Chapter 5- Nine Degrees Outside Joplin

The forecast for the second Tuesday in December said nine degrees overnight in Springfield, and Emmett spent the Monday evening reading about diesel exhaust fluid on his phone at the kitchen table, which is how most owner operators learn about diesel exhaust fluid.

He was worried about the wrong thing, which is common, and it is worth setting out what is actually true because there is a great deal of confident nonsense on this subject at every truck stop in the country.

What Cold Actually Does

Diesel exhaust fluid is a mixture of about a third urea and two thirds deionised water. It is metered into the exhaust stream, where it reduces oxides of nitrogen into nitrogen and water. Because it is mostly water, it freezes, and it freezes at about twelve degrees Fahrenheit, which in Missouri means several nights every winter and in the northern plains means most of them.

Here is the part that surprises people. The system is designed for that. Freezing does not spoil the fluid and does not change its concentration once it thaws. Frozen fluid does not stop the engine starting and does not, by itself, cause a power derate. The tank is warmed by engine coolant once the engine is running, and the lines and the dosing unit are warmed electrically, and the whole thawing process is managed by the engine computer without the driver doing anything at all.

The fluid freezes at about twelve degrees. Almost nothing follows from that on its own.

Two practical things do follow. The fluid expands as it freezes, by something like seven percent, so a tank filled completely to the neck before a very cold night is a tank under stress. Leave room.

And the more important one, which is the reason this chapter exists. If freezing itself is harmless, then a derate in cold weather is not telling you about the ice. It is telling you that something in the heating system has failed. A tank heater, a line heater, a temperature sensor, or the circuit feeding any of them. The cold does not cause the problem. The cold reveals the problem, by asking a component to work that has spent the last eight months doing nothing at all.

Which is exactly what Emmett found on the Sunday before the cold snap, in his own driveway, with a coffee going cold on the fuel tank step.

The Fault He Found in His Own Gravel

It was in the inactive fault list, which is the part of the log almost nobody reads because by definition none of it is currently happening.

A diesel exhaust fluid tank heater circuit fault, with an occurrence count of three. All three occurrences in the previous eleven days, all of them brief, none of them long enough to have put a lamp up while he was looking at the dash, and all of them on mornings when the overnight low had been in the twenties.

On a mild morning the heater does very little and a marginal circuit will pass. On a nine degree morning it is asked to run for a long time under load, and the difference between a marginal circuit and a failed one is a Tuesday.

He photographed the screen, called Milo Prentiss, and got in on the Monday. It was a corroded connector at the tank harness, cleaned and sealed for about a hundred and forty dollars in diagnostic time and materials.

What that hundred and forty dollars bought is worth stating plainly. A failed tank heater in genuinely cold weather leads to a fault, and a persistent aftertreatment fault of that family leads to a staged reduction in engine power. Not immediately, and not without warning, but on a schedule the truck sets and the driver does not. A derate on a Tuesday morning outside Joplin with a load booked is a missed appointment, a possible tow, a shop he did not choose at a rate he cannot negotiate, and a day of fixed costs against no revenue.

He found it on a Sunday, in his own gravel, in eleven minutes, because he was looking at a list of things that were not currently wrong.

The Second Thing the Winter Showed Him

The other cold weather finding that season was not a fault at all, and no fault code exists for it.

One of the twelve lines on Emmett's monthly page is the lowest voltage the electrical system reaches while the engine is cranking. It is a slightly unusual thing for an owner to record and he only started doing it because Leo Menchaca had mentioned it in passing back in the spring.

A healthy battery bank on a warm morning cranks a big diesel and holds up reasonably well. As batteries age, the voltage they can hold under the enormous momentary load of a starter motor falls. It falls slowly, over months, and it falls faster in the cold, because cold reduces the chemical capacity of the battery at the same moment that thickened oil is asking the starter to work harder.

Nothing about that process produces a warning. There is no lamp for a battery that is going to fail in three weeks. The truck starts, and starts, and starts, and then one morning at four in the morning in a truck stop lot in Kansas it does not.

Ten weeks of one number. No fault, no lamp, and a completely predictable ending.

Emmett had set his own action threshold in October, before he needed it, which is the only sensible time to set one. He decided that when the cranking figure went below nine point six he would replace the bank, regardless of whether the truck was still starting fine.

It crossed in week nine. The truck was still starting fine. He replaced all four batteries anyway, on a Thursday afternoon, at a time of his choosing, at a shop he had picked, for six hundred and ten dollars.

The comparison he makes is not against the possibility of a no start. It is against the cost of one. A no start in his own driveway is an inconvenience. A no start at a shipper at four in the morning is a missed appointment, a jump start he has to find and pay for, and a strong chance of buying whatever batteries the nearest place happens to stock at whatever they cost.

Why Both of These Were Sunday Findings

Neither of the two problems in this chapter was found by cleverness, and neither was found in the moment of crisis, because in both cases there was never a moment of crisis at all. That is the point.

The heater circuit fault was sitting in an inactive list, harmless on the day, waiting for a cold enough morning. The battery decline was a straight line on a page that anybody could have extended forward with a ruler. Both were visible weeks in advance to somebody who was looking, and both were completely invisible to somebody who was not.

If Emmett had waited until the derate, the best diagnostic help available anywhere would have told him he had a tank heater fault, which the...

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