Reverse gear is slipping

[andrewzuku]

Hey, I've got a 1UZ and Auto (A341E / 30-41LE) in a 1981 Hilux. Recently I've almost lost reverse. All the other gears are strong and working perfectly, but in reverse it will even roll down a hill forwards.

I need to rev it to around 2000rpm before it will engage at all, then it creeps backwards for about 10 meters, then disengages as if it's in neutral. Or sometimes engages and disengages over and over again.

I suspected that my aftermarket shifter's cable wasn't adjusted properly - but I've played with that and am sure it's ok.

The transmission fluid level is fine, and line pressure cable is fine.

I'm using Castrol Transmax Z, and have put about 8 litres through it 1000kms ago, but it's black already (it's meant to be green).

Any ideas on what could be happening?

 

[andrewzuku]

I had another play with it. Reverse seems to be good when it's cold, and when it's hot it feels like it jerks in and out between reverse and neutral quite abruptly.

Any insight would be much appreciated.

 

[andrewzuku]

Ok, so I've done the complete flush through with 12 Litres of Toyota T-IV ATF. Still the same problem: When it's cold, everything's fine. But when it's hot, reverse slips like crazy for about 5 seconds then disengages completely into neutral.

I'm giving up on it and saving for another transmission. I'm a little dissapointed that no-one on here had any suggestions though

 

[andrewzuku]

I've replaced my gearbox (and replaced my rear main seal while I was at it...). The vibrations from the old box are gone, but I'm still having another problem that I was having with the old box.

Reverse gear intermittently disengages, as though it's cutting in and out between neutral and reverse.

The only parts I've used from the old box are the torque converter, and the neutral switch (it goes over the selector to tell the computer what gear I'm in) since the box I bought was missing that sensor.

So obviously I suspect that sensor. But I'm not 100% sure it's the cause - reverse works fine idling backwards for as far as i want. However, if I use a little throttle, it starts to disengage intermittently.

Before I go and buy another one, I'm hoping someone else has had a similar problem and can tell me what's causing it.


Another thing that I've been wondering is if it's not that sensor, could it be the computer? Or even the diff? Can diff's work fine in the forwards gears but disengage intermittently in reverse?


the transmission is an A341E (4-speed auto from a Toyota Soarer) and I've attached a photo of the sensor I mean.

 

[spf_lexus]

I know all forward gears are syncronized and reverse is a basic spur gear. If the gear is worn it may be slipping, and when warm the metal may have reached it's slippage point. Reverse is unsynco'ed because it rarely wears out so thats so thats something to consider. I doubt it's the diff but I suppose anything is possible, if you had a blown diff the pinion depth/runout would be waaay off and it would be slamming into gears and would most likely be leaking fluid out the front seal.

Maybe try swapping that sensor for a working unit?

 

[andrewzuku]

Thanks for the suggestion

I'm chasing that sensor from the guy I bought the replacement gearbox from - Hopefully he has it lying around somewhere (when I bought the box, he had a few sensors on loosely and the bellhousing + stall converter off).

I doubt it's a mechanical problem with the actual reverse gear's teeth, otherwise it's a huge coincidence that the replacement gearbox is having the same problem as the original one.

I guess I won't be able to rule it out until I try another neutral switch though.

 

[sideshow]

reverse gear needs no electrics to work
unplug al the wiring to the gbox
u should get park reverse and 3rd and 1st i think if u manually select them
as these gears need no electronics to work
if the problem is still there then the box is stuffed i pressume
or there is some other mechanical problem

 

[andrewzuku]

Sideshow, you're a genius!

I removed the neutral switch from the gearbox, kept it in park, and plugged it in (so that I could start the car, but it would always think I was in park). I then removed all the other plugs and tested reverse gear. The problem was gone! So I plugged things in one by one until I narrowed it down to one plug. It's the speed sensor on the extension housing.

I've tired replacing that plug with one from my old box (not the extension housing speed sensor), but it's still doing it, So I'm thinking there's a break in my loom, or my computer is faulty.

I guess my next step is to call out an auto-electrician to see if the loom is ok. Luckily I have the correct wiring diagrams from this site

 

[andrewzuku]

I've tested the sensor with a multimeter and it's ok - which makes sense, since the problem only happens when the sensor is plugged in.

I'm confident the sensor is ok, since with it plugged in the car shifts through the forwards gears perfectly, but with it unplugged, I'm stuck in first (when driving in D).

I suspect the computer now.

 

[spf_lexus]

thanks for the tips sideshow. Did the ECU come with the engine/trans as a half cut or was it sources elsewhere?

 

[andrewzuku]

The ECU came with the engine and original transmission along with the wiring loom and igniters.

