Why does my air conditioning fail on long road trips?

Question: We have a 2009 Honda Civic with 71,000 kilometres. We bought it new, and it rarely leaves the town we live in. When we do drive out of the area, though, and have the air conditioning on for an extended period of time, the fan blower shuts down. It happens regardless of the fan speed.

We’ve twice tried to have the dealer fix it, without any luck.

We took it on a long trip last month, and the issue cropped up after three or four hours on the road. Even if we turn the fan off and wait a little bit, it won’t turn back on. In order to get it working again, we have to stop, turn the engine off and then fire it up again. Do you have any suggestions?

T.N. Answer: This shouldn’t be so tough, especially with your excellent description of when and how the symptoms appear. Your Civic’s heater, ventilation and air conditioning blower motor receives power via a relay that’s located in the under-hood fuserelay box. Relays are basically a large switch (a black rectangular thumbnail-size device) that’s signalled to turn on and off with a small current from another smaller switch or control unit. Your blower relay is energized by the ignition switch. This is a great way to deliver the rather large blower motor current without having it pass through the ignition switch. Many years back, Ford burned a few cars down by having too many accessories feeding through the ignition switch. This fixes that.

The negative or ground side of the blower-motor circuit is controlled by the blower power transistor, which is signalled by the HVAC control unit to provide various blower speeds. If the blower had quit permanently or began working again later — without cycling the ignition switch, this would be a possible culprit.

I think your blower-motor relay switch contacts are overheating after continually working hard. Also, after the ignition switch command is rescinded for a while, they cool off and begin working again. An easy way to test this theory is to swap the relay with a low-importance sibling in the fuse-relay box such as the rear window defogger relay (most of the relays in the fuse-relay box are identical).

The under-hood fuse-relay box is located at the left rear corner of the engine compartment. Looking at the box from the Civic’s 9 o’clock position (over the fender), remove the plastic cover and find the 11 rectangular black plastic relays aligned in four columns and three rows. The two you’ll want to gently lift out and swap are at the centre of the group (relay 1 to the left is the window, and 2 to the right is the blower). The diagram on the inside of the box lid shows this but is difficult to interpret. To be certain of the correct ones, try operating the blower motor and rear defogger as you unplug each one. Operation should cease.

If this corrects the symptom, you could just leave the relays as is. The rear window defogger operates with a much shorter duty cycle than the blower, and the sketchy relay may do fine with this easier job. Purchasing a replacement is also easy, with the old part in hand for clarification. If this doesn’t do the trick, a quick check for power and ground at the motor connection (easy to access, under-dash) when the symptom arises would indicate a good or bad motor, or a positive or negative side circuit fault.

Q: When I fill up my motorcycle, I usually get premium gas. If the person before me filled their tank with regular, and I buy only 10 or 11 litres, how much premium gas am I getting if it all comes out of the same hose?

M.K. A: There is roughly one litre of gasoline left in the dispenser’s line, so look for gas stations that have separate nozzles for each fuel grade. Or, wait for a pump where someone has just filled with premium. But it is not as bad as you may think. Petroleum industry chemist Tom Wicks told me that one litre of regular grade at 87 octane mixed with premium at 93 octane would only take it down 0.75 to 1 octane number, so you might end up with roughly 92 octane in the tank.