DME Relay / ECU Question 944/968 Hybrid
09-17-2013, 02:54 AM
#1
Racer
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DME Relay / ECU Question 944/968 Hybrid
Hey everyone, I have an interesting problem. Does anyone here know a bit about the DME/ECU wiring?
So, a little background - I have an early (1983) 944 chassis with a 968 engine. Ive had it running for the past 6 or so years and it works great. I have to admit, though, I was terrible at documentation during the conversion and this summer I have been going through the wiring and writing everything down.
So, when inspecting the wiring to the DME Fuel relay, I noticed that I didn't exactly have it wired 100% correctly; I grounded out a certain part of the relay in order for the fuel pump to run.
So, chasing sparks around the relay (see attached diagram for reference), I verified that I have power to both term 30 (Battery) and 86 (Ignition) on the DME relay. I have also verified that 85 and 86 will complete a circuit, and thus yield power to 87 (injector/ECU). When 85 and 86 are closed I also have power to 85b. HOWEVER, it does not seem that the ECU will ground that connection, so it will not complete the circuit to allow power to 87b, the fuel pump. Now, if I artificially ground out that connection downstream of the DME Relay but upstream of the ECU, the fuel pump will run and the car will run just fine. I definitely shouldn't have to do that, though.
Looking at the schematics, 85b connects to the ECU at connection #3, titled "DME-Relay Tr." I honestly have no idea how that connection is triggered. I did check the ground points on the ECU and everything checks out.
Any help would be greatly appreciated.
Thanks,
Jared
So, a little background - I have an early (1983) 944 chassis with a 968 engine. Ive had it running for the past 6 or so years and it works great. I have to admit, though, I was terrible at documentation during the conversion and this summer I have been going through the wiring and writing everything down.
So, when inspecting the wiring to the DME Fuel relay, I noticed that I didn't exactly have it wired 100% correctly; I grounded out a certain part of the relay in order for the fuel pump to run.
So, chasing sparks around the relay (see attached diagram for reference), I verified that I have power to both term 30 (Battery) and 86 (Ignition) on the DME relay. I have also verified that 85 and 86 will complete a circuit, and thus yield power to 87 (injector/ECU). When 85 and 86 are closed I also have power to 85b. HOWEVER, it does not seem that the ECU will ground that connection, so it will not complete the circuit to allow power to 87b, the fuel pump. Now, if I artificially ground out that connection downstream of the DME Relay but upstream of the ECU, the fuel pump will run and the car will run just fine. I definitely shouldn't have to do that, though.
Looking at the schematics, 85b connects to the ECU at connection #3, titled "DME-Relay Tr." I honestly have no idea how that connection is triggered. I did check the ground points on the ECU and everything checks out.
Any help would be greatly appreciated.
Thanks,
Jared
09-17-2013, 07:10 AM
#2
Just to verify, on my 968 schematics it shows connection #3 and "MFI+DI RELAY TR.", and your relay diagram is correct.
But, your car is running the way it's wired, correct? You're just trying to wire it "more correctly"?
Clark's Garage has a writeup on the DME go-ahead in FUEL-16.
But, your car is running the way it's wired, correct? You're just trying to wire it "more correctly"?
Clark's Garage has a writeup on the DME go-ahead in FUEL-16.
"For the fuel pump to start and remain running, for the ignition coil to develop secondary voltage, and for the injectors to fire, the DME computer must see an engine start signal or an engine running signal (greater than 200 RPM). When it does, it completes the circuit for the Fuel Pump / DME Relay secondary coil and the fuel pump starts. It also provides a ground or current flow path for the ignition coil primary and secondary coils... The engine cranking / running signal also goes to the injector drivers (2) which provide a ground or current flow path for the fuel injectors to open... The engine start signal is normally generated by a combination of voltage from the ignition switch to the starter solenoid and a signal from the reference sensor. However, if the starter solenoid signal is not present, an engine cranking signal can be generated by the combination of the speed sensor and reference sensor signals. If all of the previous tests have been completed satisfactorily, check for cranking signal inputs as described in the following steps."
Last edited by tamathumper; 09-17-2013 at 07:36 AM.
09-30-2013, 01:30 AM
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It's not a big deal if you don't ground it through the DME. At least on the 944 model (which I am still pretty new to). But the basic idea that I received from it being ground through the DME, is that is will kill the fuel pump in the even of the crash. I wish I could be more help, because I know how painful wiring issues can be. I hope that helps a tiny bit. If not, let me know and I will try to dig into it further. Good luck
