The Fluke DMM model 78's built-in temperature function makes it quick and easy to check engine cooling systems for proper temperature, which is critical with today's computer-controlled engines. You can also check transmissions for overheating, and heaters and air conditioning systems for proper operation. With the Fluke 78's bead thermocouple probe, you can test thermostats and fan switches without heating them in hot water on a hot plate. You get faster, more accurate diagnosis of electrically controlled cooling systems and can compare computer data stream information with actual temperatures. On many late model cars the cooling system is sealed; the only opening is in the expansion tank. Since it doesn't have water circulating through it, you can't make an accurate temperature measurement here. The only accurate test is to measure the surface temperature of the upper tank at the radiator inlet. With the Fluke 78, it's easy to do.

Fig 20 - Testing for Switch On-Off Temperature Check the operation of electric cooling fans by touching the radiator tank next to the temperature switch with the temperature probe tip. Note the temperature when the fan comes on, and again when it goes off. Check your figures against factory specifications.







 

Fig 21 - Testing for Switch Continuity Check temperature-switch continuity with the Ohms function, while the switch is in place. Test for voltage drop across the switch and from the radiator to the body ground, as described on page 8. Note: the temperature must be above the "fan-on" temperature for the fan switch to be closed.