Your light control module(LCM) may not be working. I replaced my air suspension system with regular shocks because of this problem. The LCM is a black box almost the size of a vhs tape found underneath the steering wheel.
In the Power Distribution Box in the engine compartment ( the P D BOX is " live " ) Relay # 303 ( one of the large relays ) is the air suspension relay
GENUINE PARTS MFG: RELAY, F3LZ-2C013-B. DO YOU KNOW THAT YOU COULD REPLACE THE REAR AIR SUSPENSION WITH SPRINGS? NO MORE RELAYS, SWITCHES, AIR COMPRESSOR OR AIR LINES TO WORRY ABOUT. THANKS! J.T.
Hey Grady==You don't say what kind of car you have but most don't have relays on the compressor. Ihere is a cycling switch on the accumulator that when it is low will keep the compressor from engaging. GoodlukcJoe
Besides fuses,there are relays that activate this compresorm most of them located under the hood; however, most of the compresors have a pressure control switch, if you have a small loss of refrigerant, eventually it is going to loose pressure an your compressor will not start to prevent compressor damage. Evacuate the system and refill with the correct amont of refrigerant.
In the relay box next to all the other relays.
If the compressor doesn't start remove all the relays and circuitry installed on it and measure the resistance of the windings A video detailing the process can be found in related links.
2. The 4 by 100 relay and the 4 by 400 relay.
I was looking at the power distribution box diagram for your 1998 Ford Expedition and it shows ( 6 ) relays but none of them are labelled for the air suspension In the P D BOX : fuse # 4 is a 15 amp mini fuse for the air suspension service switch and fuse # 15 is a 50 amp maxi fuse for the AIR SUSPENSION SOLID STATE COMPRESSOR RELAY ------------------------------------------------------------------------------------------------------- Just adding to my answer : I did a Google search and found that the solid state air compressor relay is mounted on a bracket near the passenger side headlight
Check under the air filter housing for the PCM (power control module) if you could open it you would see about 5 relays on a circuit board one of the relays is for ac compressor. In general change the PCM.
Mabe your compressor is not kicking on for some reason. Check to see if the front of your compressor starts spinning when you turn it on. If it don't check to see if there is power to the wires that go to your compressor. If there is power you have a bad compressor. If there is not check the fuses or relays.
Assuming you have checked the bulbs, check the brake light switch.
Have you checked the relays?