On my 89 does the grid heater only come on on cold starts or does it come on when the engine temp isn't at a certain point. I no on my dads 24v it comes on untill it is not needed i was just curious but his is a electronic

 

The heater on my 89 comes on when i turn the key then when the buzzer stops and the engine is started its off. But if its warm and i go to start it the heater doesn't come on.

 

alright thanks that is how mine works

 

The IAT sensors (Air Intake Temperature) in the plenum cover are used by the PCM to control the grid heaters and KSB. Non-IC'd trucks have a lower temp setting than the IC'd 1st Gens, but I don't recall the exact numbers. Mine used to come on around 50 degrees, IIRC.

The later trucks use a combination of readings from the intake and the airbox to help the ECM make a decision about the heaters.