Maintenance Q&A
Gauge glitches and gelling

By Paul Abelson, Senior technical consultant

I have a 1993 Kenworth W900 with a 425 Cat engine and Eaton 3.90 rears. The axle gauges were not showing the right temperatures. I replaced both gauges and their sensors on the drive axles. All parts came from my KW dealer. After installation, the readings still did not appear correct. I checked the wires on the axle sensors. If you touched them together, it pegged the needle. I boiled some water and put each sensor in, and they showed only 100 degrees.

Then I moved the gauges around in the dash to check the circuits. The transmission gauge worked in any position but not the axle gauges. They don't work at all. How can I get the gauges to work right?

My first thought is you could have either the wrong gauges or the wrong sensors, maybe both.

Obviously, transmission and axle temperature sensors are different. They may look similar and even fit in the same holes, but in the end they are not interchangeable.

In axle and transmission sensor assemblies, the actual temperature "pickup" that gathers the actual reading may be at different heights. The openings for the sensors in either assembly are above the fill hole to prevent spilling when removing the sensors. The temperature pickup itself has to be long enough to reach the oil and to extend down into it to collect an accurate temperature reading.

After quite a bit of switching parts, you told me when I called that you went back to your dealer and asked him to replace the new gauges and sensors with other new gauges and sensors. It turned out the dealer had a number of mismarked sensors in inventory. He shipped you the right part number you ordered, but the right part was not in the box. Once you and the dealer sorted the parts, everything is working fine.

On a side note, even though this question wasn't about issues with the transmission gauges, it's not uncommon for truck owners to notice a change in temperature after a service - sometimes running

50 degrees cooler. That can be a result of switching to synthetic drive train oils. In addition to being required for extended warranty on all the new drive trains, synthetic oils run quite a bit cooler than mineral oils.

This past winter, we went from T-shirt weather to frigid in just a few days several times. I generally run east-west so the differences aren't that bad. This year was crazy. I had some runs north-south, including one from Memphis to Duluth. Several times, my fuel gelled and shut me down. Once it happened while I was actually driving. I fuel only at brand-name truck stops and always use a fuel additive when I fill up, mostly either Howes or Power Services.

Why did this happen and what can I do to keep it from happening again? Those road service calls are expensive.

This often happens when running north into areas much colder than where you fueled up. Diesel is blended to work in the climate where it's sold, on the day it's delivered to the truck stop or terminal. A healthy dose of an additive will give you an extra margin of prevention, but read the additive maker's instructions and don't exceed any concentration limits. Too much could attack fuel lines and seals and even corrode metal parts.

Gelling happens when paraffin wax, a natural component of diesel, precipitates out of the liquid fuel as temperatures drop. No. 1 diesel, or kerosene, is essentially diesel with the paraffin refined out. Mixtures of No. 2 with No. 1 reduce the amount of paraffin. When free in diesel, paraffin molecules join together and eventually solidify to prevent fuels from flowing.

Fuel additives have anti-gel compounds that attach to free ends of the paraffin molecules, delaying them from joining together. When delivered, the fuel is a blend of No. 1, No. 2 and additive that will keep engines running in that climate.

Water may also be the cause. Atmospheric moisture enters tanks as fuel is displaced. When it condenses, it can freeze in fuel lines. The ice crystals are carried to today's increasingly fine fuel filters. They build up while the fuel is still cold, helping to block flow. While temperatures must drop below 32 degrees for water to freeze, gelling can start in the mid-30s.

In just a few hundred miles, we've seen temperature swings of 30 degrees or more this winter. And it's not just the north that's colder. We also saw temperatures in the low teens in Texas and the mid-South and in the low 50s in the Great Lakes.

To be sure you don't get stuck, check the forecast weather along your route all the way to your next planned fuel stop. Treat the fuel before you add it, so the new fuel mixes the additive as you add diesel. And, this is very important, don't forget the fuel already in your tanks. In many cases, fuel in tanks wasn't ready for the extreme cold ahead. If it will need more than the safe limit to treat your fuel, consider adding No. 1. But be moderate using No. 1. It has less energy, but costs more and has less lubricity than No. 2. But the lubricity is often supplemented by chemicals in the additives. LL

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