Note: Allpar does not take responsibility for the veracity of any information or opinions here, does not claim expertise, and is not responsible for any consequences. Please proceed at your own risk.
by Bob O’Neill and Bob Lincoln
The power supplied to the fuel pump, ignition coil, fuel injectors, and parts of the power module is controlled by the logic module through the Automatic Shutdown Relay (ASD); the idea is to shut off fuel pressure and the ignition systems when the key is turned off to prevent fires, dieseling, and other problems. The relay is located in different places on different years.
In the 1985 – 1987 model years, the power module houses the ASD. For the 1984 models, vehicles which use a SMEC or SBEC have an external relay. From about 1990 on, the ASD relay was in a plastic underhood relay box at the left fender well.
The ASD relay is closed (allowing power to flow through it) when the ignition is first turned on. If the signal from the ignition reference is lost while the engine is running, or if the turbo boost pressure reaches and exceeds the over boost cut-off point, the logic module cuts power to the ASD, which in turn removes power from the fuel pump, ignition coil, injectors and those parts of the power module. This causes the engine to shut down.
The logic module monitors the relay driver. If it senses that the relay driver is not responding correctly it will trigger a code. If the logic module does not sense an absence of current when the circuit is open, or 12 volts DC when the circuit is closed, it knows that the circuit is open or shorted, and triggers code 42. In later models which used the SMEC or SBEC, the logic module monitors the output of the fuel pump relay output as well as the fuel injectors. If it does not see 12 volts DC when the relay is activated, it triggers code 42.
Failure of the automatic shutdown relay is often shown as inexplicable stalling or refusal to start.
Bohdan Bodnar noted, “The ASD can be triggered by a broken wire or short in another place, e.g. the fuel pump.”
For troubleshooting purposes only (don’t drive like this!), Ed Hennessy wrote: “You can try feeding power to the fuel pump directly. Connect 12V+ from the battery through a fused link to the positive terminal of the coil. That will back feed 12V to the fuel pump through the ASD circuit. If the pump runs [it should be easy to hear], the ASD relay is bad. If not, the fuel pump or its wiring is probably the problem."
LINKS: Code 42 | Stalling problems
| Other “no start” issues
Sensors, Switches, and Other Systems | Main Repairs Page | EEKs
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