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Capillary action

By Steve Schaeber, MACS Technical Editor

Several Audi models are the subject of a technical service bulletin involving an issue with something not often thought about when it comes to heating issues; coolant migration.

Owners may first be alerted to a problem when they notice poor heater performance on certain A4, A5, A6, A7, A8, Q5, Q7 or TT models, ranging from 2010 thru 2014 model years. It all starts with a faulty coolant control valve, which on these vehicles, there can be up to three different ones found throughout the system. Normally when we think about defective coolant regulators, it’s an external leak or a stuck/binding valve that comes to mind. But in this case it’s an internal leak, allowing coolant to enter the normally dry solenoid area of the valve. When this happens, the body of the valve fills up internally, and coolant is forced out into the electrical connector. Once there, it begins to migrate through the connector, travelling alongside the wires and up into the wiring harness.


While coolant migration is not a common problem, this certainly isn’t the first time we’ve heard about it. Many years ago, GM had issues with coolant temperature sensors that would leak internally, allowing coolant to enter the harness connector. Once there, it too would migrate up into the wiring harness.

Why does this happen? It’s known as capillary action, and it’s the same physical phenomenon by which a wick works in a candle, an oil lamp, or a kerosene heater. In a candle, the fuel (melted wax) is carried through the wick to the flame where it vaporizes and then burns. A wiring harness can do the same thing, by conveying (or “wicking”) coolant, water, brake fluid, or any other liquids that it may come into contact with. Usually, the fluid reaches a point where it leaks out and drips off the wiring harness, puddling up to help lead us to the culprit. But in some cases, it can work its way all the way back to a connector, bulkhead, or even a control module.

As this condition progresses, coolant loss and higher engine temperatures will cause the PCM to illuminate the MIL and coolant level warning lamps, or even put the system into “Limp Home” mode. All the while, less and less hot air is delivered from the vents due to the low coolant level. Also, there may be a few codes to deal with, particularly P275300 (Trans Fluid Cooler Control Circuit Open) and P275500 (Trans Fluid Cooler Control Circuit High). Expect these codes to be caused by the coolant “short circuiting” the harness connectors, rather than defective components. Once the valve(s) are replaced and the residual coolant is cleaned out of the harness and connectors, the codes should subside.

Audi has a solution, basically redesigned regulating valves, which have already been implemented on the production line, so we’re not likely to see this condition continue into the foreseeable future. For the meantime, keep an eye out for valves N82, N488 and N509, as they’re the ones in question.

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