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Topic: Tire Pressure ... losing traction (Read 845 times) previous topic - next topic

Tire Pressure ... losing traction

My engine revs when I hit very hard bumps (potholes on freeways) on the highway and I suspect the tag/bag tire pressure is too high. The tach jumps up and I can hear the engine rev higher.
Cummins finds no faults or bad tach connections. FoT can't figure it our either. I've had the coach weighed fully loaded ... Steer axle 14,080 lbs., Drive axle 18,280, Tag axle 9,040. (Plate beside driver seat shows Steer 14,000. Drive 21,000, Tag 11,000 Gross wt. 41,400).
The Michelin tire chart for my 315/80R22.5 XZA2 Energy tires stops at 85 PSI as the lowest tire pressure but the chart supplied in the manual by FoT shows PSIs allows 65 PSI for the same tires. I dropped the tire pressure on the drive tires from 85 drive to 80 and the tag from 75 to 65 and the steer remained at 100 PSI. Still the same problem although I haven't hit as many sever hard bumps lately.

So. My plan is to increase the drive tire pressure back to 85 PSI and leave the tag at 65 and reweigh the coach but I'm a bit concerned that the tag tire at 65 PSI may be under inflated since the Michelin chart does not go below 85 PSI.
No Active Air on the coach.
Any thoughts? Luv this forum.
Doug, Mary & Abby the Terrier, boss of us.
2012 Nimbus 40',  #6557
2012 Jeep Wrangler Unlimited
Salem, SC

Re: Tire Pressure ... losing traction

Reply #1 –
Doug,

My first suspect would be RIDE HEIGHT.  Have you had the ride height checked?  Tire inflation would have only a very small effect on loading/unloading of the drive axle.  But if too much weight was on the tag axle/too little on the drive axle, well......
Brett Wolfe
EX: 1993 U240
Moderator, ForeForum 2001-
Moderator Diesel RV Club 2002-
Moderator, FMCA Forum 2009-2020
Chairman FMCA Technical Advisory Committee 2011-2020

Re: Tire Pressure ... losing traction

Reply #2 –
Quote from: dougself –
My engine revs when I hit very hard bumps (potholes on freeways) on the highway and I suspect the tag/bag tire pressure is too high. The tach jumps up and I can hear the engine rev higher.

So maybe torque converter dropping out of lockup?

Interesting issue and will be watching this topic.
Learn every day, but especially from the experiences of others. It's cheaper!  - John C. Bogle

2000 U320 36' non-slide / WildEBeest Rescue
2003 U320

Re: Tire Pressure ... losing traction

Reply #3 –
Electrical, wire shorting, wire harness to stressed ,  doubt  the drive wheels are going air bourne long enough for the computer to see the increased wheel speed, then down shift ? Upshft maybe.
Would be looking at wire diagram to determin what wire would cause a down shift if it shorted or momentary disconnected. Could be tuff to find, good luck.

Re: Tire Pressure ... losing traction

Reply #4 –
The first place I would look is at the transmission--- something is slipping, but I doubt it is the tires.
Current coach 1996 U320 40'
Previous coach, 1990 36' U280

Re: Tire Pressure ... losing traction

Reply #5 –
Agreed, I can't imagine the drive wheels are bouncing.

Unless someone left chassis blocks between the frame rails and there is zero suspension travel but I bet you'd notice that right off!
Jay
1989 U280 SE, 36', 3208T Cat, build 3292

Re: Tire Pressure ... losing traction

Reply #6 –
If a transmission issue (such as torque converter unlocking), it should be throwing a code. Your Allison should be locked up 100% of the time in 3,4,5 and 6th gear. It locks at higher RPM in 2nd-- that "mini-shift" you feel in 2nd.

With key on, engine off, push the up and down arrows on the Allison shift pad at the same time.  Do this a second time.

That takes you to the diagnostic codes.  So, you will see: d-1 and then a code if there is one.  If none, it will show d-1, then --.

If there is a d-1, after recording it, push the mode button to go to d-2. etc.

Brett
Brett Wolfe
EX: 1993 U240
Moderator, ForeForum 2001-
Moderator Diesel RV Club 2002-
Moderator, FMCA Forum 2009-2020
Chairman FMCA Technical Advisory Committee 2011-2020

Re: Tire Pressure ... losing traction

Reply #7 –
No way you are spinning the tires. I had a truck once with an experimental 8v92. Two turbos, lots of modifications, over 1200 HP. I could turn the tires bobtailing (no trailer). I could twist the stock drive shaft in two. Destroy the transmission and/or the differential. roll up the asphalt in the hot summer time pulling the Grapevine loaded....... but I never actually spun the tires to the point the RPMs rose.

TOM
SOB (Some Other Brand) division
1995 Wanderlodge WB40
8V92 :D

Re: Tire Pressure ... losing traction

Reply #8 –
Like Tom says, you are not likely to be spinning the tires. Easiest is to follow Brett's suggestion and read the codes. Think Michelle has the most likely answer with a loose connection letting the transmission come out of lockup. Also, check trans fluid level.

In my opinion, you are running too low tire pressure.

