Auto box okay at 60mph but getting hot at 70mph

[Lorin]

My gearbox shifts and works as it should, maintaining approx 80 degrees C in most operating conditions. However, I've noticed that at a constant 70mph it slowly goes up to 110 degrees and will stay there. If I drop back to 60mph the temp will slowly fall back to 80 degrees. It does this irrespective of the ambient temps.

Any ideas what could be causing this? The gearbox has an extreme valve body and the ATF was recently changed to Castrol Transmax Z fully synthetic and is it at the correct level (the temp issue was happening before the fluid was changed). The torque converter lock-up is working as it should, engine/coolant temps are in the normal range and the transfer box isn't getting hot. The gearbox cooler has also rccently been flushed and is working as it should,

So I'm stumped! I am running larger tyres but the truck is regeared and the truck isn't carrying much weight. Does anyone have any ideas what else it could be?

 

[Chris]

Any thing to do with drag coefficient Lorin?
Difference between 60 and 70 on a massive house brick could be the factor
You're pushing a tonne of air out of the way
May be try slipstreaming something at 70 like another cruiser and see if it still climbs
What's the max operating temp anyway?

 

[stumog]

Did Toby get similar Temps but as it's a new truck your worried and not knowing all is in fact fine?

Those mil spec 900s are a very heavy tyre much more then anything on the sivy market.

 

[Lorin]

There's no max operating temp as such. Normal operating temp as per the FSM is 80 degrees C. In reality I think people report anything from 70-90 as being pretty normal. The high temp ATF light comes on at 150 and goes off again at 120. A 30 degree increase seems quite a lot though and is not good for the gearbox in the long term. Even if it's 'normal' for this truck it isn't something that bodes well for the longer term, so it needs addressing. I have read elsewhere that 100 degrees at 70mph when towing a big trailer has been reported but that seems a way off from what's being asked of my truck.

Drag could well be the issue Chris, it as you say a lot more work for the engine/gearbox at 70mph than 60mph and those tyres are pretty heavy as Stu says.

Maybe a decent auxiliary cooler will solve it but I just want to be sure I'm not missing anything that needs addressing elsewhere.

 

[stumog]

Does your cruiser not have a extra cooler?

I would stick one off just to be safe and see. At the very least it will protect gearbox from coolant if the radio goes.

 

[Dave]

Once the converter is locked, the oil isnt really doing much apart from holding clutches. Its only when the converter is slipping and churning the oil that the temp should rise.

 

[Lorin]

Once the converter is locked, the oil isnt really doing much apart from holding clutches. Its only when the converter is slipping and churning the oil that the temp should rise.

That's what I thought but if that's right, then where is the heat coming from? A 30 degree increase for just 10mph is pretty significant. Does it suggest the torque converter isn't locking properly....? Thing is, I think it is locking, so again I'm stumped as to why I'm getting such a rise in temp.

 

[Beau]

I'd play around with various loads on the transmission and see what other results you get, then maybe you can narrow down your problem. For example, try monitoring temps going up slight hill climbs. Try dropping a gear and using higher RPM and see what effect this has, try doing 70 down a slight hill?

From what you've said, I too think it's running more on the hot side. It doesn't give you a lot of room for when you actually need to put more load on the engine/trans. On the plus side, the synthetic fluid will hold up pretty well in the hotter temps.

 

[Jon Wildsmith]

is the heat exchange in the bottom of the coolant radiator still connected?

 

[Lorin]

is the heat exchange in the bottom of the coolant radiator still connected?

Yep, it is still connected and was recently flushed through as part of the ATF fluid change.

Beau, I have done a lot of playing around and the effect of hills, running in 3rd, driving round town etc are all as would expected, with a spread f temps from 70-100 degrees. The increase in temp from 60 to 70mph comes on very slowly, maybe over about 20 mins of constant speed. It takes a similar length of time after dropping back from 70 to 60 mph for the temp to drop back down again. If I just sit at 70mph the temp stays put at around 110 degrees.

 

[Jon Wildsmith]

Yep, it is still connected and was recently flushed through as part of the ATF fluid change.

Do you monitor the engine coolant temperature as well? Your ATF temperature change could be reflecting the extra thermal load on the engine coolant when the ATF passes through the bottom of the coolant radiator.

 

[Lorin]

Do you monitor the engine coolant temperature as well? Your ATF temperature change could be reflecting the extra thermal load on the engine coolant when the ATF passes through the bottom of the coolant radiator.

I've just put in a coolant temp gauge, so I can get some figures. I'm not sure coolant temp variation will explain it though. I remember from my old truck that had a coolant temp monitor, temps didn't vary that much, certainly not by 30 degrees. Long climbs at motorway speeds used to push the coolant temp up but even at 80 mph the difference from 60 mph was never more than 20 degrees IIRC.

I've not got the truck for the next week but when it's back I will check coolant temps and see what they're doing.

