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Fan troubleshooting reminders

MACS Technical Think tank

It’s not getting any easier, is it? More complex cooling systems bring sophisticated, and sometimes unusual, computer controls and fan circuitry. Technicians usually aren’t too pleased with extensive test and troubleshooting procedures, and the OEMs have tried to reduce them to save time.
One way used to reduce the test list is to introduce more fault codes, but then you need a scan tool that can read all of them. Even the largest independent tool makers have a hard time keeping up with the latest changes and additions from the car maker.
Make sure your diagnostic tools are up to date. Many aftermarket tools include “fan coverage” but may not read all the codes. Others may not cover all models. Can your tool command the fans on and off as well as reading any related codes? It should.
Remember as well that systems are different and there is no one-size-fits-all cooling system or diagnostic path. Mechanical systems differ and operational software strategies differ widely; both can lead you astray if you don’t understand how the systems are designed to work. For example, if you are used to radiator fans that come on anytime the A/C is engaged, you’ll waste a lot of time on a car that uses an ambient temp sensor in the control circuit.

Which means of course that you need the best information sources you can get. However you get yours –on-line, printed page, inside the scan tool, or even the guy-next-door – make sure it is up to date and accurate for the model you’re working on. Knowledge is power.
Out of the ordinary
And speaking of ambient temperature sensors, a MACS member company recently related a story about a problem truck in their test fleet. They use the truck for testing various new products but every now and then the A/C system would just not work, period. The problem seemed related to rain or the presence of water—it would even happen after a visit to the car wash.
As good as the company’s technicians are, they still couldn’t figure out the cause and the truck was sent to a neighboring specialty shop that could spend more time on it. Although the system was working on arrival, the shop decided on a nose-to-tail inspection of every sensor, switch, relay, connector and component.
They got lucky and found the problem early on. The outside ambient temperature sensor, which should have been behind the right-side of the grille, was missing. Missing, as in never installed. Over time, the connector had turned upwards, and during a heavy rain or in a carwash, the connector filled with water. That created continuity across the pins, and the control modules were getting a “too cold for A/C” message, cutting the system off. When the connector dried out, system worked. Installing the missing sensor cured it for keeps. Weird, huh?
Watch out for this one
According to reports from some shops, and an August article in the “New York Times,” some ’05 through 2010 Nissan Xterras, Frontiers and Pathfinders are experiencing an internal radiator failure with dire results.
Evidently the transmission oil cooling loop that runs through the radiator may crack, allowing the trans fluid to pick up coolant before returning into the automatic transmission. Predictably, the transmission later fails in use and the vehicle may be stranded. The NYTimes article mentioned that Nissan has extended the warranty on the radiators but will only cover related transmission failures within the vehicles’ normal warranty period.
The quick lesson here is to check these vehicles carefully anytime they’re in your shop for anything. It is reasonably easy to look for coolant in the ATF or oil in the coolant, and your customer will be most appreciative of your finding a problem before it disables them.


The Mobile Air Conditioning Society’s blog has been honored as the best business to business blog in the Automotive Aftermarket by the Automotive Communications Awards and the Car Care Council Women’s Board!
When having your mobile A/C system professionally serviced, insist on proper repair procedures and quality replacement parts. Insist on recovery and recycling so that refrigerant can be reused and not released into the atmosphere.

If you’re a service professional and not a MACS member yet, you should be, click here for more information.

You can E-mail us at or visit to find a Mobile Air Conditioning Society member repair shop in your area. Visit to find out more about your car’s mobile A/C and engine cooling system.
The 32nd annual Mobile Air Conditioning Society (MACS) Worldwide Convention and Trade Show will take place January 18-20, 2012 at the Rio All Suite Hotel and Casino in Las Vegas, NV.

One response to “Fan troubleshooting reminders”

  1. James Buttery says:

    Great article! Thanks again. Really enjoyed the one about the Ambient Air Temp Sensor. I’ve also heard of Sun Load sensors giving problems like this too. 🙂

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