Pierce
Pierce and Gaylie Stewart
'93 U300/36 WTBI
Detroit 6V-92TA Jake
1140 watts on the roof
SBFD (ret)

Re: Tire Pressure ... losing traction

Reply #9 –
Quote from: wolfe10 –
If a transmission issue (such as torque converter unlocking), it should be throwing a code. Your Allison should be locked up 100% of the time in 3,4,5 and 6th gear. It locks at higher RPM in 2nd-- that "mini-shift" you feel in 2nd.

When our unlock was happening last fall, we didn't get a code.  Ours occurred just rolling along, though.  Not over bumps.  We eventually got an ABS light, though not until a few hundred miles had gone by and several episodes of drop out.  But never any Allison code, even though we did try to pull one each time and checked the VMSpc log as well. 

Steve replaced the ABS sensor (a tag one, so easy to do unlike the drive ones) and the transmission output speed sensor (since it looked a little gunked up with oil) while we were at it and didn't have another issue

Doug - you don't mention ABS light and I'm not sure what the newer coaches have for a system and if Silverleaf monitors that data, but if it does come on or you see a code logged (if you have a Silverleaf or their VMSpc) for one the ABS sensors, that might be indication of a sensor going bad, having a bad connection, or being out of alignment.
Learn every day, but especially from the experiences of others. It's cheaper!  - John C. Bogle

2000 U320 36' non-slide / WildEBeest Rescue
2003 U320

Re: Tire Pressure ... losing traction

Reply #10 –
Thanks to all.
I have not had the ride height checked. How would ride height cause the problem?
Allison and Siverleaf show no fault codes. Silverleaf appears to monitor the ABS since it shows "ABS 000 fault codes". Thanks to Wolfe and Michelle. Transmission fluid OK.
Both Cummins and FoT checked ECM wiring and found no problem.
Mark Harvey asked about tag bag pressure but he said we don't have Active Air and a road test showed nothing. No pot holes in Texas. This is why I've focussed on air pressure stuff.
I'm also concerned about too low tire pressure so I'll go back to 85 PSI on the drive tires.

I guess we'd better stop in Albuquerque at an Allison place (Stewart and Stevenson) on the way home from Santa Fe. Gotta get to the bottom of this.

Doug, Mary & Abby the Terrier, boss of us.
2012 Nimbus 40',  #6557
2012 Jeep Wrangler Unlimited
Salem, SC

Re: Tire Pressure ... losing traction

Reply #11 –
Quote from: dougself –

Allison and Siverleaf show no fault codes. Silverleaf appears to monitor the ABS since it shows "ABS 000 fault codes".

Doug - does the Silverleaf have a data log file you can review?  We actually saw our ABS sensor issue (low current, IIRC) recorded more frequently/repeatedly in the VMSpc data log file than was reflected on the dash.  There was no dash ABS light initially or it wasn't on long enough for us to notice for quite a while during the unlock/relock episodes until it finally came on and stayed on for a while.  The incidents of sensor problems recorded in the data log file generally coincided with the timing of the unlock episodes. 
Learn every day, but especially from the experiences of others. It's cheaper!  - John C. Bogle

2000 U320 36' non-slide / WildEBeest Rescue
2003 U320

Re: Tire Pressure ... losing traction

Reply #12 –
Michelle:
VMS logs faults but I cleared the file way back after having the faults corrected. There was an engine coolant sensor fault and crankcase filter fault only.
Doug, Mary & Abby the Terrier, boss of us.
2012 Nimbus 40',  #6557
2012 Jeep Wrangler Unlimited
Salem, SC

Re: Tire Pressure ... losing traction

Reply #13 –
Michelle & Folks:
I just read the Allison manual looking for a possible explanation ... like maybe the transmission is so smart it unlocks and lets the engine rev to prevent damage caused by major road jolts. Is this possible?

The Torque Converter section of the manual for my 3000MH 4th Generation transmission states "The torsion damper tries to absorb engine torsional vibration to attempt to prevent transmitting engine torsional vibration on through the transmission components (clutches, etc), or items bolted to the transmission (PTO, etc.)"

Doug ( a slow day in Santa Fe)
Doug, Mary & Abby the Terrier, boss of us.
2012 Nimbus 40',  #6557
2012 Jeep Wrangler Unlimited
Salem, SC

Re: Tire Pressure ... losing traction

Reply #14 –
Pothole plus poor connection to tranny?
The selected media item is not currently available. Dave Head & Megan Westbrook
Titusville, FL - The Great Outdoors
'98 270 buying this month
Toad is a 2018 F150 XLT
 

Re: Tire Pressure ... losing traction

Reply #15 –
The latest after a long talk with Allison Tech Center:
o When hitting very hash bumps an ABS 'event' takes place and the ABS sensor on a wheel that takes tells the transmission torque converter to unlock to prevent damage. A 'common thing' according to Allison and nothing is wrong with the transmission. No fault codes show up anywhere and the ABS light does not come on because everything is working as it should.

o FoT and Michelin both, based on my CAT scale data, advise lowering drive and tag axle tire pressure to those in the FoT Manual chart plus 5 lb. which I'done. Allison said to pursue the tire pressure issue first to soften the ride , try it out, and if necessary have the ABS seniors/wiring checked.

Again, thanks to all for your input and Michelle with the ABS story.

I'll keep y'all posted when we get back on the road later this week.

Doug
Doug, Mary & Abby the Terrier, boss of us.
2012 Nimbus 40',  #6557
2012 Jeep Wrangler Unlimited
Salem, SC