When I purchased my truck about 5 years ago, I was told it was on its third transmission since the awesome 7.3 diesel engine has 200,000 miles on it. At first I noticed that going up a hill with the truck empty would cause the tranny temp gauge to climb to 250 degrees F. Nowadays when I tow my boat or carry my camper, the temp gauge pegs over 250 and the overdrive light flashes steadily when I do any hill climbs. I have had the fluid flushed several times, and I have a large fan powered oil cooler too. I also recently noticed that the tranny heated up when the truck is just idling in place.

I just got back from burning man and during the 800 mile round trip the overdrive light started flashing at least 6 times and the temp gauge read over 210 the whole time, with it pegging over 250 during hill climbs. No fluid boiled out but I am very concerned that damage is being done.

Is it possible there is a clog in a line somehwere restricting the flow of oil? Or should I replace the tranny?

 

Do you have a radiator cooler as well? (E99 models had no rad cooler). I would try to blow out the cooler with compressed air and see what comes out. The blinking O/D light means there is a code or codes present in the system, you need to see if it's stored and get it scanned. I would replace the fan cooler with the 6.0 transmission cooler its better than any aftermarket kit out there. Is your truck all stock? Tire size, gear ratio etc..

 

could be a faulty temp sender as well.

the right guy with the right scan tool would be able to diagnose that in a hurry.
(if the truck is cold, and it's 80* outside, the sender should read 80* as well)

i would think with temps constantly hitting 250, your fluid would look burnt

 

Fluid doesn't burn at 250°F. It needs to get well over 300°F for that to happen.

First you need to determine if you have a cooler in the radiator. Are there fittings with 3/8" lines going into the bottom of the radiator? If so, you have one, if not, you need one.

The next thing to do is to see if there is junk/debris/leaves/small animals plugging the cooling fins on the coolers and radiator. If the airflow is restricted the transmission isn't going to cool.

Next is to see if there is enough flow through the cooler(s.) Warm the trans up by driving it. Get two containers, each at least a gallon. Remove the cooler line where it attaches to the rear of the trans. Point the line into the first container and have someone start the engine, leaving it idle in park.

Once the flow is steady move the line to the second container. Leave it there for EXACTLY 15 seconds then move the line back to the first container and shut the engine off.

If fluid sprayed out of the open fitting on the transmission it failed the test. Some dribble is normal. If there is less than a quart in the second container it also failed.

If it failed there are two possible causes. One is the bypass is stuck open. The other is that the bypass is open because the cooler is restricted.

 

Thanks for the suggestions. I will check the radiator this evening to see if there is a built in cooler, but I am pretty sure there is as I recall keeping it in line with the aftermarket cooler.

I also know that there is no bypass line on the tranny as I sealed it off years ago since it only works to heat the oil faster.

The oil is not burnt as it is synthetic and still looks clear and red and right at the full mark.

 

No it doesn't heat the oil faster. The ONLY thing the bypass does is to maintain lube oil flow to the rear of the transmission if the cooler circuit becomes restricted.

With this new information I have a theory. The heat is coming from a combination of a restricted cooler and the trans running with little or no lubrication due to the restricted cooler flow and no bypass. If my theory is correct you need a trans rebuild, hopefully with a new bypass that works.

Run the cooler flow test and we'll know.

 

I find it odd that you are on tranny 3 with only 200K miles on it. I just recently had mine rebuilt for the first time and I am at 278K. Anyway.... Your issue could be due to restriction in the cooler lines as the others have said. However, the cooler that does come with the 7.3's does tend to lack the abillity to do its job.
A cooler from a 6.0 is what most 7.3 PSD owners are going to because it flows more and does a much better job.
As for the flashing light.... I think that has something to do with the VSS, (variable speed sensor) that is on the rear differential. Easy and cheap to replace.

There are quite a few guys that will have more info for you. Maybe if the moderator puts this thread into the 99 - 03 Diesel forum, they can lend a hand.

 

When I purchased the truck, I became the third owner. I was told by the seller that the original owner pulled a fifth wheel and had the tranny replaced twice. Of course the seller also swore that a new tranny had been installed the last time and after I bought the truck and went through the records, I found it was merely a rebuild, not a new tranny.

 

If the trans went out that often I would be very concerned with hard pulling. Hard pulling and a heavy foot = high EGT's as well.

 

No worries about the EGTs, the truck has a 5-inch exhaust all the way with no cat.

 

I finally ran the flow rate test on the transmission and here are the results:

With flow through the aftermarket tranny cooler, 15 seconds yielded ~1/3 of a quart.

Bypassing the aftermarket cooler, 15 seconds yielded ~1/2 quart.
This seems really slow compared to Mark's suggested test results that the oil should flow at one quart in 15 seconds. There was no oil coming out of the rear tranny fitting,

What makes the oil circulate in the transmission? Is there a pump that can be replaced rather than rebuilding the entire transmission or is it the torque converter that pumps the fluid?
The wierd thing is that after I removed the bypass feature on the tranny years ago, the overheating problem did not start immediatly. It took at least 2 or 3 years before the overheating started.
Driving the truck around unloaded is not a problem, its when I carry a camper or tow the boat over a mountain that it really heats up.

 

There is a pump in the transmission. The torque converter does not pump fluid through the trans.

You may have a partially plugged cooler. It would be a good test to see what the flow is measuring it right out of the front fitting on the trans, without the coolers in the circuit.

 

I already did the test with the cooler bypassed. There is no cooler in the radiator, and when I bypassed the aftermarket cooler I installed I only got a flow rate of 1/2 quart per 15 secs.

Can I just have the pump replaced or do I need a rebuild?

 

You can replace just the pump. That may or may not fix the problem. The pump may be supplying what it is supposed to, there can be leakage inside the trans.

 

So if I go for a rebuild, what are your thoughts on adding a larger oil pan with cooling fins, such as Bd Diesel Part bdd1061710 - BD Diesel Big Daddy Transmission Cooling Pan