Update on towing 5,270# TT with '78 F150 Supercab 6.6L w/ 2.75 rear end

[Scottbral]

Hey guys,

I towed the camper home a couple days ago. It took about 4 hours but it went pretty OK. I did put the Air Shocks in, trannny cooler, tranny filter with temp sender and gauge. Running with no load the tranny was running about 145-150 degrees.

When I put the tounge weight on the truck with only 30lbs of air in the shocks the truck was level even before I put the WD bars on. It carried the weight great and handled pretty good too.

Acceleration was slow but tolerable. Once on the highway at 55mph and things got good and warmed up the tranny temp hovered around 235-240 and on hills it hit a high of 255-260 for short bursts going 50 mph up hills. I felt like this was too high so I installed a second oil cooler yesterday to see how that works. Haven't tested it yet with the trailer but seems like it dropped the temps by about another 20 deg. What should my highest safe tranny temp be?

My tires are 27", rear end is 2.75:1. With no trailer I tested the rpm on the highway in 3rd gear today and at 55mph I ran 1,950 rpm. At 60 mph I ran 2,050. At 65 I ran 2275. At 70 I ran 2,400rpm.

Whle towing the truck only came out of third if I floored it on hills. That means I was powering at around 2000 rpm pulling 5,200#. Not great.

I'm considering changing the rear end before I leave for Florida. I'm interested to know what you guys think I should switch to.

3.55:1 will give me 2651 rpm at 60 MPH
3.7:1 will give me 2763 rpm at 60 MPH
3.8:1 will give me 2837 rpm at 60 MPH
3.89:1 will give me 2905 rpm at 60 MPH
4.11:1 will give me 3,069 rpm at 60 MPH

What do you think??

Thanks

[overlander63]

The 400 has plenty of torque, but not so much horsepower. You can probably change the rear to a 3.7 rear, it will give you more ummph, but not so much the engine will be screaming at highway speeds without the trailer.

[Action]

I think the gear choices you will have are 3.08, 3.55 and 3.73 & 4.10.

>>>>>>>>>Action

[Scottbral]

Originally Posted by Action

I think the gear choices you will have are 3.08, 3.55 and 3.73 & 4.10.

>>>>>>>>>Action

You may be right - I was using this chart I found online and it seemed to make sense and be credible.

Ford Rear End Ratio Chart

[SteveH]

I'm not an automotive engineer, but I think you NEED a gear change because I think those transmission temperatures are in the uncomfortable range.

[ROBERT CROSS]

Air Shocks...Why?

[Action]

Originally Posted by SteveH

I'm not an automotive engineer, but I think you NEED a gear change because I think those transmission temperatures are in the uncomfortable range.

I would concur with this statement.

For those that like to read
How often should the automatic transmission fluid... — Yahoo! Autos
For those that a picture is worth a 1000 words
Transmission temperature/failure chart.

225 degrees and below is the place to be.

>>>>>>>>>>>Action

[SteveH]

Action, I wonder if that chart takes into consideration the new synthetic transmission fluid that most manufacturers are now using?

[Action]

Not likely. And I would use the synthetic strength fluid as an insurance policy and shoot for 225 degrees or less. Stuff happens and if the temp goes over the artificial fluid strength will be there to take the punishment.

>>>>>>>>>>>>>Action

[Action]

After all a $100 fluid change can be done 10 times at 30000 mile intervals before one gets anywhere close to a transmission replacement. By that time one would have 300K on the clock.
And that is a fer trek for any auto tranny.

>>>>>>>>>>>>>>Action

[SteveH]

My truck has a transmission temp guage, and last summer pulling in the mountains I saw it get as high as 225 on some of the grades, but I didn't know what that really meant. Glad you posted the link to the temp chart.

My truck also uses the sythetic fluid, and the manufacturer recommends it be changed every 100,000 miles, unless it's used for towing, and then they recommend every 50,000 miles. I can live with that.

[SteveH]

I had the opportunity to talk to a GM service manager today, and asked him what GM considered an overheated transmission. He could not answer my question, but asked had my "overtemp" light ever come on, and I told him it hadn't.

He them went to his computer, and told me the trans overtemp light on my truck was programmed in the computer to come on when the transmission temperature reached, or exceed, 266 degrees for five seconds or more.

So, based on that, at least GM, considers 266 degrees tranmission temperature to be overheated, at least on my vehicle.

By the way, the hottest I have ever seen the trans temp on my vehicle was 230 degrees pulling a long mountain pass in second gear with the Airstream in tow.

Of course, your vehicle may be different.

[Action]

I would take the factory setting and the light coming on for that concern the same as an "idiot" light for engine oil pressure. If it comes on, likely it's too late the fluid is burning and the fluid needs to come out.

(And I used to work for the factory)

I am kind of funny about oil. I generally never buy brand name specific. I buy by specification. And then I drain and refill frequently. My current TV has about 40,000 miles on it since I bought it 12 years ago. The tranny fluid has been replaced at least 4 times. Once to modifiy the shift schedules in the valve body just after I bought it. Admitedily I am running this TV at max load when I tow my boat. I have 2 added coolers in addition the the factory radiator cooler. I also generally tow in outside air temps that most of the peeps on this forum will never see. It's a tough life for my tranny. All I offer it is more frequent changes and a couple of extra coolers.

>>>>>>>>>>>>>>>>>>>Action

[ROBERT CROSS]

Originally Posted by SteveH

I had the opportunity to talk to a GM service manager today, and asked him what GM considered an overheated transmission. He could not answer my question, but asked had my "overtemp" light ever come on, and I told him it hadn't.

He them went to his computer, and told me the trans overtemp light on my truck was programmed in the computer to come on when the transmission temperature reached, or exceed, 266 degrees for five seconds or more.

So, based on that, at least GM, considers 266 degrees tranmission temperature to be overheated, at least on my vehicle.

By the way, the hottest I have ever seen the trans temp on my vehicle was 230 degrees pulling a long mountain pass in second gear with the Airstream in tow.

Of course, your vehicle may be different.

On the newer GM's if the trans overheats the ECM will go into "limp" mode until it gets below 260 degrees. If everything is working properly, it's hard to overheat 'em.