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Old 12-02-2020, 05:17 PM   #1
Mini-Skoolie
 
Join Date: Feb 2020
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Year: 1999
Coachwork: Freightliner
Chassis: FS65
Engine: 6 cylinder Cummins
Transmission Cooling

the bus has a problem with the transmission overheating on any hill.

Once in a blue moon when it gets to 180 degrees i can hear some pump or fan kick in and the temperature goes back down, but usually it climbs up past 200 and gets near 350 and i have to stop.

does anyone have any idea what might be cooling it down? i figured it would be pretty big because i can hear it running over the engine, but i didnt see anything like that when i looked down there.

if i could find out where it was i would just wire it up to a switch, on the highway it never overheats.
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Old 12-02-2020, 06:59 PM   #2
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your fan clutch kicks in and cools stuff down. ~200 degrees

its loud, it roars, everyone on board looks at each other like wtf. that is normal.

the fan in front of the engine just freewheels until it gets hot, then the clutch engages and it starts to move air thru the radiator.


as for running hot on hills, thats normal too. as long as its not too hot. 220ish is the line where i'll pull over and let cool down.

don't be a lead foot on hills. turn on your flashers and run up the hill at 1/2-3/4 throttle. your rig will be much cooler at the top.
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Old 12-02-2020, 08:59 PM   #3
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your temps should never get above 210.
assuming your roaring is a viscious clutch fan and you are experiencing these problems i would say a coolant system flush and recharge with new fluid?
verify guage readings.
check what transmission you have and see if it is piped into your radiator? some were split where they served the engine cooling the tranny cooling and AC if equipped?
if piped so the tranny heat also adds to the engine heat which can throw that guage off and you think just engine heat but its actually the tranny giving you issues?
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Old 12-02-2020, 11:24 PM   #4
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The FS65 has a separate gage for the trans fluid so the temp stated here is not referencing the coolant. He has a another issue with his coolant temp reading low as posted earlier today.
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Old 12-03-2020, 12:56 AM   #5
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350 degree transmission oil temp?

If my transmission oil temp got to 350, I would think that I roasted that turkey.
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Old 12-03-2020, 08:40 AM   #6
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Quote:
Originally Posted by bus-bro View Post
350 degree transmission oil temp?

If my transmission oil temp got to 350, I would think that I roasted that turkey.



i ran an original AT540 up to 385 in the mountainjs a couple years ago.. boiled out the fluid.. let it cool down, poured in a couple gallons of fluid and drove it the next 2000 miles home LOL..


but yeah 350 is hot..
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Old 12-03-2020, 09:32 AM   #7
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350 definitely didn't do it any favors.

Most of the time, when it gets that hot the trans is fully loaded, so clutches slip and glaze, steels turn blue and warp, every fluid seal in the thing is now brittle and unreliable. The hotter you go above max temps and the bad just seems to spiral out of control.

Opinions will vary on what is too hot, and a lot of it depends on the transmission, the fluid being used, and the location of the gauge/sensor. I'm going to assume you have an at545 and the sensor is in the pan?

According to allison, they have several fluid temp specs for the AT,MT, and HT transmissions. Here they are straight from the operators manual.

Sump Fluid, Minimum Continuous 100F (40C)
Sump Fluid, Typical Continuous 200F (93C)
Converter Out Fluid, Maximum 300F (149C)
Retarder Out Fluid, Maximum 330F (165C)
Sump Fluid, Maximum 250F (121C)

And those are the temps I go by.

So if you get close to exceeding any of them, I recommend to stop driving, pull over, and let it idle in neutral to cool back off.

Going that hot like you did might not have done immediate damage, and it might still drive and function. But, in my opinion, you're on borrowed time with every mile.

Truthfully, if you're boiling the fluid on every hill you encounter, the transmission is likely junk and the clutches are already slipping. So I'd start looking into a rebuild or reman. Or, if it's an at545, you can look into upgrading to a better transmission. Some have jumped to the mt643, others have made the bigger leap to one of the electronically controlled allison options.
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Old 12-03-2020, 01:09 PM   #8
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Quote:
Originally Posted by Booyah45828 View Post
350 definitely didn't do it any favors.

