My heater grid is not coming on. If I manually run a ground wire to the negative side of the two relays they both come on (the relay coil has power with the ignition on but no ground). Also if I disconnect the small negative wire and reconnect it the relay it will come on for a split second. So my question is what controls the negative wire on each of the two relays for the heater grid?

 

I know the PCM controls the grid heater cycling. I'm not sure if it's a switched ground or not.

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So after looking at the wiring diagram in the FSM, it does look like the PCM controls the ground side of the relay.

 

Computers almost always switch ground.

 

Thank you for the help. The intake heater grid was litterally hacked out of my intake manifold by a previous owner and I replaced it to prepare for the cold weather. I beleive the reason why it was not coming on is my weather here in AZ is not cold enough yet.... I found the following link to better explain how they work Grid Heaters it is unclear if the relays are triggered by ground or 12v but considering I always have 12v if the ignition is on it is a negative trigger.... Thanks again for the help.

 

i dont use the grid heater i think it is to hard on the altenator i plug in when it is very cold and let the motor warm up in the a.m. before work. i have done this for three years and not had any trouble.