Transmission Temp Question

[mpaulson]

In low range I really doubt that you would ever be able to tell the difference in the rear end gear ratio because there is so much torque multiplication going on with low range that the difference would be so unnoticeably small.

 

[Ozymandias]

Some doesn't know that if you switch manually to Second Gear then the Converter locks-up too.
So its possible to drive pretty slow uphill and downhill with locked converter.

 

[Too Tall]

Another thing that I read was a higher read ratio produces more heat. IE; a 4:11 would run hotter than a 3:73 or 3:42

 

[HorseMan-AR]

You can't get anything but 3.42 in a SRW, can you?

Thanks TransEngineer. Your explanations are always a great source of information.

In 2013, I moved to the Aisin on my RAM 3500 which I work pretty hard off road in 4x4 low. I notice the Aisin runs a little warmer than the 68 rfes' did but it feels stronger even with 3:42 gears. In Arizona summer with outdoor temps running ambient 100+ degrees, I've seen the Aisin as high as 205 degrees pulling livestock off road up 10-15% mountain grades in 4x4 low.

Highway it runs around 160 to 175 degrees in the summer. I'm always watching the temps but so far no ill effects and the Aisin runs strong.

I just ordered a 2015 RAM 3500 Aisin with the 3:42 gears. Would 3:73 gears in 4x4 low run the transmission noticeably cooler?

 

[transe]

Another thing that I read was a higher gear ratio produces more heat. IE; a 4:11 would run hotter than a 3:73 or 3:42

From a transmission standpoint, I believe a higher (numerical) axle ratio will allow the trans to run cooler. Not sure if you were referring to the axle temp or the trans temp. A lower (numerical) axle ratio requires more torque (from the trans) to maintain the same road speed, so in general I'd think the transmission in a 3.42 axle truck would run hotter than one in a 4.10 axle truck. Of course, the 4.10 axle would also be running at a higher speed, so maybe that tends to offset the difference due to torque. I'm not a cooling systems expert, so my opinion may be all wet.

I do, however, have one tool that may provide a more definitive answer. I have a program that simulates transmission thermal operation during one particularly severe portion of our powertrain endurance testing. I can change axle ratio in that simulation, and see what the effect is. So I ran the simulation three times, for a 3500 truck with 68RFE trans, at 35,000 lbs GCW, running in 90°F ambient. Now admittedly, you wouldn't want to run that truck with a 3.42 axle (in real life), but at least this gives us a comparison. I ran the 4.10, 3.73, and 3.42 axle ratios. Here are the results, starting in each case with a 180°F sump temp:
• 4.10 axle = Peak sump temp of 216°F
• 3.73 axle = Peak sump temp of 224°F
• 3.42 axle = Peak sump temp of 236°F

So it looks like my gut thinking was probably correct.

 

[Apache]

Interesting, from your simulation the lower ratio may explain my slightly higher trans temp. Probably the axle not the transmission is throwing off a few more degrees of heat.

If there is a difference it's not enough for me to trade the highway fuel economy and lower rpm of the 3:42s' for 3:73s'.

When I ordered the 2015, we noticed the 3:73 is now offered for the 2015 SRW trucks as a no charge option (DME).

 

[JLandry]

I'm not a cooling systems expert...

TranEngineer,

I know you said your not a cooling systems expert, but by chance do you have access to any engineers who might have any insight on the effect of installing a mesh window screen on front of the radiators/coolers to keep out bugs and pebbles?

I've searched the web and can only find a lot of opinions but no real world testing. It seems obvious that putting anything in front of the radiators must affect cooling capacity, but maybe a window screen is insignificant except under the most extreme conditions.

Thanks in advance,

John L.

 

[transe]

I know you said your not a cooling systems expert, but by chance do you have access to any engineers who might have any insight on the effect of installing a mesh window screen on front of the radiators/coolers to keep out bugs and pebbles?

I've searched the web and can only find a lot of opinions but no real world testing. It seems obvious that putting anything in front of the radiators must affect cooling capacity, but maybe a window screen is insignificant except under the most extreme conditions.

According to my contact in Cooling Systems, a window screen would not be insignificant. It would increase the airflow restriction through the cooling system, resulting in somewhat higher temps.

I know that a window screen doesn't seem like much (of a restriction), but try holding one out the window while driving down the road and you'll see that it does cause a significant drag. I would also point out that with a screen, you'd still get plugged up with bugs and debris (they'd just be stuck on the screen, rather than in the radiator, etc.). Of course, it's probably easier to remove (and clean) a screen than the radiator!

So bottom line, my contact says if you live in an area with an extreme bug problem, you'd have to weigh the benefits of a screen (and easier cleaning) against having slightly higher operating temps. But in general, he would not recommend adding a screen.

 

[JLandry]

But in general, he would not recommend adding a screen.

Those are some very good points to consider.

Thank you VERY much for checking into that... and the speedy reply.

Best regards,

John L.

 

[Cummins12V98]

I would go at least a 1/8" hardware cloth. IMHO screen is too restrictive.

 

[Dieselnerd]

I've run window screen on every Cummins I've owned and never experienced any higher temperatures on anything. Bugs clean off easily with a brush instead of slowly clogging up everything inside where it's very hard to clean.

Geno's even sells a kit called a condenser guard that makes it easy to install a screen.

 

[Cummins12V98]

I have used the screen in the past and have no data to back up my thoughts that 1/8" hardware cloth would be better.

 

[mpaulson]

I've also used aluminum window screen on every truck I've owned as well and have never had a heat problem. It wouldn't make the truck run hotter, it could just cause the fan to kick in a little sooner, that's all. Now granted, I haven't towed heavy in temps like you southerners do, tops for me is 37 degrees C/100 degrees F this summer and had no problems climbing 6-8% grades towing a 30' RV with the deck on the truck that you see in my signature picture, NOT aerodynamic!. I also have the bumper slot totally blocked with a 24" light bar, and still no heat problems.
Put the screen on.