Emission Control - Diagnostic Procedures Revision

Article No. 85-6-8


EMISSION CONTROL DIAGNOSTIC PROCEDURES - 5.8L M.C.U. - THERMACTOR AIR BYPASS - 1985 SHOP MANUAL REVISION


FORD 1982-85 CROWN VICTORIA

LINCOLN-MERCURY 1982-85 GRAND MARQUIS

CALIBRATIONS: All with 5.8L M.C.U. police, taxi and trailer tow.

WARNING: This modification is authorized only for the listed engines. Performing this modification on other engine calibrations is unauthorized and could create liability under applicable federal or local laws.
If the thermactor system fails, a service code 44 will appear during the MCU engine running quick test.

Before proceeding to the appropriate MCU pinpoint test for service code 44, verify the integrity of the TAB circuit by performing the following diagnostic procedure.

Figure 16 - POSITIVE THERMACTOR DUMP SYSTEM:

The thermactor air bypass (TAB) circuit #190 is controlled by a "positive thermactor dump system" on the above vehicles.

See Figure 16.
OTHER APPLICABLE ARTICLES: NONE
WARRANTY STATUS: "INFORMATION ONLY"
PURPOSE:
TO PROTECT THE CATALYST FROM OVERHEATING DURING EXTENDED IDLES. THIS SYSTEM WILL DUMP THERMACTOR AIR AFTER 90 SECONDS IF THE VEHICLE IS IN PARK OR NEUTRAL. (THIS ACTION WILL OVERRIDE MCU SYSTEM CONTROL) DIAGNOSIS:
^ CHECK CIRCUIT 190 FROM MCU TO DUMP RELAY AND FROM DUMP RELAY TO TAB SOLENOID FOR CONTINUITY.
^ KEY-OFF, VERIFY DUMP RELAY IS OPEN (HIGH RESISTANCE BETWEEN PIN 3 & 4). IF RELAY IS NOT OPEN, REPLACE RELAY.
^ KEY-ON, VERIFY DUMP RELAY IS CLOSED (LOW OR ZERO RESISTANCE BETWEEN PIN 3 & 4). IF RELAY IS NOT CLOSED REFER TO 1984 FORD/MERCURY ELECTRICAL VACUUM TROUBLE SHOOTING MANUAL, PAGE 39, 40 FOR DESCRIPTION, AND PAGE 37 FOR ELECTRIC SCHEMATIC.
^ AFTER THE ABOVE DIAGNOSIS CHECKS OK, RERUN MCU QUICK TEST, SERVICE AS NECESSARY.