If it is due to the coolant temps, will adding an auxiliary ATF cooler help things? Is there enough ATF flow through the cooler when the TC is locked? I guess the other option would be an upgraded main radiator to better reduce coolant temps.

 

[Scott]

As has been said those tyres are always going to make the 80 work harder, I used to really notice the difference in acceleration and brake the additional rotating mass when I swapped from the OEM alloys and road tyres to the OEM steels and KM2s, but it has been running around for years with them fitted during Toby's ownership so she has proved that she can cope with them.

A pretty basic question and do excuse me if it has already been discussed but are the fins on the engine radiator in good condition and are the gaps between the radiator fins totally free from any blockages? Also is the gap between the AC condenser free and is the AC condenser also free from blockages?

It sounds like the drive train is working well at 60 but 70 just tips it over the edge and causes it to work a lot harder, if there is a blockage or obstruction to the air flow over the radiators, the engine maybe able to cope at 60 but the additional load at 70 may just be tipping the balance past what it can handle. Then passing the heat on to the ATF via the in rad cooler (as Jon has suggested).

The truck has seen some adventures and been round Lincomb once or twice, so it possible there could be some debris in there if it hasn't had an in depth clean out in a while.

 

[Jon Wildsmith]

If it is due to the coolant temps, will adding an auxiliary ATF cooler help things? Is there enough ATF flow through the cooler when the TC is locked? I guess the other option would be an upgraded main radiator to better reduce coolant temps.

If it's due to coolant temperature it might be that needs cleaning as Scott suggested or it might be a red herring. iirc there isn't any ATF flow through the coolers when the TC is locked so the temperature the sensor is seeing might be just the static fluid in the pipe not the actual pan fluid temperature ...

 

[Lorin]

I did check that the radiator looked clean/free of blockages but maybe I need to inspect it more closely - being mostly obscured by the condenser it is possible it's worse than a cursory look over suggested. The condenser itself is clear and looks in good shape. Ideally I guess I should probably remove the radiator and have a good inspect. I'll check it out and report back.

 

[Sarmajornz]

The fluid flows through the cooler all the time as the oil pump is driven off the transmission input shaft, even when the torque converter is locked up.

My temp goes from around 80 degrees to 125 ish climbing over long mountain passes (the Mangamuka gorge, NZ) with switchbacks every 3-400 meters for 10 kms.
Cools back down on the other side quite quickly. No noticeable increase in engine temp. I am running after market steel wheels and 285/75R16 km2s.

Aftermarket cooler should be before radiator cooler so that fluid return temp is constant.

 

[Lorin]

The fluid flows through the cooler all the time as the oil pump is driven off the transmission input shaft, even when the torque converter is locked up.

My temp goes from around 80 degrees to 125 ish climbing over long mountain passes (the Mangamuka gorge, NZ) with switchbacks every 3-400 meters for 10 kms.
Cools back down on the other side quite quickly. No noticeable increase in engine temp. I am running after market steel wheels and 285/75R16 km2s.

Aftermarket cooler should be before radiator cooler so that fluid return temp is constant.

That's good info, thank you.

 

[Rich Sims]

I would say your torque converter isnt locking up, when at running temp you it should lock up at 50mph in overdrive light load and you will feel and see the rev counter drop down, after that there should be little to no heat build up and its the water that will be giving the oil its heat if you still have your original cooler in the bottom of the rad. What revs do you have at 55mph when at running temp?

This is under normal load low power, on hills or under the load and the torque converter isnt locked then skys the limit and its down to your cooler to keep it under controll. I think my standard warning light comes on around 120 c

 

[Jon Wildsmith]

My understanding is that on an A442F a relatively small volume of fluid that is being pumped is diverted through the cooling circuit and that flow / circuit is via the torque converter and is interrupted when the TC is locked. I have never tested for flow on the move with the TC locked to verify that but when I had an auto 80 the aux cooler (oem cooler bypassed) when checked after a good run with the TC locked would be at ambient temperature not the 100+ degrees on the trans temp gauge ...

 

[Lorin]

Rich, what you're saying would make sense, only my TC is definitely locking up. It is very apparent when it locks, usually around 50mph and trans temps if high always drop when the TC locks. I am also able to lock in any gear via a switch and again it is very obvious. The trans temp warming light on an 80 comes on at 150C and goes off at 120C.

Jon, it seems your test is pretty conclusive then - indicating no flow through the cooler when the TC is locked. Given where the temp sender for the aux gauge is located and the fact there's no ATF flow to the cooler/radiator, it doesn't seem likely that the observed rise in trans temp is due to a spike in coolant temps. I have just been lent an IR thermometer so I'm going to check the pan, engine radiator and transfer temps when the aux gauge is showing the box as running hot. The truck is away having the underside treated at the minute though, so I won't get a chance to have a look till Friday. I'll report back then.