Test 2 Switch No. 1 Low Temp. Switch (Code 51)

NOTE: Observe the following instructions when performing the Subroutine Tests.


1. Do not measure voltage or resistance at the MCU module or connect any test light to it, unless otherwise instructed to do so.

2. Turn ignition key off and isolate both ends of a circuit whenever checking for shorts or continuity.

3. Connectors shown in schematics are viewed from front of connector.

4. Disconnect solenoids and switches from harness before checking continuity, resistance or by energizing by applying battery voltage directly.

5. Follow each step in order beginning with the first one in the appropriate routine. Follow each step until the problem is located, then correct the problem and perform the appropriate Functional Test to make sure that the problem has been corrected.



Test 2-Switch No. 1 Low Temperature Switch (Code 51)

Fig.5 - MCU Wiring Circuit, Inlet Engines:

NOTE: Make sure that engine coolant temperature is above 95° F during testing.


1. Measure resistance across low temperature switch contacts. If resistance is less than 5 ohms, proceed to next step. If resistance is greater than 5 ohms, replace low temperature switch.

2. Check circuits 309 and 60, Fig. 5, between MCU unit and low temperature switch by measuring resistance. If less than 5 ohms, replace MCU. If greater than 5 ohms, repair circuit. For cold drive problem (code 51) proceed to step 4.

3. Disconnect harness from low temperature switch, then repeat "MCU System Running Test" under "Functional Test." If service code other than 51 appears, proceed to step 5.

4. Check low temperature contacts by measuring resistance. If resistance is greater than 5 ohms, problem is not in MC unit. If less than 5 ohms, replace low temperature switch.


NOTE: Temperature switch contacts should be open when temperature is less than 90° F.


5. Check circuit 309, Fig. 5, for a short to ground by measuring resistance. If less than 10,000 ohms, reair circuit 309. If greater than 10,000 ohms, replace MCU module.