Troubleshooting Car Air Conditioning
This article briefly describes the control system of computer controlled air conditioning systems which are typical of Chrysler vehicles which do not have automatic temperature control. This system is also similar to that of other manufacturers' products. The following components are used:
1). An electromagnetic a/c clutch.
2). The fan relay (the relay is a double-pole-single-throw type; 1/2 is used for fan control whereas the other 1/2 is used in the relay logic circuit used to control the a/c clutch).
3). The wide-open throttle (WOT) cutout relay.
4). The pressure cycling switch.
5). The switches used to select air conditioning: defrost and a/c.
6). The fan switch.
The point marked "to ecm" is drawn to a positive value via a pull up resistor on the computer's board. The pressure cycling switch is used to prevent ice formation on the evaporator's core (this would obstruct air flow and cause system overheating). The WOT cutout relay is a normally closed one; it is energized (opened) by the ecm if the ecm detects a WOT condition -- this reduces engine load during high power demand. The fan relay is energized by the computer when cooling is required, a/c is required, and, under appropriate conditions, for radiator demisting (to reduce steaming when the vehicle's engine is on, but the vehicle is not moving). The clutch is an electromagnetically controlled "brake" which is attracted to the compressor when current is supplied; the friction material ensures that, if the clutch is working properly, the compressor will fully engage. The surge suppressor on Chrysler products is often a pair of zener diodes mounted back-to-back in the (polarized) connector going to the clutch. Operation of the system is as follows (assume that the car is not near WOT):
- If the fan is on, adequate pressure is present in the system (pressure cycling switch is closed), and either a/c or defrost is selected, the point "to ecm" is drawn close to ground. The computer interprets this as an air conditioning demand.
- A timer (typically, 300 ms to 400 ms) is set.
- If engine rpm is below a threshold (typically, around 900 rpm), the rpm is raised to the threshold.
- Once timer expires, the fan relay is energized.
The compressor is now on. The compressor will continue to be on unless WOT is reached or the pressure cycling switch opens. The process then repeats.
The following readings were taken using an OTC 500 meter and OTC current measuring clamp; they are off of a 1986 Le Baron:
- Clutch resistance: 5 ohms, as measured at the clutch connector.
- Operating current: 2.4 amps (per Chrysler service manual: current in excess of 5 amps indicates a shorted clutch). The current will vary with system voltage.
- Voltage drop to battery's negative post from clutch: < 500 mv.
A common failure is the fan switch's resistance increasing. Indeed, these switches are known to melt from overheating. On 1980s products, this switch is a replaceable item (about $18) -- the entire pushbutton assembly does not require replacing. Here are some problems I ran across in my Le Baron:
1). BAD FAN SWITCH The switch didn't overheat, but caused an insufficient current to be delivered to the clutch. This caused the clutch friction material to overheat, which caused the clutch to float on a layer of gas. The symptoms were (1) gradual reduction in cold air delivery and (2) smoking refrigerant oil (leaking from a 9 years old compressor front seal).
2). BAD PRESSURE CYCLING SWITCH AND CONNECTIONS The pressure switch had bad intermittent connections; also the terminals in the external connector were corroded. The symptom was the a/c cycling on and off about three times per second (recall that the computer's timer is set to something between 300 and 400 ms). Copious use of television tuner cleaner and polishing the blade terminals on the switch would temporarily eliminate the problem. The fix consisted of replacing the switch (aftermarket bought from a parts jobber -- about $30), replacing the terminals in the connector (available in any hardware store), and spraying the connectors' boot with silicone lubricant (for a weatherproof seal). The original terminals were merely crimped; I crimped and then soldered the connections.
Air Conditioner Troubleshooting Suggestions
You will require the factory manuals (or equivalent information) and a high impedance multimeter. If a digital meter is used, it must have a bar graph display since the digital display with the dual-slope integrating analog to digital converter will react too slowly to changes in the circuit. A min/max function is also useful to have.
The voltage drop from the clutch to the battery's negative terminal MUST be low when full operating current is seen -- about 700 mv maximum is ok. In my case, after all repairs were made, the voltage was between 400 and 500 mv. For one test, I disconnected the clutch and replaced it with an unpowered test light -- this allowed me to check voltage drops with the a/c always on (pressure cycling switch would never open). Although full current was never approached (the light draws only about 300 ma), I did find bad connections. Measuring individual voltage drops is the ONLY reasonable approach.
Once problems are found, resistance measurements can be used to home in on target components. For example, I found bad electrical contacts by isolating the problem to the pressure cycling switch, then turning off the engine, disconnecting the WOT cutout relay connector, and measuring resistance from the battery's negative post to the WOT connector. Moving the terminals on pressure cycling switch's connector showed a huge decrease in resistance. Television tuner cleaner didn't reduce the resistance too much, so the terminals were replaced.
Ed Hennessy said one problem with 1980s Chrysler a/c systems is leaky hose fittings, which leave a tell-tale slimy residue near the metal fittings at the rubber hose connections. Another is a leaky compressor shaft seal, which can (contrary to what 'they' may tell you) be re-sealed with a gasket.