I'm not 100% sure what the transmission I have on there now is from, but it's a 30-41LE and is identical to the one I took off.

The problem started happening with my original gearbox out of the blue one day.

 

[sideshow]

if u have faulty speed sensors on gbox u should get some codes coming up
strange y speed sensor can effect reverse gear

maybe u have wired the neut and rev and 1st and 2nd wires into the ecu wrong
some ecus can vary slightly where these goes
so when u find the diagrams make sure u go over afew of the common wires and see if they all match up

 

[nemisonic]

Re-check your throttle cable again

Hi I have occasional reverse slippage as well. I am looking into this using the shop manual. So basically I am including a few things you can try, but realistically you need A) a shop manual and B) a referral to a transmission shop from your mechanic... the transmission shop has the equipment to run these tests... obviously so does the Lexus dealer!

SC400-Automatic transmission-Reverse:
The manual indicates to check in this order: 1)throttle cable 2)transmission control rod, and 3)oil strainer.

I've read elsewhere that the line pressure is adjustable by turning a screw... if the clutch plate is not exerting enough tension it will slip. It stands to reason that the hydraulic pressure has to be high enough to press on the clutch and engage reverse gear. So, here's the procedure to check line pressure:

For checking line pressure (LH/driver side), you are probably best served by visiting a transmission shop. But here is some information:
SST: 09992-00094 (p/n for the "fancy" pressure guage)
Condition Gear kPa (psi)
Idling D 382-441 (55-64psi)
Stall D 183-203(183-203psi)
Idling R 637-716 (92-104 psi)
Stall R 1726-2060 (250-290 psi)

Problem:
(!)Lexus service note: if measured pressures are not up to spec, recheck the throttle cable adjustment and perform the tests again as a retest.

*If the measured values at all postions are higher:
-throttle cable out of adjustment
-throttle valve defective
-regulator valve defective
*if the measured values at all positions are lower:
-throttle cable out of adjustment
-throttle valve defective
-regulator valve defective
-oil pump defective
-O/D direct clutch defective
*if pressure is low in the D position only
-D position circuit fluid leakage
-forward clutch defective
*If pressure low in R position only:
-R position circuit fluid leakage
-Direct clutch defective
-First and reverse brake defective

Accumulator pressure (RH/passenger side)
{This is a wicked complicated test.... take it to a shop.}
What to check if the Accumulator pressure not good enough:
-throttle valve out of adjustment
-throttle valve defective
-solenoid modulator valve defective
-SLN solenoid valve defective
-accumulator control valve defective

But wait, it gets better. The shop manual has a procedure to tell you if the issue is in the transmission or in the electronics. There are a lot of diagnostics built in so you can have an idea of what's wrong when you go to a transmission shop. This is a really long list of things to do... but essentially it boils down to:
A) disconnect the solenoid wire from the side of the transmission
B) shift the transmission into all the gears and make sure it goes into them (drive test)
C) then cancel/clear the diagnostic codes

Anyway if you go to the lexusownersclub.com and search the sc400 forums there are links to a shop manual resource, some may be online.

Good luck! Let us know how it turns out...

 

[andrewzuku]

Thanks for the suggestion Sideshow. I double-checked the wiring diagram and pin-outs, but everything's where it should be.

Thanks for the comprehensive reply Nemisonic

I ended up testing all the wires coming and going to the transmission with a multimeter and they all had continuity. I suspected my ECU so I bought another one of those, but it wasn't the problem either.

So with a new ECU, new Transmission, no problem with the loom, and no error codes from the engine or transmission, I was out of ideas. The only thing I know for sure is that when the 2-wire speed sensor is unplugged, reverse works perfectly. The 3-wire speed sensor isn't connected to anything at the moment (I don't have a speedo yet).

I decided to use a Normally-Closed (NC) relay to disconnect the sensor when I put the shifter into reverse. The car drives perfectly now (as it always had in the forwards gears) and reverse is strong and trouble-free.

I'm not 100% happy with that work-around, but I'm completely stumped for other ideas.

 

[Nick M]

I know all forward gears are syncronized and reverse is a basic spur gear. If the gear is worn it may be slipping, and when warm the metal may have reached it's slippage point. Reverse is unsynco'ed because it rarely wears out so thats so thats something to consider. I doubt it's the diff but I suppose anything is possible, if you had a blown diff the pinion depth/runout would be waaay off and it would be slamming into gears and would most likely be leaking fluid out the front seal.

Maybe try swapping that sensor for a working unit?

What are you talking about? The automatic is a planetary gearset with no synchronizers at all. Reverse is gotten by holding the carrier. Unplug your range selector and see how it drives. This is step two after fluid checks. After that, DTC's. Toyota 340 series will function normal without any electronics.

The range selector is the picture you posted