5-speed A750 trans won't shift in to 5th gear. (7 Viewers)

[LJE]

Worth mentioning, there was no rain in the time between putting in the new (used) ECU and this reoccurring.

Possible the new(used) ecm want bad on its own but that it did in the exact same manner in a head scratcher, unless caused by existing problem in the truck.

At this point, I've changed the transmission control module, the ECM, and the complete trans. The only thing not changed is the trans wiring harness.

Pulling codes at @cruiserpatch 's place now and this is all that comes up. No longer a solenoid code. View attachment 3787981

Oh snap - that's a different fault! That's probably a good thing. I bet that means it's not sure if it's in low gear or not - which, as discussed earlier in this thread, means that 5th gear could be locked out in software then! I'd start by cleaning the sensor connector contacts.

 

[jpneely]

Is there a sensor that tells the computer if it’s in low range or not? Could that or its harness be the common source? I’m just talking out my backside right now.

 

[LJE]

Here's the diagnostic page for it. This thread also lays out the location of the skid control ECU (above your feet on the driver's side): Fix for ABS, VSC TRAC, VSC OFF, & Brake lights remain on no CEL, or how to repair an ECU - https://forum.ih8mud.com/threads/fix-for-abs-vsc-trac-vsc-off-brake-lights-remain-on-no-cel-or-how-to-repair-an-ecu.1122305/

 

[MongooseGA]

Oh snap - that's a different fault! That's probably a good thing. I bet that means it's not sure if it's in low gear or not - which, as discussed earlier in this thread, means that 5th gear could be locked out in software then! I'd start by cleaning the sensor connector contacts.

Here's the diagnostic page for it. This thread also lays out the location of the skid control ECU (above your feet on the driver's side): Fix for ABS, VSC TRAC, VSC OFF, & Brake lights remain on no CEL, or how to repair an ECU - https://forum.ih8mud.com/threads/fix-for-abs-vsc-trac-vsc-off-brake-lights-remain-on-no-cel-or-how-to-repair-an-ecu.1122305/

View attachment 3788057

OK so I did my best OBGYN impression and got my hands all up under her dash, got the ecu down and did the test. Voltage with key on is showing 4.4v, not the 10-14v it should be.

Move to check on the following sensor/ connection:

(Pic provided by bearded Alex)

Put my hands on the L4 sensor and connector on the transfer case, shown below (the one on the right of the 2 visible)

There may be a little bit of gunk in there. I didn't have any contact cleaner so I went and picked some up. Now my exhaust is super hot and there's no great way to access this again without being on the exhaust, so I'll give it a bit/ overnight before I shoot it with some cleaner.

Assume if cleaning the connections does nothing, I'm looking at either the sensor itself or back to wiring harness again.

 

[Malleus]

You sir, have the patience of Job. Good on you.

 

[MongooseGA]

Is there a sensor that tells the computer if it’s in low range or not? Could that or its harness be the common source? I’m just talking out my backside right now.

Until this issue (and subsequently this thread), I wouldn't have thought about it, but yes. I know earlier trucks will automatically lock the CDL when in low range, so I guess it makes sense that there would be a little dude telling the brain I'm in crawl mode.

 

[MongooseGA]

You sir, have the patience of Job. Good on you.

The irony being I'm more impatient than a cat that knows her dinner time was 1 minute ago.

 

[MongooseGA]

Thinking on this some..

The L4 pickup code was present when I initially hooked it up to TS while it was at the trans shop. This was present simultaneously with the P0771 shift solenoid code. At that time, I cleared all codes, drove the truck, and the only one that can back up was the P0771 solenoid code. Also present (and was today) was that ABS system malfunction code.

What does this imply? Was the L4 pickup issue present first, and ECM fried itself getting to compensate/ rectify? Is this a potentially unrelated, coincidentally concurrent issue than the ECM? I would wonder if ABS module was source of issue, but the L4 pickup was what measured bad today, in front of the ABS module (or skid control module? The unit discussed in the thread linked earlier). This module, as I understand it, manages ABS, VSC, and TRAC. If it's getting bad info from transfer case, would it possibly be doing the same thing the ECM had done and be working on ending itself? Maybe the ABS system malfunction is a generic code byproduct of the issue being a bad signal from transfer case?

I'm always interested in the logic behind these decisions made by a car.

 

[MongooseGA]

CDL Sensor/Switch

www.george4wd.taskled.com

I just found this during some googling. I can definitely test out the sensor/ switch in the morning.


Also, is there supposed to be a 4-Lo light on our dashes? I've never seen one but looks like the 80s have em.

 

[jpneely]

CDL Sensor/Switch

www.george4wd.taskled.com

I just found this during some googling. I can definitely test out the sensor/ switch in the morning.


Also, is there supposed to be a 4-Lo light on our dashes? I've never seen one but looks like the 80s have em.

I’m fairly certain my 05 does not have a low range light. Only CDL light. I’ll go check after kids bed and bath time if no one’s confirmed by then.

 

[MongooseGA]

I’m fairly certain my 05 does not have a low range light. Only CDL light. I’ll go check after kids bed and bath time if no one’s confirmed by then.

