If you have an HP 5971 or Agilent 5972 MSD and you can't get the diff pump to turn on there are only a few possibilities. 1) You have a massive leak and the diff pump controller board won't allow the diff pump to turn on. 2) The DC power supply in the MSD is dead and can't run the pump controller board, and 3) The diff pump heater is dead.
To trouble shoot the diff pump heater, follow this procedure.
Follow normal vent procedure as defined in MSD manual. Turn the GC off, un plug it from the wall. Turn the MSD off, unplug the power cord from the wall. Allow sufficient time for the diff pump and oven zones to cool. Unplug the rough pump. Remove the column to allow the MSD manifold to vent. Remove the heat shield from around diff pump. Put on gloves, the diff pump can be rather messy. Remove the aluminum tee assembly from diff pump. This tee assembly holds the thermocouple vacuum gauge and is the primary fitting for the rough pump hose. Remove the entire tee by loosening the red nut and disassembling the small ring clamp. Remove the large ring clamp from the diff pump and allow it to set on the bench.
For the 5972 you can remove the diff pump cable by pulling it directly away from the plug on the chassis.
For the 5971 Remove the fan assembly by removing two screws, one at the top and one at the bottom. Set the fan assembly on the bench away from the diff pump. Disconnect the diff pump cable by unplugging the diff pump connector from the diff pump controller board (Power Distribution Board, PDB). Access the connector through the big square hole (where the fan was).
Once you have the diff pump removed from the MSD and on your bench, here are the pinouts:
Two red wires - too hot sensor
Two white wires - too cold sensor
One large wire (perhaps green) – ground
Two large insulated wires - diff pump heater
To test the diff pump heater, measure resistance between the two diff heater pump wires.
Zero ohms - heater burned and fused to heater case
Infinite ohms - heater burned with open circuit
30 to 50 ohms - heater ok
For diff pump service, remove the inner parts (baffle, cap and retainer clip) drain all fluid from diff pump into a clean beaker, cover and save for re-use. Diff pump fluid is expensive and may be re-used. Replace the inner parts of the diff pump. Cover top flange of the diff pump with two layers of aluminum foil. Replace the large ring clamp to securely seal the aluminum foil to the top of the diff pump. Wrap the entire pump in plastic wrap or place inside double plastic bags. Pack well and ship it to us. We'll test it and then replace the diff pump heater for you.
