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?

Historically, Porsche has used relays to isolate expensive electronic components from electrical system faults. That's why the DME relay exists. It protects the very expensive Motronic engine management computer from damage caused by the components it controls. Replacing an inexpensive relay two or three times in the life of the vehicle is much preferred to replacing the very expensive engine management computer.

How does the DME Relay work?

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 cranks, the DME-ECU activates the second relay in the DME Relay housing, which powers the fuel pump as long as the engine is running.

In 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 inside the DME Relay housing are active when the engine is running, so 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 the relay's solder joints, and combined with vibration, they 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), reinforce the ones soldered to the internal frame with copper wire filaments, and store the repaired relay in a door pocket as an emergency spare.

Solid Stae Relay

Others spring for a solid-state replacement relay, viewing it as a permanent solution. Some have had great experience with the solid-state relay; others have had problems. 

I don't think the relay fails often enough to justify the premium-priced solid-state version, and with a spare in the door pocket, you're 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; unbranded ones made in China have a different internal design, and their reliability may be suspect.

8/13/2026 Recently, Bosch, the supplier of the Porsche-branded part, appears to have moved production to China.

Old and new part comparison

 

In this case, the Porsche-branded made-in-China part looks to be identical to the part formerly made in Germany. 

This is one of the rare cases where it appearsBosch moved

Old and new part circuit board.

their design and 

tooling, so the relocated part should be as good as the original.

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