I have a 2012 Ford E-350 cutaway with a Starcraft Allstar bus body (70k miles). The front AC is the OEM Ford equipment, and the rear AC is from Trans/Air (I don't have exact model numbers, but I'm looking for them). There are two compressors driven by the serpentine belt, one for each unit.

The front AC has not worked for several years (at least), but we ignored that because the rear unit is more than powerful enough for the entire vehicle. Now the rear unit is also not cooling. (Most of the rest of this question will focus only on the rear unit.)

The rear unit is controlled by two knobs (fan speed and temperature) on the dash, which are connected to a set of five relays in the bus electrical cabinet above the driver's head. The five relays are, in order: fan high, fan medium, fan low, compressor clutch, and condenser fans. (The fan relays are all controlled by the fan speed knob, and the remaining two relays are controlled by the temperature knob.) I have verified that all five relays function properly, but the compressor clutch does not engage.

Listening very carefully (with the engine off to make it easier to hear), I turned the thermostat knob back and forth to listen to the relays. As shown in the wiring diagram below, the pink wire (highlighted purple here) triggers the condenser and compressor relays. I only hear one relay click when I turn the knob, even though I expect to hear two clicks. I know the condenser relay is coming on, because I hear the condenser fans, which means the relay that is not coming on is the compressor relay. (As noted above, I have tested the relay itself and verified that it is working. I have also checked the fuses.)

wiring diagram

My first assumption was that the high/low pressure switches (labeled "HP/LP" in the diagram; connected via the blue wire) are preventing the compressor from turning on (by not providing power to the relay). I further assumed that this would be low pressure due to a refrigerant leak, given the age of the vehicle.

I tried to add refrigerant using an AC Avalanche recharge hose and some R134a with stop-leak. (My plan, if this worked, was to use it as a stopgap measure until I could schedule a proper service to fix the leak in a few weeks.) However, the system did not actually take any refrigerant - the can did not get any lighter, and the "pressure check" button did not pop up on the end of the hose.

Thinking that maybe I was doing something wrong or there was some kind of blockage in the system, I then tried to recharge the front (Ford OEM) AC, with the same result - the can can feels just as heavy, and the button never popped up.

At this point, I can think of several possible reasons that I've had no success:

  1. Despite following the directions exactly, I'm doing something wrong.
  2. Both systems are fully pressurized, and there's something else wrong.
  3. These systems both require so much refrigerant that this 12oz can can't provide enough pressure into the system to raise the pressure enough to start the compressor.

Short of taking it to someone who has a proper AC recharge machine, is there anything else I can do to try to diagnose/repair this issue?

4 Answers 4


Unless you're completely familiar with vehicle ac systems using all the equipment for diagnosing, troubleshooting and knowledgeable about repairs, your first mistake is using sealer; a no-no as you may not find any reputable new car dealer or repair shop using sealer. It simply contaminates systems and makes repair shops either turn away business to avoid contaminating their equipment or allows them to offer rebuilding/overhauling a system to rid it of sealer. This would include the contaminated compressor, filter/drier, condenser and evaporator coils and hoses. A very expensive repair from diy repairs using sealer that rarely works. Every vehicle ac system using r134a may have up to three things circulating permanently in a sealed system; r134a, refrigerant oil and dye. Most if not all dealers and repairs shops will never share the advantage of factory installed dye. It allows anyone with an inexpensive uv blacklight to find the source of ac leaks by shining the light everywhere on ac system plumbing and parts in a dark garage, shade or at night to allow dye to glow from damage or leak. I don't know if Ford uses dye in factory installations. GM does, a greenish yellow dye. Examples of dye are easy to find, right on service valves where refrigerant, oil and dye escapes from service. Shining a uv light on a system with dye should result in a bright glow. No dye, no glow. Shining the uv light everywhere on the ac system should reveal the source of a leak or leaks. Presuming the evaporator coils buried in the console and elsewhere in a dual system, are usually trouble free but not fittings. Compressors can leak; at the case separations, relief valve, service fittings, front seal hidden by the clutch. Dye will be seen at the source of a leak as well as refrigerant oil staining the leak area.

Most if not all vehicle ac systems have compressor protection; a thermal fuse built into the electric clutch and a high/low pressure switch. The clutch coil thermal fuse, if blown will not allow 12v to energize the clutch coil. Loss of refrigerant results in lower or complete loss of operating pressures. Lower low side pressures below approximately 40 psi translates to zero return of circulating oil that won't lubricate the compressor. Allowed to run below low pressures without oil lubrication will accelerate compressor damage. Refrigerant oil is circulated by system refrigerant. No refrigerant = no oil = no compressor lube. The high/low pressure switch protects the compressor from self-destruction by opening the 12v line to the compressor clutch. Once a repair is made with refrigerant no longer leaking, the pressure switch resets automatically without manual intervention. A sealed, repaired system; a complete vacuum after a repair shows zero needle movement from evacuating a system of all air and moisture. If an evacuated system can hold a vacuum for minutes, a repaired system would be considered ready for installation of refrigerant. Oil and dye are usually installed during repairs.

Measure the compressor clutch coil for a blown thermal fuse. Coils are around 2-5 ohms. An open coil means it's blown and not repairable. Low and high side pressures, on standby with a full system of refrigerant should be close to outside temps; 75 psi/75F, 85 psi/85F, etc. Summer temps should show standby pressures well above 40 psi. As you may or may not know, an operating compressor is always sucking refrigerant gases; at idle, low side pressure should be no lower than 30-35 psi, freezing temps if familiar with refrigerant temperature/pressure charts of a specific refrigerant. High side pressures vary from 125 psi to over 250 psi in hot and humid stop and go traffic. As mentioned, ac repairs are not for average diyers and require knowledge of refrigeration and use of equipment.

  • The Trans/Air manual explicitly says "we don't recommend using dye" but nothing about sealers. I have the service manuals, including refrigerant pressure charts for this system (freely available from Trans/Air). I discussed it with my mechanic and he thinks I know enough to suggest that I try it myself before I come to him. Perhaps trying the sealer was a mistake we overlooked (as was assuming the AC Avalanche tool would work), but unless someone else has already clogged the system with a previous use of sealer, it doesn't explain my problem. I am looking for a proper set of gauges now.
    – Moshe Katz
    Commented May 15, 2023 at 8:34

You need to get an AC manifold gage set and learn how to use and interpret it. It is dangerous for you and your vehicle to add refrigerant without knowing what the gage reads and interprets. In most cases, AC problems like yours are best handled by an experienced professional with the proper equipment.

  • I know to use a manifold gauge set, I just don't currently have one and won't have access to one for a while. I do plan to have a professional look at it, but my regular shop can't do it for several weeks and we need AC now.
    – Moshe Katz
    Commented May 15, 2023 at 0:05
  • 4
    Then I would suggest that you purchase or rent a gauge set. Other than checking everything electrical, checking pressures is the first step of diagnosing. If you're in the US, many parts stores will rent these out for free when you put a refundable deposit on them.
    – Jupiter
    Commented May 15, 2023 at 0:24

If you have access to the pressure switches which engine is running (mind where your arm and fingers are), you could check which of them is closed. If you cannot put the multimeter probes in them, pull the electrical connector and see which one makes whatever relay was closed to open.


My mechanic and I looked at it together last week and determined that my third suggestion was correct:

  1. These systems both require so much refrigerant that this 12oz can can't provide enough pressure into the system to raise the pressure enough to start the compressor.

Both systems were almost completely empty of refrigerant, with around 1-2 ounces in each - the front system is supposed to have just under 2lb and the rear should have around 5lb. They have now been refilled and dye has been added.

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .