Transmission not downshifting while in D. P0750, P0753, P0773 and a random P0510 months ago

[CletusVanDamme]

Hi everybody,

I apologize in advance for the novel. I've been searching around the forums for awhile now trying to find a similar issue and found a few but the threads die and I never see if the solutions were successful for my 1997 Lx450.

The issue is when I come to a stoplight or stop sign the transmission doesn't downshift back into first. I manually shift into 1>2>D and there are no problems there and shifts smooth manually. This does not happen all the time. I will not have an issue for days and then it will come back but not at every stop.

Codes that come and go are P0750, P0753, P0773, and one time a P0510 that appeared and disappeared while driving home from a grocery run.



I usually take on projects myself and have used this forum extensively with the factory service manual doing repairs ranging from replacing taillight to doing a full headgasket job (thanks Otramm for the videos!0. Life has been busy and i didnt want to just start throwing money at the problem so I decided to drop the car off at an independent Toyota mechanic. After having the truck for 6 weeks they said nothing is wrong with it and none of the codes showed up. On the drive home P0773, P0753 came up. I decided to try having the local dealership take a look because I knew they would follow the factory service manual procedures which they did. They called me a few days later saying they were positive that it was the ECM. I asked are they absolutely positive its not a wiring issue between the ECM and the Transmission or an issue with the solenoids. They confirmed and explained how they got to that conclusion saying the solenoids all tested perfectly within spec, and found no issue with wiring between ecm and tranny. They also told me they can't get an ecm because they have been discontinued but if i find one they will install it. I found a "refurbished" unit with a decent warranty from what seems like a reputable supplier. They install the new ECM and i get the car back yesterday. The Dealership foreman did all the work and assured me that its fixed. 35 miles later P0750, P0753 appear and the truck does not downshift, but manually it works. I call the dealership and dropped the car off this morning. Shop foreman says it's probably the new "refurbished" computer they just put in.

Is it possible that two separate ecms have the same exact problem with the same set of codes?

Is it possible to test the solenoids without dropping the pan?

Could that TPS code P0510 I got months ago be a part of the problem?

Any other suggestions would help a lot. My plan of hiring a professional and not throwing money at the problem is costing me and as of now up to $1400 bucks for diagnosis, ecm and installation.

Thanks everybody,

Cletus

 

[Malleus]

Yes, it's possible two ECMs have the same problem; not likely, but possible. I would have misgivings about a dealership (or anyone else, for that matter) who determined the ECM was at fault by process of elimination alone. There are many ECM test procedures which have to be completed before that can be determined.

It depends on what they mean by "test". In order to bench test the individual solenoid, you have to remove it, and that means dropping the pan. If they claim to have "tested" the solenoids by performing some kind of function test on the transmission, you're out of luck, unless you can successfully argue that kind of test isn't proper. According to the FSM, it isn't, but you'll likely lose that argument with a dealership, because they're "experts".

The first 100 or so pages of the transmission section of the FSM is troubleshooting. This is the place to start to understand the operation and possible failures in operation. These kinds of problems are difficult to diagnose unless you're present when they occur. so you're the best source of indicator data. The FSM is in the Resources section, and it's not at all difficult to follow.

 

[Malleus]

To get you started, I've attached the fault codes for 0753, 0770, 0773. It's unlikely the TPS code is related to your shifting problem.

 

[Malleus]

Without knowing any more about your fault, I'd start looking at the two transmission harness junction connectors. They're above the starter.

It's unlikely you have intermittent solenoid failures on all solenoids, but it is possible you have faulty connections which cause the same problems at the ECU. The harness connectors to the ECU are under the glovebox, so it's unlikely those are corroded, unless you have a sunroof or windshield leak on that side of the truck. All the transmission harness connectors are on top of the transmission.

The solenoids are all routed through a single connector, "E2":

 

[CletusVanDamme]

Without knowing any more about your fault, I'd start looking at the two transmission harness junction connectors. They're above the starter.
View attachment 3313786

It's unlikely you have intermittent solenoid failures on all solenoids, but it is possible you have faulty connections which cause the same problems at the ECU. The harness connectors to the ECU are under the glovebox, so it's unlikely those are corroded, unless you have a sunroof or windshield leak on that side of the truck. All the transmission harness connectors are on top of the transmission.