You have a CDL light??

NVM

What about a neutral light for when your case is in neutral with trans in park?

 

[jpneely]

Never noticed it before but it does say AT/P when transfer case is in neutral and trans is in park. Besides that, no indication of where the transfer case is.

So I guess that means there’s definitely a position sensor somewhere.

 

[LJE]

Never noticed it before but it does say AT/P when transfer case is in neutral and trans is in park. Besides that, no indication of where the transfer case is.

So I guess that means there’s definitely a position sensor somewhere.

The sensor for the neutral position is actually separate to the one for low/high range - it's the one right next to it on the T-case.

 

[MongooseGA]

Never noticed it before but it does say AT/P when transfer case is in neutral and trans is in park. Besides that, no indication of where the transfer case is.

So I guess that means there’s definitely a position sensor somewhere.

I believe the sensor to the left in that picture I posted is the neutral sensor.

OK, thank you for taking the time to do that. That's helpful. I will maintenance and test both of those sensors tomorrow.

 

[jpneely]

Somehow missed your posts about already knowing there’s sensors and where they are. My bad. I’m all caught up now and hoping some contact cleaner and jostling around gets 5th back for ya.

 

[MongooseGA]

Today I sprayed the connectors for those t-case switches with connector cleaner, and pulled both switches to inspect further.

The L4 pickup switch looked decently clean. The neutral switch was very gunky inside.

Brought both inside the house to put a meter on them. Both are showing bad. I wired up 2x 9v batteries in series to 'clean' the L4 sensor as described in the linked article above. There was improvement. Cycling the switch with the leads hooked up shows some change, but it does not drop to '0' as it should.

Fortunately, my good friend @CW100 has graciously left his spare t-case in my garage, so I pulled the 2 sensors from that unit to test. Unfortunately, they also both showed as bad, or the meter only displays '1' whether the switch is activated or not. (Don't worry Chris, I put them back in your case).

So now I'm shopping.

 

[MongooseGA]

Ordered a new L4 pickup switch, should be here this week. I also ordered some fancy meter probing accessories since my Fischer Price multimeter didn't come with anything useful.

 

[LJE]

I also ordered some fancy meter probing accessories since my Fischer Price multimeter didn't come with anything useful.

Someone buy this man a proper multimeter!

Where we're at currently & what we know for sure: The transfer case position switch gets actuated when the T-case is shifted into low gear, which then connects one of the pins of the skid control ECU to (effectively) battery voltage. That's why you should get battery voltage (10-14v) when you measure voltage between that pin (with the ignition ON) and body ground with the T-case in low gear, per the FSM. When the T-case is in high gear, that switch is left 'open', so it's left unconnected (though, probably with an internal pull-down resistor I would assume). That means if you do a resistance check on the switch pins, when the switch is not pressed you should get infinite resistance, and when it's depressed the resistance should drop to (effectively) zero. I've confirmed that behavior with a functioning switch out a spare T-case that I have.

What's a little confusing to me is that if @MongooseGA clears that code in Techstream, and then leaves the T-case in high, the computer shouldn't be able to tell that there's a faulty switch - so it shouldn't throw a code. Therefor, it would have no reason to lock out 5th gear (we're still working under the assumption that it locks out 5th if that code is present).
He's not sure if he cleared that code when he had access to Techstream or not though, so I'm hoping a new switch and clearing of the codes will let the transmission function normally. If he wasn't more computer-adverse than my grandparents, he could test this pretty quick - but alas.

Side note: My guess the ECM or skid control computer can probably detect that there's an issue with this switch by doing a 'sanity check' by comparing engine RPMs, transmission gear, and wheel sensor speeds. It can probably guestimate whether or not it's in low gear by doing this.
If it's not getting a good 'hey, I'm in low gear!' signal from the transfer case position sensor, but yet the sanity check tells it that it is (high engine RPMs, high gear in the transmission, but low wheel speed sensor readings), it probably throws this code. That's why I wouldn't think that staying in high gear with a bad sensor would cause the code to pop - the sanity check would come back fine.

That's me pulling things out of thin air though, so take that with a healthy grain of salt.

 

[MongooseGA]

Computer adverse?

MFer, I know how to do the google and the electric mailing just fine!

 

[MongooseGA]

OK here's where we are today:

I had the old L4 pickup switch sitting on my coffee table for a few days just mocking me. Yesterday I decided to re-test it with my new meter kit. I found that it actually behaved properly (this is after cleaning with cleaner and electrical power) and readings were what they should be on a good switch. So I reinstalled the switch in to the truck and took it for a drive. CDL was engaged as the rear shaft is removed for access to this switch, so VSC lights would be expected. I was kind of surprised to see the ABS light reappear though. Still no 5th gear and also odd, the post-battery-reset VGRS blinking light did NOT go away at 25MPH as it should. No sure if this is at all related to the CDL being on and the VSC being off. I'll try again when the shaft is replaced.

So for now, I'm spending the day getting techstream put on my old college win7 laptop. So far it's kind of a pain but I'm getting somewhere.

I want my truck back.