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Differential temp
The coaches have gauges for engine coolant temp and trans temp (often errantly labeled “Oil Temp” which infers engine oil temp). But I’ve never seen a discussion of differential temp.
I’m not a trucker, but I burned up the 10 bolt rear end on our Suburban pulling the race trailer (about 11,000# or more that trip … gulp) through the Appalachian Mountains from Richmond, VA to Wisconsin. After that experience I discovered trans and rear end temp gauges for light, medium, and HD trucks.
As I understand it, many semi’s have differential temp gauges. As I also understand it, most tandem axle semi’s drive through both axles and would likely have a temp gauge for each differential. As I also understand it 18 wheelers max out at 80,000#, so with two differentials splitting the load each is pulling 40,000# at max gross.
Our coaches drive through a single differential … pushing 48,000# to 52,000# plus often a toad or trailer or even a stacker up to 20,000#.
If we’re running with these kinds of loads on a single differential, shouldn’t we be worried about the differential temp if running in hilly or mountainous country? Shouldn’t the coach have a differential (rear end) temp gauge?
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15 Replies
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Some coaches did come equipped with a differential temperature gauge and owners with that gauge have used them to their advantage. One has posted how the change from conventional gear oil to a synthetic has shown a drop in the differential temperature.
We have to keep in mind that our coaches are designed for a specific gross weight and also have a combination vehicle weight which defines towing limits for example. I would guess if we do not tow anything, or if we do, it is as light as possible we likely will not do any harm to our drivetrain. But as is true with everything, as we reach the upper limits of towing, tire weights, engine temps, etc. we are subjecting those components to extreme conditions and they are likely to compromise the service life of those components.
I would guess if a differential temperature gauge was available at one time then one could be added.
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The diff temp is the second most critical gauge in my cluster behind only the speedo. My 1990 coach came with a diff temp gauge which alerted me to it’s importance. Under extreme conditions with these coaches, it is not difficult to exceed 300F diff oil temp. I was at Mira Loma several years ago when the red Newman-Haas Indycar coach that pulled a kitchen equipped stacker was receiving a new rear end at about 100,000.
I’ve had numerous problems with drive axle seals on both coaches, but a recent (80,000 miles ago) change of seal brand may have helped. No actual diff repairs yet.
When I acquired my 2000 coach, I added a diff temp gauge. I pull a stacker and regularly run gross weights of approx 70,000# in high temp conditions (120F) at high speed (75mph). In an effort to reduce temps, I have eliminated rear air flap to attempt better air flow, switched to synthetic lube (25,000 mile change schedule needed and verified by oil analysis) and gone to super-single wheels on the drive to allow better airflow over exposed wheel bearings, seals, and brakes. All helped.
The brakes are not the problem for me as the diff temp now seems largely dependent on ambient temp and speed. At 60 mph, nothing is critical. When I’m running 75 mph, I watch the diff temp and if it gets to 260F, I begin reducing speed in 2-3 mph increments until temp stabilizes at 260F. I have had to go as low as 65mph.
In short, anyone who tows any kind of significant weight at high speeds should absolutely have a diff temp gauge and, if running dino oil, should probably be using lower target temps than mine.
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Joe: How hard was it to add the differential temp gauge? Was there already a bung for the sending unit? How did you get a wire up to the dash?
You mention 25,000 mile changes; is that for the differential lube?
I don’t know why anyone would want to run dino oil or lube anymore (unless it was absolutely necessary). Synthetic tends to run cooler, is slipperier, and you shouldn’t have to wait for it to warm up and not be so thick.
Speaking of the rear differential, does anyone know if the gear set in the differential has had the REM surface finishing process done? Everytime I take my racing trans or rear gears the gear guys ask if I want them “REM’d”. I used to ask why, they said it reduces heat and friction and said “all the front runners do it”. I’ve played with REM’d and non-REM’d gears at the PRI show a few times and it appears there is a marked difference.
