AC compressor not engaging,computer or chip?
Thread Starter
1st Gear
Joined: Jun 2006
Posts: 1
Likes: 0
From: Dayton Ohio
AC compressor not engaging,computer or chip?
Hi all!
This is my first time posting anything any where on the forum,so please bare with me.
My problem is that my ac compressor is not engaging. I have power
thru the relay ,the high pressure cut out switch,and pressure cyling switch. At the compressor I have 2 to 3 volts,not enough to pull the clutch in. In the FSM wiring Diagram there's a splice to the wire from the pressure cycling to the ac clutch that goes back to the ecm ,terminal g17 ,labeled ac/on input. I thought the wires might have shorted to ground,so I disconnected them at the switch,the clutch and the ecm.nothing ran to ground. the wiring is ok.I hooked the wires back up to the press. cycling sw. and the clutch .Left the ecm disconnected,turned the ing. on and had twelve volts at the clutch.
So the problem is the computer to my thinking(been wrong before).Would it be in the main body,Or the chip. Do I need a new computer?! ac clutch ran before I changed engines
This is my first time posting anything any where on the forum,so please bare with me.
My problem is that my ac compressor is not engaging. I have power
thru the relay ,the high pressure cut out switch,and pressure cyling switch. At the compressor I have 2 to 3 volts,not enough to pull the clutch in. In the FSM wiring Diagram there's a splice to the wire from the pressure cycling to the ac clutch that goes back to the ecm ,terminal g17 ,labeled ac/on input. I thought the wires might have shorted to ground,so I disconnected them at the switch,the clutch and the ecm.nothing ran to ground. the wiring is ok.I hooked the wires back up to the press. cycling sw. and the clutch .Left the ecm disconnected,turned the ing. on and had twelve volts at the clutch.
So the problem is the computer to my thinking(been wrong before).Would it be in the main body,Or the chip. Do I need a new computer?! ac clutch ran before I changed engines
Le Mans Master
Joined: Dec 2000
Posts: 7,839
Likes: 22
From: San Diego, Ca
It would be better if you posted this in Tech.
Assuming this is a '90 (and if it's not, let us know the Year), ECM grounds or drives the Compressor Relay when it gets an a/c request from the Programmer (electronic air) or the dash controls as long as there's a "safe" signal from the pressure switch. You've referred to that as a high pressure switch, but it really does more than that. It's a 3 wire thermistor with a 5 volt reference from the ECM, ground and return signal which rises with pressure. Each volt is about 100 psi, and the ECM generally wants .8 volts on start up, but it also wants to see a voltage or pressure increase when the clutch engages. If it doesn't have that much, or the pressure doesn't rise, it won't ground the Relay and it should (may - can't remember for the '90) set a trouble code. It also won't drive the Relay if the pressure or signal is too high - about 4 volts or 400 psi. You can see this on a scan or can troubleshoot the wiring at the sensor.
ECM also monitors the low pressure switch for cycling or how many times it's clicking on and off. Too much or too quickly and that will set a low gas trouble code. Once set, nothing will work until you clear memory - which you may have done by disconnecting the ECM. Otherwise, disconnect and reconnect the battery.
You don't say whether or not you checked the charge. If you have a manifold gage set, hook it up and see what the static charge is - needs to be at least 50 psi to close the contacts in the low pressure switch (and it should have this much if there's enough in it to get it running). Otherwise, checked for closed contacts with your meter. If it's open, the gas is gone or the switch is bad.
Assuming this is a '90 (and if it's not, let us know the Year), ECM grounds or drives the Compressor Relay when it gets an a/c request from the Programmer (electronic air) or the dash controls as long as there's a "safe" signal from the pressure switch. You've referred to that as a high pressure switch, but it really does more than that. It's a 3 wire thermistor with a 5 volt reference from the ECM, ground and return signal which rises with pressure. Each volt is about 100 psi, and the ECM generally wants .8 volts on start up, but it also wants to see a voltage or pressure increase when the clutch engages. If it doesn't have that much, or the pressure doesn't rise, it won't ground the Relay and it should (may - can't remember for the '90) set a trouble code. It also won't drive the Relay if the pressure or signal is too high - about 4 volts or 400 psi. You can see this on a scan or can troubleshoot the wiring at the sensor.
ECM also monitors the low pressure switch for cycling or how many times it's clicking on and off. Too much or too quickly and that will set a low gas trouble code. Once set, nothing will work until you clear memory - which you may have done by disconnecting the ECM. Otherwise, disconnect and reconnect the battery.
You don't say whether or not you checked the charge. If you have a manifold gage set, hook it up and see what the static charge is - needs to be at least 50 psi to close the contacts in the low pressure switch (and it should have this much if there's enough in it to get it running). Otherwise, checked for closed contacts with your meter. If it's open, the gas is gone or the switch is bad.
