We would be taking the truck to VA for the holiday but as of now we're planning to drive my 560. The dog was never supposed to go in that car...
Taking a beat on this while parts arrive so we can rule them out before digging further. It'll likely be after the holiday before they're all installed.
Hopefully this is resolved before this thread is as long as my 7 year build thread.
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Malleus
Far west of Siegen
I'd get the screwdrivers. You can use them for more than this task, and one of them is exactly the same size as the Wiring Harness Repair manual describes for the tool. I use mine all the time for this; they've never failed me.
- Thread starter
- #312
I was at HF yesterday to buy a trans issue gauge for another project and picked up a set of tiny drivers. Unfortunately, they were not enough tiny.
Malleus
Far west of Siegen
ACE hardware has the Stanleys; at least they do here. I have a second set (Task Force) I got from somewhere (not memorable enough to remember, obviously, probably Amazon), that are exact copies.
- Thread starter
- #315
Ah! I did find the page in the first Pic but hadn't gone far enough to see the shift function in the second. I will try this! Thank you for posting.
Malleus
Far west of Siegen
I love TS
First off - major props to @MongooseGA for his patience in running down pins and probing them. I think I had him checking at least 10 different connections yesterday!
What we know: The SR solenoid is getting triggered properly in 1st, whereas with the old ECM it was entirely 'dead' (hence no 5th and super rough downshifts from 2nd->1st originally before the ECM swap). So that means the transmission is capable of using the SR solenoid to go into 5th if commanded by the ECM - so we're confident that this is an entirely different problem than last time. We already suspected this, but this confirms it.
The ECM is also getting all the proper signals from the transmission shifter to know that MongooseGA really is in 'D", and not in 4th. Interesting side note: The shifter linkage connects to a switch on the trans that rotates as you move through the gear positions, but both first gear and 4th gear are triggered by little switches right under the top center console trim plate that get bumped by the handle/lever. I was really hoping that for some reason one of these small switches had gone bad, and thus the ECM was thinking that the truck was in 4th, but no dice. Voltages at the ECM pins coming from all these switches all behave properly when in each respective gear.
I've read somewhere else online that if the transmission temp is too low, it'll keep it out of 5th, and that there's two separate temp sensors - one that is monitored to pop the A/T TEMP light on the dash, and one that controls transmission shift parameters. I'm not sure which one Mongoose's Scangauge monitors, but I asked him to check these temps with techstream (it displays both). These sensors are integrated into the internal transmission harness.
And of course, there's still the fact that he gets the L4 switch error code randomly, even without trying to shift into low range on the T-case (which is the only time that switch is actuated). That really makes me think that there's an intermittant short somewhere. The FSM doesn't list this issue as something that should prevent upshifts from 4th -> 5th, but it does say that it's something that would prevent the torque converter from locking up. Not sure if the truck would actually shift from 4th to 5th without being able to lock the TC/with this code present.
Interestingly enough, the FSM has you probe the ABS/skid control ECU to measure for that L4 switch when you look up the corrosponding code (and has a wiring diagram that shows the switch connected to that ECU), but as I've been pouring over the electrical wiring diagrams for the truck (which has been a learning curve for sure!), that switch is shown as actually connected directly to the main ECM on pin E9-4.
MongooseGA - in addition to checking the transmission temps in techstream, if you're not sick of me telling you to probe pins yet, it might be worth checking voltage between E9-4 & E6-1 (ground) & seeing what you get when you have the T-case lever in 'H' and 'L'. Should be ~0V in high, battery voltage in low.
ADDENDUM: Does your cruise control still work?
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This thread has been really interesting to follow. This forum is just a fantastic wealth of knowledge.
In the spirit of the holiday season, I (very) loosely summed things up to a classic tune.
On the twelfth day of Christmas,
my true love gave to me
Twelve hundred Mud members viewing,
Eleven CDLs a-locking,
Ten gear selector a-checking,
Nine harnesses swapped out,
Eight pins a-testing,
Seven ECMs a-frying,
Six FSMs a-queried,
Five Techstream codes,
Four fluid changes,
Three valve bodies,
Two shift solenoids,
And a 4-speed A750!
In the spirit of the holiday season, I (very) loosely summed things up to a classic tune.
On the twelfth day of Christmas,
my true love gave to me
Twelve hundred Mud members viewing,
Eleven CDLs a-locking,
Ten gear selector a-checking,
Nine harnesses swapped out,
Eight pins a-testing,
Seven ECMs a-frying,
Six FSMs a-queried,
Five Techstream codes,
Four fluid changes,
Three valve bodies,
Two shift solenoids,
And a 4-speed A750!
