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  • Air Conditioner Troubleshooting
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Air Conditioner Troubleshooting

	From the Chrysler FAQ
	From: bohdan@ihgp.ih.att.com (Bohdan L Bodnar)
	=======================================================
	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 circuit shown below is representative:

					  ------> to switched battery
					 _|
					 _) a/c clutch and surge suppressor
					 _)
					  )
					  |
					 --- 1/2 fan relay
					 ---
					  |
					 -/- WOT cutout relay
					 /--
					  |
					  |-----------> to ecm
					  |
					 -/- pressure cycling switch
					 /--
					  |
				  ---------
				  | 	  |
	   a/c switch  /	   / defrost switch
				  | 	  |
				  ---------
					  |
					  |
					   /   fan switch (closed if fan is on)
					  |
					-----
					 ---   chassis ground
					  -
	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):

	1). 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.
	2). A timer (typically, 300 ms to 400 ms) is set.
	3). If engine rpm is below a threshold (typically,
		around 900 rpm), the rpm is raised to the threshold.
	4). 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 -- dealer only) -- 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.

	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.
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