Ignition relay may be causing heat problem

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This article was published 08/01/2010 (5901 days ago), so information in it may no longer be current.

QUESTION: I own a GMC Sierra 5.3L. The truck has command start and works every time.

The question I have is sometimes when I enter the truck, the heat is on but it seems to be only working on the top vent. The circulation light is not on and I cannot turn on my rear defrost.

I shut the truck off and restart it and you can hear the heat click down to normal (window & floor) and the circulation light is on, as well as power to my defrost. This happens on and off, and 99 per cent of the time it works if I shut the truck off.

Would this be a fuse problem? Relay? Your thought is greatly appreciated.

ANSWER: The ignition switch on your truck has three separate circuits that are turned on at the same time when the key is moved to the run position. Circuit No. 3 is a large pink wire that provides power to the ignition, engine computer and fuel-pump circuits. Circuit No. 4 is a large brown wire that feeds some accessory fuses and the accessory circuit of the body computer. Finally, circuit No. 1 is a larger white wire that feeds power for body-computer operation and ignition.

For the heater controls in your truck to work properly, all three circuits have to be powered up at the same time and then the body computer can send a data message to the climate-control module to operate the blower and door-control motors.

You say you have a command start, and that is likely the source of the problem, but there are many different remote starters on the market, and somehow Command Start has almost become a generic name, just like Kleenex has. Regardless of the brand of system, it needs to have three relays wired into the three ignition circuits so they are all powered up at the same time.

Some systems use external relays while others have the relays located in the "black box". I would think that one of the relays, the one that feeds the accessory circuits, isn’t turning on all the time and that would cause the problem. If the relays are external, it shouldn’t be difficult to locate the suspect relay. If they are internal, you are probably looking at either removing the remote-start system or having a new one installed.

QUESTION: I have a ’98 Subaru Outback that has never given me any trouble until recently. In the spring, I had an engine flush done at a Hurry Lube sort of place and was told I should be using an additive to keep my engine cleaner. So since then I’ve used some fuel-injector cleaner most times after getting a gas fill. My problem is when I fill my gas tank (I never let it get below three-quarters empty), the first time I try to restart the engine at the gas pump, the car doesn’t want to start. And when it does finally start, it quits unless it is running at r.p.m. above an idle. After several attempts, the car will finally sound and run normal until the next time I fill up with gas. It strikes me as very odd that this problem only occurs after filling up with gas. Hope you can suggest what the cause of the problem is.

ANSWER: The most likely cause of your problem is a saturated carbon canister. When you fill the gas tank, the air inside the tank has to exit someplace. To prevent these fumes from escaping to the atmosphere, the vent line for the gas tank goes through a liquid/vapour separator and a restrictor valve and then out to the carbon canister, where it is stored until the engine starts up. Then the fumes from the canister are pulled into the engine intake when the engine is warm and burned in the cylinders.

The liquid/vapour separator may be plugged, or you could be filling the gas tank too full, which would cause too many fumes or even liquid gas to be forced into the carbon canister. The engine would run rich until these fumes are burned and then it would operate normally again until the next fill.

Try filling the tank only until the fill nozzle clicks off. If that doesn’t work, then the tank vent system needs to be checked and this usually requires removing the fuel tank.

Jim Kerr is an experienced mechanic, instructor and member of the Automobile Journalists’ Association of Canada.

kerr.jim@sasktel.net