31-JUL-2006 - Manufacturer Service Bulletin: TROUBLESHOOTING THE TKS DIODE
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Situation A blocking diode is used in the electrical system of MerCruiser V6 and V8 engines with TKS (Turn Key Start) carburetors. When the engine is warm but is not running, a temperature switch allows positive current to flow to the TKS heater module, while the diode prevents the positive current from reaching the electric fuel pump and the ignition system. This prevents the carburetor from returning to cold start mode when the engine is turned off but still warm. Under some conditions the blocking diode in the electrical system can fail. Two failure modes for the diode are possible. One is that the diode is shorted internally and will pass current both ways, and the other is that the diode is open internally and will not pass current through either way. • If the diode is shorted internally, the engine will continue to run after the key is turned off. If this condition is found, removing the diode from its connector will shut the engine off. • If the diode is open internally, the TKS module will not have positive current when the engine is warm and the key is off. The engine will be hard to start when it is warm. Correction If an engine will not turn off or becomes hard to start when warm, check the blocking diode with an ohmmeter to determine if the diode is the problem. If the diode has continuity both ways, it is shorted. If the diode does not have continuity either way, the diode is open. If either condition is found, the diode needs to be replaced for the engine to operate properly. Replace the diode and test the engine to verify: 1. The engine turns off with the key switch and the lanyard switch
