406 V6 – ZF 4HP20 problem

[minardista]

Hello,

I have a Peugeot 406 V6 Coupe with the ZF 4HP20 auto box. I've been doing plenty of reading but I can't quite find a consensus on the internet as to the right angle of attack for dealing with my problem...

I've got the Sport/Snow lights blinking on the dash, 'automatic gearbox fault' warning on the computer and the transmission locked in third gear. So, of course, something is amiss. I take it to a mechanic for diagnostics and...no fault codes.

What?

So the gearbox is faulty but the ECU is not giving any fault codes to pinpoint what might actually be wrong. So, my question...

What is the most effective sequence of guess-the-problem fixes, considering Occam's razor and avoiding spending big money on 'fixing' things that might not be broken?

My list thus far...

1. Drain and replace gearbox oil (Mobil 1 LT 71141)
2. ??????
3. Replacement oil filter
4. Torque converter rebuild
5. Invent a time machine, go back and buy a manual instead

Am I on the right track here?

Merci beacoup

[Hell Razor5543]

CitroJim has a guide on how to strip down and repair that gearbox.

https://eastment.net/4hp20/4hp20_introduction.htm

[Rp0thejester]

I like the Merci beaucoup bit, must sound funny in a strong Edinburgh accent

[Hell Razor5543]

I like the Merci beaucoup bit, must sound funny in a strong Edinburgh accent

The strangest accent/language combination I heard was an Indian family. When they spoke in their native tongue it was melodious; utterly beautiful. However, when they spoke English it was with the thickest accent you could imagine. They chose to move to Glasgow!

[Rp0thejester]

Try a Romanian from Newcastle!! Y I man was hilarious!

[Rp0thejester]

In case it's not clear, you need to have a sense of humour when owning a french car. Or just go crazy, your choice....

[xantia_v6]

I suspect that the mechanic's diagnostic tool is not correctly reading a PSA ECU of that age.

Ideally you should use Peugeot Planet.

Is the fault apparent (flashing lights) before the engine is started?

[minardista]

I suspect that the mechanic's diagnostic tool is not correctly reading a PSA ECU of that age.

Ideally you should use Peugeot Planet.

Is the fault apparent (flashing lights) before the engine is started?

No, only when the engine is running. Switch off/on cycle temporarily clears the fault. How long the fault takes to kick in varies...sometimes it'll take about a minute before it starts, sometimes a few seconds. But it's always in the low gears – in fact, I'm fairly sure it only happens in first, as I don't think it's happened when going any faster than 20mph. If I can get onto a motorway without the fault happening it'll trundle along there just fine and happily change between third and fourth.

It also seems to happen a bit less often when Sport mode is engaged before moving out of Park.

I guess my hope is that there's a fix for this that doesn't involve taking the whole gearbox apart...perhaps that's too optimistic

[Armidillo]

Well you can't even change the filter without pulling the box out of the engine bay...

[CitroJim]

I guess my hope is that there's a fix for this that doesn't involve taking the whole gearbox apart...perhaps that's too optimistic

As said, it's vital to get a read of fault codes from the ECU using the correct equipment. Otherwise you're just groping around in the dark...

The fault you have sounds like the ECU is detecting an error in the input speed versus the expected/calculated output speed which may indicate a brake/clutch is slipping - see later.

If your car has done more than about 120,000 miles then a rebuild will likely soon be necessary...

A quick assay of the oil can be valuable. Drain a small amount, ensuring no external muck contaminates it.

Firstly, look at it. It should be light to dark straw colour and should be translucent with no clouding. Clouding indicates friction material in suspension.

Then smell it. LT71141 smells awful, like cats pee, and may make you feel sick but it should not smell like burned toast. If it does then brakes/clutches may have been slipping and getting a bit hot.

If it's red, some muppet has changed it and filled it with Dexron - bad news...

Look for any sediment in the oil, suggesting debris from worn friction material.

Usually, the torque converter lock-up clutch starts to fail and break up, filling the oil with debris which starts to block the filter and a vicious circle develops. More debris causes a fall in pressure as the filter plugs and then brakes and clutches cannot engage with the needed force and they start slipping, liberating friction material and so on until the filter plugs completely and the 'box fails catastrophically.

I have known one HP20 fail with a broken valve in the valve block which causes two brakes to engage simultaneously, burning out both in the process but this is rare.

Rebuilding, as you can see from my little guide, is not difficult. The hardest bit is lifting it and the engine out and the fact the gearbox is very heavy.

Hope that helps.

[RichardW]

Rebuilding, as you can see from my little guide, is not difficult.

I'm over in Lanark and have PP / Diagbox if you want to get it plugged in...

[minardista]

Thanks everyone.

The mileage is part of the reason I'm a bit miffed – just ticked past 70,000 – so I was hoping yanking the thing out for a rebuild wouldn't have been needed just yet!

I'll do a small oil drain and see what I find.

I'm over in Lanark and have PP / Diagbox if you want to get it plugged in...

Thank you Richard. I may well take you up on that offer – just as soon as I get a new set of rears fitted. If the gearbox isn't in good nick, those...

[CitroJim]

Indeed, 70K is low for any serious trouble to be apparent so fingers crossed, a session with Richard will nail it down...

[minardista]

Rebuilding, as you can see from my little guide, is not difficult.

I'm over in Lanark and have PP / Diagbox if you want to get it plugged in...

Now I have some rear tyres fitted that don't look like racing slicks, it would be great if I could limp down to Lanark in third gear and find this mystery fault! Sent a DM.