Auto Gearbox Cooling

Transmissions, Transfer Boxes, Axles, Drive Shafts
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Auto Gearbox Cooling

Post by JG Shields » 22 Oct 2010 08:28

Please help me with an answer.

What is the standard operating temperatures for the auto gearboxes when driving in 4x4 and what is the purpose/need for the Patrol auto geabox oil to have a double cooling systen i.e. the vent radiator and the water cooler inside the main radiator?

Some arguments claim that the water radiator-cooler serves its purpose when crawling in 4x4 with hardly no airflow over the vent radiator to provide adequate cooling for the oil. Other arguments claim that the oil unessesary heats up the water in the radiator and may result in overheating problems. Subsequently some owners by-pass the water radiator cooler. Either way, I can forsee pro's and con's.

Nissan must have had a logic reason(s) for the design. What are they?
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Re: Auto Gearbox Cooling

Post by Freakazoid » 23 Oct 2010 14:50

I would also like to know. I have the odd overheating problem in sand when I am gunning it. The auto light starts flashing and the bix starts jumping around. The service guy says it goes into limp mode and I just have to wait for it to cool down. I'd like a solution, as I don't want to stand around looking at other guys 4by'ing.

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Re: Auto Gearbox Cooling

Post by Gerrit Loubser » 24 Oct 2010 07:06

JG Shields wrote:What is the standard operating temperatures for the auto gearboxes when driving in 4x4...


I don't know the temperature specs for the oil in the 4.8 Patrol's autobox specifically, but typically the temperature range in normal operation would be 80 to 90 deg C. If the oil never exceeds this temperature range, it (the oil) will last for a very long time indeed (perhaps more than 100000 km).

In more severe use temperatures up to 115-120 deg C could be accepted, but if such heavy duty use is the norm, the transmission oil should be changed more frequently. The rule of thumb is that the change interval should be halved for every 10 deg C above 90 deg C that the transmission runs in the particular duty cycle.

Temperatures above 120 deg C lead to very rapid degradation of the transmission oil and should be avoided.

JG Shields wrote:Some arguments claim that the water radiator-cooler serves its purpose when crawling in 4x4 with hardly no airflow over the vent radiator to provide adequate cooling for the oil.

I don't agree. The engine cooling fan ensures air flow through the entire cooling pack at the front of the engine when the vehicle is moving too slow to generate much flow.

Neither the engine radiator, nor the transmission cooler in front of it are much use without air flow...

JG Shields wrote:Other arguments claim that the oil unessesary heats up the water in the radiator and may result in overheating problems.
The engine coolant temperature would typically range from 90 to 110 deg C. In very severe use, the transmission temperature should still not exceed 120 deg C, so there is quite a small thermal gradient between the two fluids. With a small thermal gradient, not much heat transfer takes place.

In extreme conditions, the transmission will place a bit of a heat load on the engine cooling system and would be cooled a little, but I believe the real reason for sending transmission oil through the engine radiator is actually to heat it up to working temperature (at least 80 deg C) as soon as possible after a cold start.

When driving away gently after a cold start, the engine would start to warm up to operating temperature much quicker than the transmission. This would initially generate a large temperature gradient between the engine coolant and the transmission oil that would help to heat up the latter.

Transmission shift quality is quite dependent on the correct oil viscosity, which in turn is of course temperature dependent.

JG Shields wrote:...and what is the purpose/need for the Patrol auto geabox oil to have a double cooling systen i.e. the vent radiator and the water cooler inside the main radiator?
As per the above comments, the engine radiator cooling can only help a little in extreme situations (its other function being to heat the oil to working temp), while the heat exchanger in front of the radiator will take care of the lion's share of the cooling due to the much greater thermal gradient between ambient air and the transmission oil.
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Re: Auto Gearbox Cooling

Post by JG Shields » 24 Oct 2010 12:20

Thanks for the explanation Gerrit! The reason for my question was the fact that I experienced my temp guage exceeds the normal position slightly on two ocasions recently. However it never overheated.

One - I was pulling a trailor up the mountain pass on route to Cradock from the coast. The ambient temp was 38 degr. I eased back on the pedal and the temp returned to normal.
Two - I was crawling in standing-still traffic, pulling a loaded trailor with 35 degr ambient temp. Again the guage crept up slowly above normal but returned to normal as soon as the traffic starts to move and the airflow became better.

The latter just did not feel right to me. My logic tells me it should not happen at all, especially if I listen to the other 4.8 owners who never realy complain about overheating. Lately it also felt to me as if my viscus fan came into operation too soon and to agressively. This was just a "gatsometer" reading based on no actual measurements.

