anyone know where there is a schematic for the fuel system supply ? I need to see the route that supplies power to the injectors as I only seem to have 1 volt to both injector terminals at the moment ?

 

If I recall correctly, the fuel injector outputs are paired; 1-2 and 3-4 fire simultaneously. Each pair is driven by an output transistor on the DME. If one of the injector's coils is shorting, it can overdrive the transistor. Try unplugging both injectors from the pair and then checking output.
I'm not sure if this scheme was used across the board for all models, but the early car wiring diagrams show it; the injectors are wired paired in parallel. The 968 diagrams show individual circuits for each injector, so they may have added individual output transistors on the later cars. The factory service manuals are a bit of a help, but you might look for the Bosch fuel injection handbook for a really in depth discussion on how things work.

 

With the key on you should get 12 v (less on a "S"or"S2") on one if the pins to battery ground. The other pin is grounded by the DME so it should be "dead" unless you are running the starter then it should be a ground. (I think

 

actually it is a common fallacy that one injector terminal has 12v+ and the other has -ov .

As I understand it both terminals should have 12v+ when ignition is turned on and and are grounded by the DME (ECU) when the engine is turned over or running

The 4 injector supplies appear to be in two pairs but the supply for all 4x comes from the same bus bar inside the ECU . Its split into pairs to reduce the current that would be carried by a single supply cable

I need to see from a schematic where the 12v+ at the injector connectors derives from right back to the ignition switch or wherever .I may need to bypass them

I'm beginning to suspect the injector loom is fried Its 20+ years old and very hard and brittle. The connector terminals show signs of excess current being drawn too.

 

Just to be clear -- Both pins would have 12v on them, if you could get at the pins (or wires) with the clip still attached to the injector. If you are measuring the voltage in the sockets in the clip with it disconnected from the injector, only one will have voltage. Right?

 

It would seem that is correct although there is a lot of conflicting advice on the net.
From what I have read it would seem that the injectors are fed simultaneously right up to and including my 1989 S2 but from 1990 S2 onwards they are fed individualy in sequence and the ignition is also different from 1990 onwards . Individual coils I believe ?

I have found an excellent schematic for the ECU but nothing very useful for the fuel system yet

 

Hi try this link

http://porsche.elevate.nl/technical/Schemes/

Click on the "engine' scheme for the 944.
Hope it help.

Vic

 

Hi Vic thanks for the link . I didn't get an email notice of your post.