I’m thinking that given the few miles we put on our coaches, it probably isn’t worth the effort. Any of you guys running big truck fleets 50,000 to 100,000 miles a year, did you use the REM process on the differential gears and the manual transmissions?
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I have a different read on this subject. We never ordered rear axle temp gauges on any truck we had, and never needed them. Rear differential failures are almost unheard of today in vehicles that are well maintained. If you never change the lube, or have a seal leak and run them low, that is a different problem.
One more gauge for drivers to ignore…
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Yes, 25K on my diff lube or iron gets high.
Yes, there is a bung. I’m running a 2 1/8″ vdo gauge in the upper left area that no longer contains 2 gen gauges…they got moved to the main kitchen panel and are digital. Also have outsde temp gauge there.
Can’t remember where we found a wire, but I might have appropriated something. I know I’m not using my rear (or front) radar detectors.
1999 XLV X-6872, liberty #440 -
Actually, you probably don’t have my dash configuration. I have a bastard XL with an XLII dash…I guess there were a few at the end of the run.
As I recall, on my 1990, I replaced the 2 air pressure gauges with a single double-needle gauge and added the diff temp gauge but it was a Marathon. Not sure what you’ve got.
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Joe: What kind of temps do you see? Diff temp rise significantly when towing a heavy load or going up a hill or mountain? Do you get valuable information from it? Worth out while to put one in?
I was waiting for Mark from Duluth to comment on this thread. Interesting they didn’t have diff temp gauges on their tractors. My thoughts were that a semi fully loaded to max legal gross is splitting 80,000# between two differentials and we’re squeezing 48,000# to 60,000# through just one differential. That was my concern. I’m not enough of an engineer to know what kinds of problems that could create other than to know that more weight results in more stress and heat. Jon or Tom (maybe someone else) commented that the drivetrain should be sized for the expected load, which I would tend to agree with. I wonder if our single differential has the same capacity as each of the differentials on a semi or if our’s is of a larger capacity.
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Dick, with what you’re towing, just switch it over to synthetic. I’ve towed a lot more weight than you and for longer distances with no problems. the change was recommended by both Ben and Jeff. Geo.
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my temps are in my post of 6-18
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If I am reading the Meritor specs for our axles correctly our axles have ample capacity but the capacity is heavily impacted by the percent of grade. In the spec I was trying to interpret a 3% grade shows a capacity 27,000 pounds greater than an 8% grade.
That is not surprising because as the weight or grade increases the work the axle is doing increases.
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This is not rocket science. If a gauge was really needed, they would put one in.
There has been a push on by some truck buyers to go to a single differential, with essentially a tag behind it. Supposed to be for weight savings so they can haul heavier loads, and save fuel too. Mainly flatbed operators from what I have seen.
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Would it be fair to say that towing a GMC Arcadia at appox 5000 lbs is no big deal ?
Towing a 28 ft stacker with contents is another conversation.
Rear end gear oil probably never wears out. What happens is over a time period is contamination from micro metal particles and some moisture.
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Dick, my “classic” Country Coach came with a Diff Temp gauge. I replaced it along with the others when I upgraded to the Speedhut gauges. Speedhut actually individualizes each gauge with a label of what the gauge measures. When pulling Black Mountain the temp is reading 145 degrees it drops back when not pulling the 6-7% grade.
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Mark, my brother and I were over the road owner operators pulling reefers back in the 70’s-80’s. He kept watching Roadway trucks back then with tag axles, so he decided to order a new 379 Pete with a tag axle. After a few trips out west, he wore his drive tires badly. Big motor and a heavy foot didn’t help.
The only advantage we could find, was we could fool the Mississippi axle scales by adding or removing air from the tag axle LOL>
Never a problem with the single axle itself pulling.
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This will show my age…. I had an H Model Mack with an air tag and it was a good setup for the scales, I could load or unload the steer weight as needed. Twin screws were the choice back then, but the ole Mack ran through a lot of snow in western NY as long as you kept weight on the drive.
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