Backfeed in lighting system could be causing flasher issue

by Jim KerrBackyard Mechanic . May 18 2018

Question: I was wondering if you could help us out. My son has a 1996 Mercury Mystique that is still in decent shape. Just the other day, the flasher mechanism started making a rapid clicking sound as soon as the ignition was turned on. If you select the turn signal in either direction, the clicking slows to the rate of (and is in sync with) the turn signal. However, as soon as you put the turn signal lever back to the neutral (or centred) position, it resumes the rapid clicking.

We thought the flasher had gone bad and replaced it, but that was not the problem. I’m not sure what else it could be. Can you help diagnose the problem?


Answer: The flashers used on most vehicles are electronic rather than the older-style mechanical points and electrical heating coils. Because they are electronic, they can be affected by stray voltage backfeeds throughout the lighting system.

The first step in diagnosing your problem is to turn on the vehicle lights and walk around the car checking the operation of all the lights. If any are not working or appear dim, repair that problem first. A burned- out bulb may be all that is wrong, but bad connections where the bulb plugs into the socket or a bad ground connection to the car’s body can make a bulb glow dimly. The bad connection causes the electricity to flow through paths not normally taken, and all kinds of strange problems occur.

Be sure to check the cornering lights on this car. A burned out bulb here can cause a backfeed, and most people never think of checking the cornering lights. Also, check the signal indicator lights on the dashboard and daytime running light operation.

If all the bulbs appear to be working, then a loose or bad body ground connection is probably at fault. Clean and tighten all the ground connections for the lights. Most will be found on the inner fenders or radiator support, but there are also ground connections behind the trim panels in the trunk.

I suspect you will find you have a bulb burned out or a bad bulb-to-socket connection, but don’t get discouraged if it is not. Sometimes this type of problem can take some time to locate and every bulb may need to be removed to check connections.


Question: I have a 2009 Toyota Camry, a four-cylinder with 153,000 kilometres. Lately it was experiencing gas starvation, which occurred twice. Fuel was leaking out of the charcoal filter canister. I bought a used canister from a pick-a-part yard. The same dilemma occurred. When I fill it up with gas, it’s always full up to the neck. I stopped using this car because of the problem. I need your advice.


Answer: From your description, I am not sure if the problem is wrong connections at the carbon (charcoal) canister, or an incorrect fuel-filling procedure. Let’s start with the refuelling. When you fill the car with fuel, stop when the gas nozzle kicks off the first time. It is OK to top up the tank a little to round off the price, but don’t keep adding until the filler is full. If the tank is completely filled, fuel can flow back into the air vent line from the fuel tank to the carbon canister, saturating it with fuel and allowing fuel to leak out.

Since the 1970s, vehicles have used sealed fuel tanks. The fuel cap has no open vent (although there is a safety pressure release valve). The only way for the tank to get air when fuel is used is for it to pass through the filter in the carbon canister and back down the vent line to the tank. To prevent fuel flowing from the tank into the canister, there is a raised area at the top of the fuel tank and a fuel/vapour separator in the vent line to allow only vapours to the canister and air back into the tank. If the hose connections at the canister or at the fuel tank have been accidentally switched, such as when an in-tank fuel pump was changed, it is possible for fuel to flow to the canister. Have the hose connections checked for proper routing.