Pressure Gradient Switch - Checking Procedure

Applies To 5000 TURBO Group


1985 26

Subject Key Points Number

OXS System Checking procedure for pressure gradient
switch (engine code letters KH) 85-26-02
Part Identifier N/A (supercedes Product Circular, Group 26, No. 85-01)
August 5, 1985

Beginning with November 1984 production the 5000 Turbo is equipped with a pressure gradient switch for cold acceleration enrichment.

Pressure gradient switch, operation

When engine coolant temperature is below approx. 65~(149~F), vacuum is supplied to the pressure gradient switch through the thermo-pneumatic valve, located on the left rear of the cylinder head.

During acceleration, until the specified coolant temperature is reached, the pressure gradient switch will close for approx. 0.4 seconds. This will temporarily ground the OXS frequency valve and provide a duty cycle of 100% for acceleration enrichment.

Pressure Gradient Switch, Checking:

Pressure gradient switch, checking

Test conditions
^ engine coolant temp. below 10~C (50~F)

- unplug electrical connector from pressure gradient switch

- connect voltmeter between terminals 1 and 2
- switch ignition ON
^ specification: approx. battery voltage

- unplug electrical connector from frequency valve
^ specification: approx. 4 volts (measured on pressure gradient switch connector) If specification NOT OK

- check for open circuit in wiring according to current flow diagram

If wiring OK
- unplug vacuum line and electrical connector from pressure gradient switch

Pressure Gradient Switch, Checking:

- connect hand vacuum pump to open hose connection on pressure gradient switch
- connect ohmmeter across both terminals of open electrical connection on pressure gradient switch
- apply vacuum to pressure gradient switch, then release vacuum from pump
^ specification: ohmmeter must briefly read 0 ohms (indicates continuity)

If spec. NOT OK
- replace pressure gradient switch and re-check

If spec. OK
- check thermo-pneumatic valve as follows

Thermo-Pneumatic Valve, Checking:

Thermo-pneumatic valve, checking

Test conditions
^ engine coolant temp. below approx. 10~C (50~F)
- unplug vacuum lines from valve

Note

If coolant temp. above test specs. remove valve from cylinder head and cool down separately
- connect hand vacuum pump to angled hose connection (arrow)
- temporarily seal off straight hose connection
- apply vacuum with pump
- open sealed connection
^ vacuum must release

If vacuum DOES NOT release
- replace thermo-pneumatic valve

WIRING COLOR CODE:

WIRING COLOR CODE
8LACK - BK
BROWN - BR
CLEAR - CL
RED - R
YELLOW - Y
GREEN - G
LIGHT GREEN - LT. G
BLUE - BL
VIOLET - V
GRAY - GY
WHITE - W
ORANGE - OR