Bottom Line
Maintenance Q&A
Gauging accuracy

By Paul Abelson, senior technical editor

Q. The rear axle temperature gauges stopped working on my 2006 Peterbilt. I checked the ground and hot wires and they are fine. I tried a new sending unit. It registered on the gauge when I applied heat. The shop that changed the lubricant at 200,000 miles used a hot air blower on the sending units, and both registered on the dash gauges. Heat in the drive axles will not excite either sending unit. I am at a loss as to why.

A. I checked with my “brain trust,” two Technology and Maintenance Council Silver Spark Plug awardees. Tom Tahaney retired from Kenworth as a regional service manager, and Carl Tapp is maintenance VP at PAM Transport.

First of all, Peterbilts and Kenworths have electronic multiplexed gauges. All sending units use the same twisted pair of wires to signal the gauges. Those gauges figure out which signals are theirs. After replacing, they need to be recalibrated. The controller for the entire dashboard may need recalibration after work is done.

Heating the sensors with a heat gun, using hot air (at up to 1,200 degrees), will not be a valid test. Heat guns will give you false readings because they heat the sensor tube assembly with very high temperatures.

When we spoke, you indicated you did use boiling water to test the sensors. To get higher temperatures than water’s boiling point of 212 degrees, you could heat either engine oil (not recommended in pots used for cooking) or cooking oil, keeping a cooking thermometer in the oil to note temperatures up to 260 degrees. That will show you what temperature the gauges start registering. You can also compare temperatures on the thermometer with the gauges.

At what temperature would you notice a change on the gauge? Most people don’t know. By the time you would see a significant change on the gauges, any bearing damage could already be done. Many fleets do not use rear end temperature gauges.

You mentioned an oil change. Synthetic lubricants have lower operating temperatures but even with conventional lube it’s doubtful you’ll be seeing much of a reading.

The location of the sensors in their housings is important. This is also a concern with coolant sensors. A number of engines have burned up because of low coolant, when the heat never registered on the gauges. Because of their locations, there wasn’t any coolant contacting the sensors.

If the rear axle sensors are not submerged in lube, they won’t read. You also need to make sure that the lube levels in the housing are correct.

Sensors that are the wrong specification might be installed in the wrong location. Were the replacements for the failed sensor the correct ones?

If gauges were replaced, were they OEM gauges or from the aftermarket? Some aftermarket gauges have different read spans and don’t indicate lower temperatures. The needle may not move until it gets hot – and how hot varies with the gauge supplier.

Q. I need new batteries. I’ve read about the absorbed glass mat batteries. I also read that you can’t charge them with a normal charger. What do I have to do to my alternator to use them?

A. Your standard alternator/voltage regulator will charge these batteries with no problems. Problems arise when using external chargers. 

Chargers with an initial high amperage charge, typical heavy truck chargers, will cook AGM batteries. Unlike conventional lead-acid batteries, AGMs need a slow current buildup. Smart chargers will work well. They provide a trickle charge initially, build amperage as the battery gets stronger, and then go back to a trickle.

Q. I’ve been reading the pros and cons of wide-base tires versus duals. I’m looking to purchase a new trailer and trying to decide if I should buy them. Any recommendations? 

A. Wide-base tires reduce weight and fuel consumption. One wheel and tire weighs about 75 percent of two wheels and two tires. And single wheel and tire costs are higher, but overall they’re less than two wheels and two tires.

Sidewall flex is a major contributor to rolling resistance. By eliminating half the sidewalls, half the energy needed to flex sidewalls is eliminated. The result is between 4 percent and 5 percent better fuel mileage. You also get improved ride, directional stability and lateral stability. Tanks and bulk tankers have a lower center of gravity because of their greater track width, which cradles the tanks at a lower height.

Initial driver concerns included the inability to “limp home” and the lack of replacement tires. Never limp anywhere on one remaining dual. It overloads and overstresses the remaining tire and does severe internal damage. You risk a catastrophic blowout with the remaining tire.

Today, wide-base tires are as durable and reliable as duals. They are available at virtually all truck stops and tire centers. But they are sensitive to tire pressure, and they wear irregularly if they’re underinflated. They can be recapped, but only at qualified facilities.LL


Senior Technical Editor Paul Abelson is a life member of OOIDA, holds an Illinois CDL, is active in the Technology & Maintenance Council, and is a recipient of the Lifetime Achievement Award from the Truck Writers of North America.

Send your question to Paul Abelson, senior technical editor, in care of Land Line Magazine, PO Box 1000, Grain Valley, MO 64029; e-mail them to truckwriter@wowaccess.net or fax questions to 630-983-7678. Please mark your message Attention: Maintenance Q&A.

Although we won’t be able to publish an answer to all questions in Land Line, we will answer as many as possible.

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