Most of the time, when it gets that hot the trans is fully loaded, so clutches slip and glaze, steels turn blue and warp, every fluid seal in the thing is now brittle and unreliable. The hotter you go above max temps and the bad just seems to spiral out of control.

Opinions will vary on what is too hot, and a lot of it depends on the transmission, the fluid being used, and the location of the gauge/sensor. I'm going to assume you have an at545 and the sensor is in the pan?

According to allison, they have several fluid temp specs for the AT,MT, and HT transmissions. Here they are straight from the operators manual.

Sump Fluid, Minimum Continuous 100F (40C)
Sump Fluid, Typical Continuous 200F (93C)
Converter Out Fluid, Maximum 300F (149C)
Retarder Out Fluid, Maximum 330F (165C)
Sump Fluid, Maximum 250F (121C)

And those are the temps I go by.

So if you get close to exceeding any of them, I recommend to stop driving, pull over, and let it idle in neutral to cool back off.

Going that hot like you did might not have done immediate damage, and it might still drive and function. But, in my opinion, you're on borrowed time with every mile.

Truthfully, if you're boiling the fluid on every hill you encounter, the transmission is likely junk and the clutches are already slipping. So I'd start looking into a rebuild or reman. Or, if it's an at545, you can look into upgrading to a better transmission. Some have jumped to the mt643, others have made the bigger leap to one of the electronically controlled allison options.

im really surprisde i didnt de-laminate the friction discs and end up sitting a few miles later.. I suppose i should pull it out and rebuild it
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Old 12-04-2020, 12:44 PM   #9
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A transmission cooler would help.

You probably cooked a lot of rubber seals and gaskets at that temp.
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Old 12-04-2020, 11:26 PM   #10
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The radiator fan influences transmission temperature more than one might at first guess.
Think of it this way: There are two items that regulate the temperature of the water in the engine cooling system: The thermostat and the fan.

The thermostat regulates the water in the top of the engine and radiator.
The fan regulates the water in the bottom of the radiator and engine.
And where is the transmission cooler located?
Bingo! At the bottom.

So, I installed a manual switch to control the fan. It is in parallel with the automatic sensor/switch so I can bypass the automatic one and run the fan whenever I want. (And the fan will still come on automatically, regardless.)

Now, when I see the first glimmer of uphill road in front of me, I start the fan. One might say I give the transmission-branch of the cooling system a head start on keeping the bottom of the system cool. This makes a huge difference on the transmission temperature all the way up the hill.

All for a five-dollar toggle switch and a few feet of wire.
In a pinch, you can simply put both terminals on the automatic sensor/switch together and have the fan run all the time.
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Old 12-05-2020, 05:00 AM   #11
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The switch works great on mechanical engines or those which just use a temperature switch in the engine, but those with ECM controlled fans you want to carefully look over how the fan is wired, if a relay is used you may be ok with a bypass switch, if not then overriding can cause damage to the ECM or throw a code if the ECM detects an invalid state on the fan.

Depending on where you just blast 12 volts at there may be a fly back circuit which could be damaged. Again I’m older to separate fan switch types it’s a non issue but on electronic engines do research before just putting in switches
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Old 12-05-2020, 10:46 AM   #12
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Good point by both of you.

Newer electric engines you might find they can have a fan switch added/enabled in the ecm. Might just have to add a pin into the connector, or you might find a harness under the dash unused for it.

But if you have a mechanical fan, there isn't much they can do aside from adding auxiliary fans and coolers to the system. Turf has a nice add on system he made, look at his build for pics/info. But if you're transmission is overheating that often, you have other underlying issues that need addressed, and a lack of cooling capacity isn't likely the problem.
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Old 12-05-2020, 11:35 AM   #13
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i got called out ..... what?!?!


i modeled that system on what elliot is saying a couple of years ago.

i added a big cooler, and electric fan on a switch or thermostat.

update:
fans removed, manual switch removed

the big cooler works fine by itself... maybe too well. my trans runs cool all the time, maybe 150's. i should add a bypass, so that it can heat the fluid up more before it sends it to the cooler.

