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Old 03-24-2008, 08:17 PM
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Hall of Fame The DME Relay explained

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:

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

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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:

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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.
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Old 03-24-2008, 09:28 PM
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DME Relay Pinout

Here are the pinouts from the 84-89 Carreras and the 944s/964s/993s (which I think are all the same).

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Pin 85  [1] - Ground to the DME/injector relay coil
Pin 30
 [2] - +12V directly from the battery
Pin 86
 [3] - Switched +12V from ignition switch (bus 15)
Pin 87b [4] - +12V to the fuel pump, switched by the fuel pump relay
Pin 87
 [8] - +12V to the DME and fuel injectors, switched by the DME/injector relay
Pin 85b [9] - 0V switched to the coil of the fuel pump relay from the DME, pin 20


Note that Porsche/Bosch changed the pin numbers of the relays in the documentation. In the older docs like I have for my 85, they're labeled with the 85/30/86 numbers. In later docs (and on the newer relays) they have the 1, 2, 3 numbers.
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Old 03-24-2008, 09:33 PM
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Replacing a bad DME with jumpers to get home

So your POS DME relay went bad and you were too cheap to buy and carry a replacement? Well, you can remove the relay from it's socket and connect a wire between pins 30 and 87 to turn on the DME and between pins 30 and 87b to turn on the fuel pump. This is a hack, so do it only to get home, put a crowbar in your wallet and buy a new DME relay. Then fix your old one.

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Old 03-25-2008, 05:46 PM
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Am I to gather from all these posts that your car is being improved upon once again?
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Old 03-25-2008, 06:05 PM
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hopefully he's running out of 'reasonable' improvements
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Old 03-25-2008, 09:04 PM
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That is awesome.
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Old 03-25-2008, 09:51 PM
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That my friends is just how a technical write up should look like.

Thanks Chris
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Old 03-25-2008, 11:57 PM
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Quote:
Originally Posted by racer View Post
Am I to gather from all these posts that your car is being improved upon once again?
Actually, it's mostly just out of my interest in this part of the car. This was the very stuff I loved doing before I realized there were better career opportunities in software. That said, I did just add 10 hp to my car for free. I'm working on a new post about how.
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Old 04-19-2008, 02:07 PM
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Jazz I gotta question...

Testing for a no start on my car, and just testing the wiring from the 3.2 install as well. When I test my where the DME relay plugs in, or in my harness, my #30 has power at all times. It is at 12+ volts with the key on or off. My plug # 86 is hot with the key on, and my #87b is hot with the key on. My # 87b is also connected to the red/green wire in the front trunk. None of the other plugs are hot at all with the key on or off. Pin #87 is supposed to be hot with the DME relay in.

And the kicker is with the DME relay removed my fuel pump still runs. That's not supposed to happen right?
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Old 04-19-2008, 04:32 PM
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Quote:
Originally Posted by pookie View Post
my #30 has power at all times. It is at 12+ volts with the key on or off.
Correct. This comes directly from the battery.

Quote:
Originally Posted by pookie View Post
My plug # 86 is hot with the key on
Correct. This comes from the ignition switch

Quote:
Originally Posted by pookie View Post
and my #87b is hot with the key on. My # 87b is also connected to the red/green wire in the front trunk
This is a problem. This is the OUTPUT of the DME relay to the fuel pump. It is supposed to go to one side of the fuel pump fuse, and the other side of the fuse goes to the pump. My guess is instead of hooking this to the fuel pump, you connected it to a +12V source in the fuse box. Again, the red/green wire isn't a +12V supply FROM the fuse box TO something else, it's +12V FROM the DME TO the fuse box and then to the fuel pump

Quote:
Originally Posted by pookie View Post
None of the other plugs are hot at all with the key on or off. Pin #87 is supposed to be hot with the DME relay in.
Yes. This is switched +12V to the ECU itself.

Quote:
Originally Posted by pookie View Post
And the kicker is with the DME relay removed my fuel pump still runs. That's not supposed to happen right?
Not at all. Start at the fuel pump and work backwards and find where you cross wired the fuel pump to a +12V switched source.
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