Air Conditioning - Electronic Thermostat Control Unit
88yugo01NUMBER: 8-02
DATE: 7/13/88
DISTRIBUTION: All Service Departments
SUBJECT: AIR CONDITIONING/ ELECTRONIC THERMOSTAT CONTROL UNIT
VEHICLES: Equipped with Yugo factory air conditioning system.
GENERAL: This system cycles the compressor to accomplish the cooling function. The compressor clutch receives power through the clutch relay, which is controlled by the Electronic Thermostat Control Unit.
UNIT The Control Unit receives inputs from the ignition
FUNCTION: switch, dashboard temperature control, evaporator blower switch, and the evaporator thermistor probe. The Unit processes these inputs and cycles the compressor accordingly. Compressor clutch relay control current is the only output from the Unit.
UNIT The Control Unit is located on top of the evaporator
LOCATION: case behind the center console in the passenger compartment. It can be accessed by reaching through the driver's side above the accelerator pedal.
DESCRIPTION: THERMOSTAT CONTROL -- Part Code 8744 Part #5005 - Early type, blue case, 9-pin Part #5078 - Late type, black case, 8-pin
Control Unit circuitry and troubleshooting procedure are attached for your reference.
SYMPTOM: If the unit is not working properly, symptoms are: NO COOLING, COMPRESSOR CLUTCH WILL NOT ENGAGE
ELECTRONIC THERMOSTAT CONTROL UNIT -----------------------------------------------------------
ADVANCE
NOTE: The most common A/C system failures are the result of refrigerant leakage or loss. The hi-lo pressure switch will interrupt the clutch relay coil ground circuit at pressures below 30 psi to protect the compressor. Check system charge before proceeding with electrical diagnosis; a fully charged system will show static gauge pressures approximately equal to ambient outside temperature if the compressor has not been running (on an 80 degree day, the high and low-side pressures should stabilize near 80 psi).
TOOLS: 10mm socket, Volt/ohmmeter, short jumper wire
PREP: Remove 10 mm nut holding unit to evaporator case. Radio chassis &/or right side console trim panel may be removed for additional access. Then pull unit and harness out to left aide of console.
NOTE: Turn ignition switch > OFF before disconnecting or reconnecting unit to avoid damage to electronic components.
TROUBLESHOOTING PROCEDURE
STEP
1: Lift locktab an unit and disconnect unit from harness.
Inspect each cavity in connector; be sure terminals are
secure and contact strips are visible as shown below to
make positive contact with pins of control unit. Probe
connector only from wire side, not cavity side, to avoid damage to terminal contact strips.
2: Ignition switch > ON, temp control > MAX COOL.
Check for 12v @ connector position 6. (Also check for 6-12v at connector position 9 on early type system) If voltage present, go to Step 3. If no voltage at 6, check wiring & fuses. (If no voltage at position 9, check red wire to power source at splice)
TROUBLESHOOTING PROCEDURE
STEP
3: Evaporator blower switch > HI.
Check for ground at connector positions 5 & 8. If ground present, go to Step 4. If no ground at 5, check wire to blower slide switch. If no ground at 8, check wire to relay mounting bracket.
4: Ignition switch > OFF, reconnect unit to harness.
Connect jumper wire between positions 1 & 4. Ignition switch > ON, blower switch > ON. If clutch engages, go to step 5. If no engagement, go to step 8.
5: Ignition switch > OFF, disconnect unit from harness.
Dash temperature control > MAX COOL. Check resistance between connector positions 3 & 4 is less than 200 ohms. If OK, go to Step 6. If over 200 ohms or infinite (open), check wiring and connections in loop; if OK, replace control switch.
6: Ignition switch still > OFF, unit disconnected.
Check resistance between connector positions 1 & 2 is:
9950 @ 50~F 4650 @ 80~F
7650 @ 60~F 3650 @ 90~F
5950 @ 70~F 2950 @ 100~F
If OK, go to Step 7. If over 10% variance, check connector in thermistor wire loop & recheck resistance at thermistor pigtail.
7. Ignition switch still > OFF, unit disconnected. Check resistance between ground and each of connector positions 1, 2, 3, & 4 in infinite. If OK, go to Step 8. If continuity present, check all related wiring to find source of short.
8. Ignition switch still > OFF, unit disconnected. Connect jumper wire between connector positions 6 & 7. Ignition switch > ON. Check if compressor clutch engages. If clutch engages, replace electronic control unit. If no engagement, check wiring & connections to clutch.
NOTES: * Early and late style Thermostat Control Boxes are not interchangeable.
* When connecting the early style (blue case, 9-pin) box be sure that you are connecting pine 1 thru 9, not pins 2 thru 10; the two gray wires for the thermistor probe should be at the left end (pins 1 & 2).