the problem i had with the manual switches was i'd forget about them. i felt smart in traffic or climbing a hill..... but then you stop at a store....walk away for an hour and come back and the batteries are dead. all of a sudden... not so smart.

i hooked the new fans to the same tstat as the fan clutch for a while, but since the big cooler, i just havent got hot.

to avoid another accidentally dead battery..... the wires got pulled out.

my cooling comes from the added volume from the cooler, not the fan. the fan made so little difference, its not worth it.


oh... and i swapped in a locking trans and got rid of that 545 slipomatic
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Old 12-05-2020, 01:03 PM   #14
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Just a note that I added an oil cooler to my racecar using the oil cooler from a RX8.
They have a built-in simple mechanical thermostat so it bypasses the cooler until the oil reaches like 150f or something. You can google it, but it worked great.
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Old 12-05-2020, 01:37 PM   #15
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i got called out ..... what?!?!
Yup, you're my example of how it should be done. It's how I would have done it. You can feel honored, or go and seek help for both of us lol
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Old 12-05-2020, 01:42 PM   #16
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Quote:
Originally Posted by BeNimble View Post
Just a note that I added an oil cooler to my racecar using the oil cooler from a RX8.
They have a built-in simple mechanical thermostat so it bypasses the cooler until the oil reaches like 150f or something. You can google it, but it worked great.
Good find.

I found and bookmarked a few years ago that some air cooled volkswagens had an oil thermostat that routed oil to the cooler or bypassed back to the engine.
VW Engine Oil Thermostat
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Old 12-05-2020, 03:22 PM   #17
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Yikes! Thank you for bringing up the matter of computer controls! I am 68 years old and live on an analog planet, so I never gave it a thought.

I now see we are working on a 1999 Cummins, so it is probably a 24-valve which is computer-controlled. Telling these apart is easy, since they switched to electronics at the same time they upgraded to 24 valves:
Six individual valve-covers = mechanical 12-valve.
One long valve-cover = electronic 24-valve.

Over-cooling an automatic transmission can indeed be an issue. Note the minimum sump temperature of 100 F kindly listed in an earlier post -- it should be more like 200 F for happy travels.
This is why an extra transmission cooler should always be installed before the regular one which is part of the engine cooling system.
This way, if the extra cooler lowers the temperature too much (light load, level road, winter weather), the engine water will bring it back up to happy working temperature before it returns to the transmission.
This is one lesson they hammered into us at Chrysler's auto mechanic school in Detroit back in my day.
(That, and soaking wheels in water to tighten the spokes by swelling the wood. )

As for inadvertently leaving a switch on and draining the battery while parked... easily avoided by taking the power from a switched source (as the factory indeed does with this circuit) -- ignition switch off = everything off.

By the way, on Millicent, I also have installed a battery master switch. It is located near the driver's left knee, so I easily flip it on and off when I get into and out of the seat. This takes care of the rear-most dome-light or anything else that might suck away electrons while parked.
In this photo, I have my fingertip on the switch.


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Please stay home until this pandemic is more safely behind us. And wear a mask, properly, when you must go out for groceries. The life you save may be mine. Thank you!
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Old 12-05-2020, 03:41 PM   #18
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On a computer bus you can usually leverage the A/C input to start the fan, I know for sure you can do it on Navistar , Cummins medium duty ECM should also have the A/C input. When it detects voltage it assumes you are in a truck with AC and need the fan on continuous. It may need enabled in the program
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Old 12-09-2020, 04:55 PM   #19
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The simplest way to ensure that if the engine is off the auxiliary loads are off is to add a voltage sensitive relay for the auxiliary loads. Also known as a charge relay because it only closes when the battery is seeing charging voltage thus the engine is running. Most boats will have them to isolate either engines charging/starting circuits from each other. There are 12V and 24V charge/sense relays available or you can make your own version. BlueSea will have them.
Good luck with your project.
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Old 12-09-2020, 07:25 PM   #20
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im really surprisde i didnt de-laminate the friction discs and end up sitting a few miles later.. I suppose i should pull it out and rebuild it
I have a good used GEN 3 3000 series I would make you a good deal on if you are interested.
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