Any automatic gearbox experts here?

[nsa]

1996 4WD Toyota 4Runner/Hilux Surf. It has an Aisin A340F gearbox with throttle cable kickdown.

A couple of months ago it started shifting straight from 1st to 3rd, and overdrive operation was intermittent. It shifts up to 3rd and then into overdrive 4th way too early. It won't shift from 3rd or 2nd when I slow down, but if I come to a stop it will select 1st after a bit of shuddering. I can get through the gears by using L, 2, D and manually selecting overdrive and shifts happen without too much drama. Sometimes it will come out of O/D on its own but generally not. Kick down works fine from 3rd to 2nd and 2nd to 1st. The gearbox doesn't make any unusual noises. The car is OBD1. There are no codes flashing.

I've been through the FSM diagnostics/forums/Youtube and changed:

Fluid and filter (fluid came out cherry red, no debris in the pan)
Shift and T/C solenoids
ECU (combined engine and transmission unit) after I discovered burnt capacitors on it
Wiring from ECU to solenoid and TPS checked. I checked resistance at lower/upper bounds but not gradual resistance change. Will check that.
MAF cleaned
Valve body replaced with one known to be good
Torque converter believed to be OK

I watched a specialist gearbox mechanic change the valve body today, but he didn't check or change the accumulator pistons and didn't think the problem lay there. One came out with the VB so he replaced it. I forget if the seals were in tact but the springs went back OK.

So, to summarise:

• Gearbox changes up way to soon
• Won't change down gears automatically unless <5mph or using throttle cable activated kickdown

Two things I need to try next:

• Proper TPS check and possible replacement because they are cheap
• Inspect valve body accumulator pistons, just in case

Can anybody see obvious symptoms and possible causes? My guess is that either the ECU is getting bad signals from a speed sensor or the TPS, causing it to select a higher gear than necessary, or fluid is not being directed to the right places at the right times. I am hoping it's not something deeper in the gearbox. Note there was no sign of debris either in the fluid or pan magnets.

Edited by nsa on Saturday 26th October 12:52

[swampy442]

First off, its not a kick down cable, its manual line pressure control, later model boxes control this electronically.

Secondly, have you pulled the error codes from the wagon? Are there any speed sensor issues? From my experience on other Toyotas, theres 2 sensors, the speedo one is near the prop shaft, and the ECU one is towards the front of the box.

[nsa]

[Noted. Throttle valve cable. There's no slack in the cable and is seems to work correctly.

There are no error codes. It's OBD1 so what I have is limited to:



I had a 63 code but it went after I replaced the ECU. I've had the rear speed sensor off but I can't find the front one. Speedometer still works. This is the older 3rd gen Hilux Surf, before it went to using the wheel sensors to read the speed.

I had the transmission pan off yesterday to check the accumulator valves, no problems there. The car also drives fine with the solenoid connector removed. 1st, 3rd, O/D as specified in the FSM.





All torqued back on to correct spec.

Edited by nsa on Tuesday 29th October 04:16

[swampy442]

When I'm home the weekend Ill confirm the speed sensor location, Ive got a 340 sat in my unit.

[nsa]

I found the front speed sensor. Disconnected it and there was no difference to the symptoms. I also replaced the TPS and checked voltages at the plugs and ECU terminals. All seems OK; 0.30v off throttle up to 5v, and IDL reads 0v at idle, 12v on throttle.

I rigged something up to test when the ECM is telling the car to change gear. Downhill off throttle it changed up through first, second and into third (direct drive) at 17mph. Revs were around 1,000 at this point, which led to shuddering. On flat road and light throttle it will change up to third around 20mph and the car will shudder. On full throttle it changes normally, up to second around 35mph. Obviously on the way back down it changes incorrectly because it is off throttle.



So, the gearbox is changing gear correctly, it's just that the computer is telling it to change too soon on the way up and too late on the way down. I think the transmission control unit on this car (integrated in the ECM) reads signals only from the TPS and speed sensor. I'm pretty sure the TPS side is OK, but, if the speed sensor is at fault, I don't understand why when I disconnect the No2 speed sensor the symptoms don't change, because the car should be taking signals from the No1 speed sensor at the back of the gearbox, so if No2 sensor was bad, No1 sensor would fix the problem. I'll replace No2 anyway.

I took the car to a transmission specialist here but he wants to test the engine mounts and transfer box, which seems way off the mark.

Any help much appreciated.

Edited by nsa on Friday 8th November 12:02

[LarJammer]

You seem to have a handle on the electronics aspect. Have you (and is there a way) to check the pressure?

[nsa]

This gearbox changes gear by solenoid rather than pressure, as in older boxes. The throttle valve cable controls line pressure but it has no bearing on when the box changes gear apart from to initiate kickdown when the throttle is depressed >3/4 or whatever.

I'm pretty sure it is shifting correctly, it's just being told to change at the wrong time, as evidenced by the LEDs showing how the ECM is controlling the solenoids. There are apparently only two factors involved in choosing which gear the car should be in - propshaft speed and throttle position. MAF and engine coolant temp might have a bearing on it, but I can't imagine any mode that requires the gearbox not to shift down when engine revs are <1,000.

Today I'm investigating resistance between the speed sensor signals at the ECM and the wiring between ECM and the speed sensor.

I see, like me, you have a '97 NSX. It feels like an annus horribilis for me with electrical problems. My heater computer gave up this year and after a lot of faffing I had to get Kaz to repair it. Blown capacitors ruined the PCB but he revived it.

Edited by nsa on Saturday 9th November 03:48

[swampy442]

As far as I know, the sensors are independent, one for the speedo and one for the ECU

[nsa]

This turned out to be a case of not following the supplied fitting instructions. I replaced the #2 solenoid and at the same time I decided to replace the lockup solenoid. Gear solenoids are closed at 0v, lockup solenoids are open. Unfortunately I put a gear solenoid in where the lockup solenoid should be. I'm surprised that it fit, and that the car even drove with a locked up TC.

I only found out because Toyota did a lot of diagnostic work and ended up putting multimeters on the TCM solenoid pin outs. They could get the car to drive normally when they put 12v to the lockup solenoid, which opened it. They didn't actually work out what was wrong, and suggested I needed a new gearbox, but when they showed me on a test drive I knew straight away what I'd done.

So the three garages that said it was a gearbox problem were right, but the gearbox specialist didn't pick up the problem. Neither did another gearbox specialist when I had the valve body replaced. I can't blame them I suppose. On the plus side I have given the car a major service and sorted a few unknown issues in trying to figure out what was wrong. I'm very confused why I didn't notice the car didn't drive right just after I replaced #2 solenoid.

The fitting instructions are very clear. I have the early style, with cable: