NSA

My fuel pump won't run when ignition is hot and I'm currently at the DME relay. If I jump the 30 (12v in) and 87b (12v out to fuel pump), the pump will run when the ignition is hot, but when I try two different DME relays, the pump just won't run. Car is a 924S / 968 hybrid. 968 engine and DME, 924S basic wiring. The car has run fine for years but I had this problem after sorting out another wiring issue and I suspect I've broken a connection somewhere.

The DME has power going to 86 (hot) and 30 (always).

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

How do I check 85b? It has 0v, but I guess this isn't the same as checking the ground.

Any ideas? Please don't say my DME might be fried.

plumbum

I started to do what you are into doing and ended up pulling apart the relay and found a crack in the solder..i scrapped it with a screwdriver enough to get me home and repair it with a solder gun...it is now my spare (It was stamped 1991...it has done its job) and a new one was installed.

Hear is a picture of the crack in the solder I located (I know you said you tried a couple...but if they are used who knows.)

mytrplseven

A lot of members buy DME relays that are after market or older versions and they tend not to work. I'd recommend buying one with the part number 993 615 227 00. It's the updated version that's important.

NSA

Thanks. I'm going to dig out a wiring diagram for the relay and see if it's faulty before replacing it again. Both relays rattle like there's something loose inside, is that normal?

plumbum

They are big mechanical relays, it takes a second to pop the cover off and have a look tho

veleno

I had a similar problem, turned out to be the ecu and alarm module

Vincent C.

the latest dme relay from porsche is 993 615 227 01..... i would buy it from porsche !!!!!
good luck.....

jpk

So, just to be sure, you know that the DME does not power the fuel pump unless it sees 300 or so RPM from the engine speed sensor or the key is in the start position. Just turning the key to run, you should not see any power to the fuel pump.

GasPowered

Hi jpk,

On Clark's Garage, it doesn't state anything about "300 rpm's" or "key in the start position".

The reason I'm following up is that I have the same issue. (apparent lack of fuel)

I did the Clark's Garage "jumper" test. I jumpered 37 & 87b, the fuel pump did not come on, so I ordered a new one.

Would you please confirm what you stated is correct and that Clark's is missing info?

Cheers


NB: from Clarks Garage...

4. On late model cars, remove the DME relay located on the relay/fuse panel under the hood.
5. On the panel where the relay was removed, jumper terminals 30 and 87b. If unsure about the terminal numbers, look at the terminal labeling on the bottom side of the relay. When the jumper is installed the fuel pump should start.

jpk

I learned all about how the Motronic system works from here:

http://www.frwilk.com/the944/dmerelay.htm

This link talks about the DME relay functions, but there's a lot more about how the DME works on this site.

GasPowered

Hi JPK,

Thank you for the link.

Looking at that information, I believe you can test the fuel pump by the jumper method. I believe the "2-300rpm" is if the ignition is "on" so as to prevent fuel spillage in case of an accident.

The diagram towards the bottom of the page indicates jumpering 37 & 87b to test the fuel pump.... Just like Clark's. Thanks


Cheers

william_b_noble

the pictured solder joint failure is the classic failure mode of the DME relay - resoldering is a temporary repair - the permanent repair is to use a jumper wire from the relay body to the circuit board so the mechanical stresses are not carried by the electrical connection. A properly repaired DME will never again have the pictured failure.

NSA

I jumped the DME relay last night and the car started and ran fine, so it's either the relay that is bad (this is an aftermarket replacing an old OEM one), or I have a bad connection somewhere that is not telling the fuel pump to come on when the ignition is hot.

I'm going to test the relay tonight using the Clark's Garage info.

How can the relays cost $30? That must be one of the most expensive ever for its size. Surely it can't be that hard to make up reliable relays for less?

931guru

I guess that this is your first Porsche...

StoogeMoe

lol. He doesn't know about the fan relay yet.