Now for the embarrasing part. Since I transplanted the 4.8 engine into the previously ZD30 Patrol, I intended to replace the radiator with a new one, which I bought but safely packed away in my garage - FOUTJIE! In addition to the radiator replacement, I intended to add the second blower fan to the front (ZD30 only have one. 4.8 has two blower fans) of the cooling pack but it would required a few modifications which I also postponed because of lazyness. On Friday I decided this was it and I disasembled the entire front of my car, fitted the additional blower fan, moved the oil cooler slightly lower, cleaned the aircon radiator (what a surprise to see the masses of dirt from this radiator), replaced the main radiator with the new one and made minor modifications to the fan-cowl, which is a hand made cowl build to combine the 4.8 fan position with the ZD30 radiator configuration. This configuration never bothered me since the ZD30 and 4.8 radiators are exactly the same size and capacity. The end result is very interesting.

1. The old radiator was in a shocking condition with visible dirt blocking the outside. I am convinced that the inside core is also blocked.
2. The amount of dirt from the aircon radiator surprised me.
3. My car runs like a clock and I hardly hear the viscus, irrespective of the fact that I grined the car in a test drive yesterday.

Conclusion - If a 4.8 Patrol starts with overheating problems, clean the ..!@#$%.. cooling system. The auto gearbox cooling system also remained stock standard. I believe I solved my problem by attending to the basics instead of questioning the design. Gerrit, in terms of your explanation, I assume that gearbox oil overheating can therefore happen as a result of main radiator problems.
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Re: Auto Gearbox Cooling

Post by Gerrit Loubser » 24 Oct 2010 12:53

JG Shields wrote:Gerrit, in terms of your explanation, I assume that gearbox oil overheating can therefore happen as a result of main radiator problems.
JG, I don't think so. If the engine coolant gets so hot that it starts heating the transmission oil when the latter is at operating temperature, then the engine is already in serious trouble.

One point that I neglected to mention in my previous post is that the transmission cooler in front of the engine radiator also acts just like a heat load on the engine cooling system due to the fact that the air temperature impinging on the part of the radiator core behind the transmission cooler will be higher than ambient (due to heat transferred to the air from the transmission oil). This will reduce the temperature gradient available for engine cooling.

In reality, the system temperature of any of the cooling systems in a car fluctuates quite a bit as ambient temperatures and heat loads change, but the average vehicle temperature gauge masks the driver from this effect. People love to brag that their temperature gauges remain rock steady despite the vehicle working hard. These gauges typically have a highly non-linear response and only react when things really get dramatic, thus making them little better than idiot lights.
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Re: Auto Gearbox Cooling

Post by Freakazoid » 25 Oct 2010 11:07

Just for reference. You are not supposed to change the oil in the 4.8's auto box ever. If you do it is apparently expensive exercise in excess for R3500.

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Re: Auto Gearbox Cooling

Post by Hermi » 26 Oct 2010 11:31

Not true

Look at your manual ,and you will be surprised at the intervals.
Just get the oil from Nissan (Ettiene)and take it to Triangle gearboxes in Bellville.

I rather spend the money on a service once or twice in my lifetime than a new box.


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Re: Auto Gearbox Cooling

Post by JG Shields » 26 Oct 2010 13:29

When I did the 4.8 engine & gearbox transplant I replaced the oil (Nissan supply) @ R180/L x 10.5L.

There are conflicting guidelines/material regarding replacement interval, however most of the sources I consulted also states that the gearbox oil should be replaced with particular intervals, depending on the predominant kind of driving.
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Re: Auto Gearbox Cooling

Post by Bart Aarsse » 14 May 2014 15:31

Morning
i have converted my nissan 4.8 into a proper 6*6 campervan
When driving ( ploughing) through very loose sand the temperature does go up
i want to put a smaal autograbox oil cooler into my bull barr
the Cooler will teh be at a higher level the the radiator cooler
Do you think that i need to put an air release valve??
regrads
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Re: Auto Gearbox Cooling

Post by Chris Skinner » 14 May 2014 17:56

I dont want to hi-jack the thread, but you may find this interesting.
I tow a 3ton boat&trailer (plus full load = add another approx 600-800kg fuel and provisions etc) to ponta do ouro and back a couple of times year, and I dont think there are many examples of heavier work for the auto-gearbox. The vehicle really has to work over the hot sandy dunes once past the border. Although I never seen the engine temp gauge rise more than a needle width, I was worried about the temp in the gbox, as high temp and auto bxes are not complimentary to long life. So, I set up a temp measuring instrument directly at the gbox and have real time temps at my dash. I have been monitoring my gearbox temps for approx two years now. My intention was to never let the box get to 100degC, so in order to cater for a little instrument error, I actually will not let the gauge read over 90deg to be safe (not my intention to one day fork out for a new gbox as I believe they are fairly expensive).
What I have found is that normal operating temp (not towing) is between 70-75deg. When I tow with some reasonabe caution (90-100kmph) the box gets to between80-85deg, but if I drive hard, ie kicking down to lower gears and high revs, it starts to climb to 90 and would possibly go well above if I let it.
On the sand dunes I have to drive hard in some places over the approx 15km stretch to Ponta, but I have never seen the temp above 90deg - to me this means that the additional cooling system, ie front radiator, really does its job (some earlier posts on this thread talk about isolating the gbox radiator - I think that would be a big mistake! :naughty:
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