DELCO-REMY Electronic Module Retard Ignition System: Operation

In most cases, the ignition system operates as a standard HEI system. The timer core, rotating inside the pick-up coil and pole piece assembly, generates an electrical signal to the HEI-EMR module. This signal turns on and off the primary circuit in the ignition coil. As the primary is turned on and off, a voltage surge occurs in the secondary, firing the spark plugs. Spark timing is varied by the vacuum advance unit to meet varying speed and load conditions.

In the above conditions the EMR vacuum switch (vacuum-operated electrical switch) is in the open position. There is no retard or delay in distributor firing.

At coolant temperatures below 120°F, the distributor thermal vacuum switch permits vacuum to be present at EMR vacuum switch. This closes an electrical switch, connecting the EMR circuit to ground. When the EMR circuit is grounded, the firing of spark plugs is delayed for a calibrated number of crankshaft degrees.

When temperature of the coolant reaches 120°F, the distributor thermal vacuum switch cuts off vacuum to the EMR vacuum switch. The electrical switch then opens the EMR circuit (removes ground), and spark plug firing occurs without delay, being controlled by speed and vacuum.

The Electronic Control Module (ECM) does not control spark timing as is the case with EST systems, but uses the distributor reference signal, modified by the "TACH" signal conditioner for other emission-related purposes. If the HEI-EMR module is removed and/or replaced for any reason, the ignition timing must be checked and set to specifications.