LX 450 A/C Troubleshooting (2 Viewers)

[SucioP]

After putting the truck back together f/ a rebuild in the fall, I couldn’t get the A/C to work. AC blew ice cold before tear down.

Here’s where I’m at: new compressor / clutch, after market dryer relocate, new condenser, system purged and charged at local AC shop. They dicked around with it but wouldn’t get in the dash. Said it had good pressure and held charge but compressor would never kick on. Running through FSM procedures for No Engine Idle Up When AC Switch On:
1. Magnetic Clutch - Engages on jumper to battery power
2. AC Control Assembly - Skipped this as the light comes on when I engage AC and all other controls work.
3. System Amplifier - Will come back to this as its way up in the driver side dash and it was never out of the truck.
4. AC Amplifier- came out of the truck when I pulled the wiring harness. Test procedures is as follows. 2nd test is 14-ECM Terminal ECT.

What and where is the ECM Terminal ACt?

 

[jpoole]

There are some very knowledgeable AC folks on the forum, I'm not one of them, but a more basic perspective can also be valuable. My quick read gathered that:

"some AC system mechanical parts were changed, the system was recharged, and now it won't work"​

​

If they overcharged the system it could cause this scenario to take place. If you disconnect the sensor for high pressure (and possibly jump it?) you may be able to easily confirm that it's not a high pressure situation causing the issue. By rough memory I was able to do this to confirm a low pressure situation on my 80s AC so it at least could be possible for high pressure also, though I'm not going to do the leg work to find the sensor/procedure for you but it may be easy to find on the forum via Google search or others may chime in with that info?

Of course, the shop that worked on it probably checked this as it's simple but it may be worth confirming that you don't have a low or high pressure issue before going too far in other directions since the charge did change but not many electrics did from the sound of it.

Good luck,

 

[SucioP]

There are some knowledgeable AC folks on the forum, I'm not one of them, but a more basic perspective can also be valuable. My quick read gathered that:

"some AC system mechanical parts were changed, the system was recharged, and now it won't work"​

​

If they overcharged the system it could cause this scenario to take place. If you disconnect the sensor for high pressure (and possibly jump it?) you may be able to easily confirm that it's not a high pressure situation causing the issue. By rough memory I was able to do this to confirm a low pressure situation on my 80s AC so it at least could be possible for high pressure also, though I'm not going to do the leg work to find the sensor/procedure for you but it may be easy to find on the forum via Google search or others may chime in with that info?

Of course, the shop that worked on it probably checked this as it's simple but it may be worth confirming that you don't have a low or high pressure issue before going too far in other directions since the charge did change but not many electrics did from the sound of it.

Good luck,

Thanks. Seems unlikely it should be an electrical issue but since the compressor isn’t cutting on at all, that’s the procedure the FSM recommends.

Shoulder shrug emoji….

 

[jpoole]

If one of your pressure sensors is unhappy the compressor will not engage at all just as you describe. My line of thinking is to go over any thing that has recently changed, especially if it's easy to do so, before breaking new ground. For instance, did they remove and forget to plug in the low pressure sensor while they were moving/changing the dryer? etc. If you work through what I recommend with the pressure sensors you will easily knock out some fundamental troubleshooting.

 

[flintknapper]

Without the compressor engaging the only 'Pressure' the shop could have taken would be 'Static Pressure' which in not particularly useful except to tell us that there IS or ISN'T enough pressure in the system to satisfy the requirements of the low pressure switch (binary switch) which will allow the compressor to switch on. Typically this would be between 20-30 psi minimum.

Your binary switch will disable the compressor if the pressure is too low and likewise if too high.

Your shop stated the 'pressure' was good and the system held a charge.

From that... we can extract two things:

1. The Static Pressure correlated correctly for the ambient temperature. Meaning there was enough minimum pressure to allow the compressor to engage and that there was 'some' amount of liquid refrigerant in the system (though static pressure can not tell you how much).

2. The system had no appreciable leak(s).

___________________________________________________________________________

If I were troubleshooting this vehicle (with what history we have) I would work it backwards from the compressor in order to separate mechanical issues from electrical.

1. Apply 12vdc directly to the compressor clutch to see if it will engage. IF it will, move on to next step.

2. Take static pressure of system to see that it is at least 30 psi. It should be roughly the same as the ambient temperature (a bit higher, but at least the same as ambient). IF that is good, move to next step.

3. Jump past the binary switch (engine running, A/C controls asking for ac) and see if the compressor engages. IF it does, the issue is either with your binary switch or electrical components/wires upstream of the binary switch. IF it does not engage the compressor then the 'fun' begins. The issue is almost certainly electrical and you'll need to start testing components and inspecting wiring/connections/fuses/relays.

 

[SucioP]

Without the compressor engaging the only 'Pressure' the shop could have taken would be 'Static Pressure' which in not particularly useful except to tell us that there IS or ISN'T enough pressure in the system to satisfy the requirements of the low pressure switch (binary switch) which will allow the compressor to switch on. Typically this would be between 20-30 psi minimum.

Your binary switch will disable the compressor if the pressure is too low and likewise if too high.

Your shop stated the 'pressure' was good and the system held a charge.

From that... we can extract two things:

1. The Static Pressure correlated correctly for the ambient temperature. Meaning there was enough minimum pressure to allow the compressor to engage and that there was 'some' amount of liquid refrigerant in the system (though static pressure can not tell you how much).

2. The system had no appreciable leak(s).

___________________________________________________________________________

If I were troubleshooting this vehicle (with what history we have) I would work it backwards from the compressor in order to separate mechanical issues from electrical.

1. Apply 12vdc directly to the compressor clutch to see if it will engage. IF it will, move on to next step.

2. Take static pressure of system to see that it is at least 30 psi. It should be roughly the same as the ambient temperature (a bit higher, but at least the same as ambient). IF that is good, move to next step.

3. Jump past the binary switch (engine running, A/C controls asking for ac) and see if the compressor engages. IF it does, the issue is either with your binary switch or electrical components/wires upstream of the binary switch. IF it does not engage the compressor then the 'fun' begins. The issue is almost certainly electrical and you'll need to start testing components and inspecting wiring/connections/fuses/relays.

Thanks @flintknapper

The clutch engages jumped f/ battery. Check.

I haven’t checked pressure based on the shops statement but will do so.

How would one go about “jumping the binary”?

 

[jpoole]

I just put this in Google:

80 jump AC pressure switch site:forum.ih8mud.com​

Plenty of content shows up. I think I followed the 2nd post when I did this on my 80 some time back:

AC Compressor doesn't engage

Hey all, I've read around on AC-related threads, but couldn't find quite what I'm looking for. Here's the basic. 1991 FJ80 (3FE) DD named Vera with non functioning AC. I knew about the AC when I bought it, but it was November in Alabama, no biggie, I'll get to it. Now it's May in Alabama...

forum.ih8mud.com