The solenoids are all routed through a single connector, "E2":
View attachment 3313784

Thank you so much for the quick reply. Before dropping it off at the two shops I pulled out the glovebox to check for corrosion at the ECU and also E2 on top of the transmission and the connectors looks clean however did not check the transmission harness junction connectors. The reason being is when I did the head gasket last year I unplugged them thinking I would take the wiring harness out and I ended up not removing the harness and struggled for many hours trying to plug them back in because of the way I had the vehicle in my garage I had terrible access. It put me over the edge and had to walk away a few times. I have to face the fear and go back in there.
I may have to cut my loses with the experts and bring the truck back home and work on it when I have the time. These discontinued parts are a real bummer.

 

[Malleus]

FWIW, @Engineer8000 is the resource for fixing circuit boards. Much better than junkyard crap.

 

[Malleus]

Thank you so much for the quick reply. Before dropping it off at the two shops I pulled out the glovebox to check for corrosion at the ECU and also E2 on top of the transmission and the connectors looks clean however did not check the transmission harness junction connectors. The reason being is when I did the head gasket last year I unplugged them thinking I would take the wiring harness out and I ended up not removing the harness and struggled for many hours trying to plug them back in because of the way I had the vehicle in my garage I had terrible access. It put me over the edge and had to walk away a few times. I have to face the fear and go back in there.
I may have to cut my loses with the experts and bring the truck back home and work on it when I have the time. These discontinued parts are a real bummer.

FWIW, removing the LF wheel makes getting to those two junction connectors much easier than reaching them from above or below.

 

[CletusVanDamme]

FWIW, removing the LF wheel makes getting to those two junction connectors much easier than reaching them from above or below.

Just wanted to add an update.

I decided to dig in myself after the problem was not being fixed by two shops. I went through the wiring harness and checked all the connections and cleaned them out. After cleaning the connector on the transmission for the solenoids I noticed it wasn’t clicking like all the other connectors. I was surprised how much force was needed to make it click in but it finally clicked and locked. All transmission problems were solved by this one tab that was not properly clicked into place. I wish I would have problem solved this myself but at the time I was busy with life and wanted somebody else to deal with it.

Anyway, I drove it around awhile and I contacted the dealership that did the last diagnostics telling me it was the ecm. They basically told me they are the experts and that they properly diagnosed the issue and I’m s*** out of luck for the work they did that didn’t solve the issue. The foreman also said he doubts I fixed the issue since he’s been doing this 25 years. Whatever, I cut my losses and I’ve got 4000 miles on it since the connector clicked and the truck is running perfectly. I was able to return the ecm and but I had to eat the labor for the dealerships work. I bitched out and didn’t want to argue with him.

So, if anyone reads this in the future make sure all your connectors are clicked and locked in. I think as my truck got warm and hitting bumps the connector would barely lose it’s connection controlling solenoids at random times which explained the intermittent shifting problems.

 

[Tres Toys]

Thanks for the update and glad you finally got that tough to diagnose tranny issue sorted. Kudos to you for digging in there and figuring it out when the dealership couldn't. I am currently dealing with a similar PO753 intermittent issue where it sometimes doesn't shift back to first and require one cycle of manual shifting to 'D' to reset. I pulled one of the solenoids and it did not seem to be functioning when I tested it out of the vehicle. I put a seemingly working used solenoid from a 4Runner I had laying around in and it fit, but wasn't an exact physical match. It seemed to improve things for a while, but the issue has come back. I pulled the trigger today and ordered a new $$$ Denso solenoid, so fingers crossed it will fix it. If not, I might have to start chasing wires and clicking connectors.

 

[alia176]

Just curious are y'all pulling codes out of OBD-II using a typical OBD-II scanner or Techstream? Also, what indication is given by the ECU/TCI if a fault code pops up? Does the CEL come on or does the POWER light come on in the instrument panel?

 

[Malleus]