DME Relay - The number one reason why your Porsche 993 won't Start

The Bosch Digital Motor Electronics (DME) Relay failure. 

993s & 964s, like all cars, have known failure items that usually show up in the most inopportune moments. For this reason, I keep a spare DME Relay or sometimes called a fuel injection pump relay part number 993.615.227.02 in my car door pocket. This relay has internal solder joints that fail without notice. If your car’s starter turns the engine over but it won’t start 99% of the time it is this relay located in the relay box in the frunk of your 993/964. 

Interestingly, the above part number fits:

964 Carrera 2 and Carrera 4;

993 Carrera, Carrera S, Carrera 4, Carrera 4S, Targa and Turbo,

924 S and 944 S, S2 and Turbo, 968 Coupe;

So all these cars share the same weak link in their ignition systems.

Why have a DME relay in the first place?

In the 964 and 993, the DME-ECU has a permanent 12-volt feed that maintains it in standby mode to preserve its internal memory, fault codes, Readiness flags & adaption data. When the ignition is switched on the DME-ECU sends a current to the DME Relay which then switches the DME-ECU from standby to ready mode to control the engine. Then when the starter motor is cranked, the DME-ECU activates the second relay in the DME Relay housing that powers the fuel pump for as long as the engine is running.

In the other cars, power comes from the ignition switch directly to the DME relay. This then powers up the ECU which itself then controls the second part of the relay that relates to the fuel pump.

(https://philipraby.co.uk/porsche-dme-relays-explained)

Examples of failed solder joints in a DME relay

How the DME Relay fails:

The two relays internal to the DME Relay housing are active when the engine is running, and because of this, the DME Relay heats up when the car is in use. Then when not using the car it cools down. Over time this heat cycling weakens solder joints in the relay and in combination with vibration, solder joints fail. If your car does not start, or stalls out intermittently it is easy to determine if it is the DME Relay by simply removing the relay, popping its cover off, and looking for the failed solder joint(s) or simply replacing the relay. Some, me included, reheat the failed solder joint(s) and reinforce ones soldered to the internal frame with copper wire filaments and store the now repaired relay in a door pocket as an emergency spare.

Solid Stae Relay

Others spring for a solid-state replacement relay with the view that it is a permanent solution. Some have had great experience with the solid state relay others have had problems with them. 

My view is the relay does not fail often enough to spring for the premium-priced solid-state version and with the spare in the door pocket one is good to go in an emergency so I just replace the relay with an identical electromechanical one. 

Be sure to buy one made in Germany or Eastern Europe, the ones made in China are of a different internal design and their reliability can be suspect.

#start #starting #turnover #won't #stranded