[Jazzbass]

Most of us Carrera and 944 owners learn the first lesson of Motronic when we get our cars fairly quickly: always carry a spare DME relay. When the DME relay dies, the car no longer works. I've already done a write up about how to fix a bad DME relay, but I wanted to post here to discuss what exactly this thing is and what it does.


DME Relay Explained

So what is the DME relay? Well, it's actually two relays in one - one of the relays switches power to the DME and the fuel injectors, and the other relay switches power to the fuel pump. That's right, SC guys, we don't have that magic red relay in the fuse box controlling the fuel pump. Instead, we have a crappy, failure prone two part square relay under the drivers seat (the red relay in a Carrera fuse box is typically the A/C relay). Since 944s, 964s and up have fuse boxes designed after the 1950s, they typically have their DME relay in there. Despite being in a better location than in the older Carreras, 944/964 DME relays are still crappy and failure prone.

Here's a picture of the actual internals of the relay, showing both relay coils:



This is the schematic from the factory manuals showing the relay in circuit:



The schematic shows clearly how both sides of the relay are wired up. The switched +12V coming in on pin 86 is the first step in turning everything on. When you turn the ignition switch to position II or start, +12V is switched to this wire. Pin 86 is connected to one side the the right relay coil, and the other side of the coil (pin 85) is connected directly to ground. So, switching the ignition switch on turns on the right relay.

Turning on the right relay allows +12V to come in on pin 30, go to the right side of the left relay coil, and then out of the DME relay on pin 87. The +12V coming out of the relay on pin 87 feeds power to the DME itself on pins 18 and 35 and to the positive side of the fuel injectors (the ground side of the injectors is controlled by the DME). As you might imagine, this is a pretty high current circuit. Power coming in on pin 30 is coming directly from the battery on a 4mm^2 (~11 ga) wire. Note also that this power is UNFUSED - this is why your DME can get destroyed if there is a short in it. That big 4mm^2 wire can supply as much power as a bad DME could possibly want to burn itself up inside.

So the right side of the relay controls the DME itself. What about the left side? That's the fuel pump relay. With the right side of the relay energized, the left side now has +12V on one side of it's coil. To turn on, it needs the other side of the coil (pin 85b) to be grounded. This is controlled by the DME itself. When the left side of the relay is turned on, current flows from pin 30 out of the relay on pin 87b, through the fuel pump fuse, and to the fuel pump itself.

Pin 85b of the DME relay connects to pin 20 of the DME. The internal circuit of the DME controlling pin 20 looks like this:



Transistor T480 is a simple NPN darlington transistor being used as a switch to pull DME pin 20 to ground when activated. For the non-EE types, it's sort of like a relay, but it's an electrical switch instead of a mechanical one. T480 will turn on (and connect pin 20 to ground) when there is voltage at base of the transistor. This base voltage is supplied either from the DME microcontroller (uC) coming in at ADV7 or from the starter signal coming into the DME at pin 4. The point of this circuit?

Well, the DME doesn't want the fuel pump running unless the engine is running. The uC in the DME will only turn on T480 if it sees the engine turning at over a certain RPM (I've read 200). So, if your engine stalls, the first thing the DME uC does turn off the fuel pump. However, when you go to start the car, the starter doesn't turn the engine fast enough to allow the uC to enable the fuel pump in this manner, so a bypass is needed for starting. It's a pretty simple solution, too.

The +12V coming from the ignition switch going to the starter solenoid is also routed into the DME on pin 4. D752 and D481 form a "diode-OR" connection which turns on T480 when either input line has voltage. With positive voltage at the base of T480, the transistor switches on and connects pin 20 if the DME (and thus pin 85b of the relay) to ground, turning the fuel pump side of the relay on. Viola.


Troubleshooting tips

So, if you're troubleshooting start up/DME relay problems, the common issues are likely:

1. Bad DME relay. The solder joints inside crack from fatigue. See this thread for more info.

2. No power to the DME relay on pin 86. The factory alarm in Carreras disables the car by interrupting power line. If you are having alarm issues or have removed your alarm, make sure you run this this wire to a location in the fuse box connected to ignition switch bus 15. This last part is very important - bus 15 is hot in the "on" and "start" positions of the ignition switch. I've heard of people who remove their factory alarm and connect this line to another ignition switch bus that was hot in the "on" position, but not in "start". So the DME relay turned off every time they went to start the car. In my 85, the factory location for bus 15 is the unfused side of fuse 8.

3. Bad main power line. The big 11ga wire coming directly from the battery can corrode and not supply power to the DME. Without power on this line, you're SOL.

4. Bad speed sensor. This will cause the DME to not work for many reasons, but one of the side effects will be that without a good engine speed signal from the speed sensor, the DME uC will turn the fuel pump off.