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Posted: Wed May 10, 2017 9:57 am
by jimh
Below I give some advice on the operation of the Mercury outboard engine WARNING HORN or AURAL ALERT SOUNDER as used on many Mercury outboard engines, particularly c.2000 and older two-stroke-power-cycle models. The circuit is shown in a pictorial wiring diagram below. (The original drawing contained some notations of wire color that were contradictory. I have corrected the confusing wire color notations that were found in the original.) I offer some comments on how the circuit is supposed to work, how to interpret the alert signals, and how to diagnose faults.

Fig. 1. Aural Alert System typical of older two-stroke engines; drawing revised to correct errors in original version. . PUR means VIOLET wire insulation color.
MercuryWarningHornWiringDiagramREVISED.jpg (82.6 KiB) Viewed 3763 times


The AURAL ALERT SOUNDER (warning horn) [d] takes 12-Volt power from the IGNITION KEY [e] ACCY circuit ("A") on a VIOLET (or purple or "PUR" conductor). Any return to ground on the sounder TAN circuit causes it to sound. The circuit runs through connectors [g and f] on a TAN/BLUE conductor and is carried by the remote control and engine wiring harnesses back to the engine and connects at terminal block [c].

On the engine at terminal block [c] the engine TEMPERATURE SENSOR (or thermal swtich) [a] is wired in parallel. If the engine temperature sensor closes its circuit to ground, the sounder will sound with a continuous BEEEEEEEEP as long as the thermal switch is closed to ground.

A TAN wire carries the circuit to the ALARM MODULE [b]. A return to ground through the alarm module will also cause the sounder to sound. Also attached to the alarm module is the OIL RESERVOIR FLOAT LEVEL SENSOR [h]. When the oil level in the reservoir falls below the float switch threshold, the circuit carried on the two BLUE wires is either completed or opened. (I cannot tell from the diagram which circuit state corresponds to which tank state.) When the tank level is low, the BLUE circuit causes the ALARM MODULE to generate an alternating OPEN/CLOSE to ground at the TAN wire (or possibly TAN/BLU wire), causing the sounder to have a BEEP BEEP BEEP cadence.

The circuit is presumed to operate as follows:

--when an overheat condition exists, the thermal switch [a] closes the TAN or TAN/BLU circuit to ground and the sounder sounds a continuous BEEEEEEEEP alert signal;

--when a low oil condition exists, the float switch [h] either opens or closes to signal to the alarm module [b] that it should create an alternating ground at its TAN output circuit. This closes the sounder to ground in an alternating fashion causes a BEEP-BEEP-BEEP alert signal.

--if both an overheat and a low oil condition exist, the sounder sounds a continuous BEEEEEEEEP.


If there is a continuous BEEEEEEEEP and the engine temperature is cold, disconnect the thermal switch [a] circuit from the terminal strip [c]. If the this stops the BEEEEEEEEP then the thermal switch has failed; if the BEEEEEEP continues then disconnect the alarm module circuit TAN conductor from the terminal strip. If the BEEEEEEEEP stops then the alarm module [b] has failed.

If there is a repeating BEEP-BEEP-BEEP and the oil reservoir tank is full, disconnect the float switch sensor [h] BLUE circuit from the alarm module [b]. If this stops the beeping, then investigate the float switch. If this does not stop the beeping, then temporarily connect the two BLUE wires to the alarm module. This should stop the beeping. In either case, investigate the float switch. In the BEEP-BEEP-BEEP does not stop not matter if the BLUE circuit is open or closed, the alarm module has failed.

The OIL RESERVOIR FLOAT LEVEL SENSOR [h] may be operated by magnetic forces. Check the oil reservoir tank to see if the magnet on the float has fallen off; check to see if the cork float has become saturated and lacks sufficient buoyancy to carry the magnet afloat; check the float switch to see if it has failed. Check the wiring associated with the circuit between the tank and the alarm module.

Because the ground return circuit for the AURAL ALERT SOUNDER is carried a long distance from the helm, through two wiring harnesses, and through a connector, any open circuit in that wiring will disable the sounder from working. Any short to ground will cause the sounder to sound. These are other means of failure in the circuit.


Posted: Thu Feb 07, 2019 9:17 am
by Dutchman
Is this article applicable to all Mercury outboard engines?

Or only the older two-strokes?


Posted: Thu Feb 07, 2019 2:17 pm
by jimh
I doubt the drawing, taken from an older publication of Mercury, would be applicable to ALL Mercury engines ever made. I doubt that ANY engine manufacturer has ever achieved such universality in their aural alert warning systems that the same method, once employed would then be used forever in all models. This would go against the nature of all engineers to want to continually improve on their predecessor's designs.

The caption for the pictorial drawing above notes: "Typical for older two-stroke engines." This should give some indication of the applicability of this drawing.

I am not a historian of Mercury engine electrical alarm systems, so I cannot say exactly when they began or stopped using this type of components and configurations--but it was used in many "older two-stroke engines."

The information was developed from older literature, for older and simpler two-stroke-power-cycle outboard engines using these rather primitive electrical devices. Mercury was still making those older-design, high-emission, two-stroke-power-cycle outboard engines until c.2000, and perhaps even after that for sale in countries other than the USA.