DME Control of Fuel Pump '86 911

[helgeseg]

I've just finished installing a 3.2 motor from an '86 into a '77 chassis. I want to get the fuel pump power from the DME/fuel pump relay rather than the fuel pump relay on the '77 (which does work). Problem is that pin 20 from the DME computer is not going to ground to energizing the fuel pump side (85b) of the DME/Fuel pump relay.

So the question is: What are the inputs to the DME that effect pin 20?

Thanks,
Erik Helgeson
'86 930
'82 911SC
'77 911 - Problem child.....

[Steve Grosekemper]

You need #30 (batt) and #15 ignition input as well as a RPM and TDC signal that come from the two flywheel sensors.

If you don't have ignition or injection signal you are most likely not getting a speed signal from the flywheel sensors.
#1 mistake people make when doing these conversions is crossing the speed and references sensors.
Try switching them and see if it runs...
Should be:
Head temp
Speed -DG
Reference (TDC) BG
(top to bottom)

At the flywheel the speed sensor is forward and lower.

Then test the sensors, there should be 1000 +/- 500 ohms resistance between pin 25 and 26 of the sensor (bottom 2 pins)
If that is OK then hook a DVOM (ACV) to 25 and 26 and crank the engine.
Unless you have an oscilloscope...
The constant voltage is the speed signal and the flashing signal is the reference signal.

It must have a speed, reference, battery and switched power to the computer to activate the DME relay in order to power up the fuel pump.

There are other things it could be but here are the biggies that should keep you busy for a while.

A lot easier with an oscilloscope and FI tester...

Good Luck,

[helgeseg]

Steve,
The the car seems to run fine, and I rang out the cables to the speed sensor and the TDC sensor to the DME connector so I'm pretty sure I got that right.

Right now I'm using the old relay circuit (relay #1 on your SC) for fuel pump power and it works fine, but I'd like get fuel pump power from the DME relay. The reason for this is that I think the DME provides a rev limiter (right?) and it just seems like the right thing to do.

On the SC, fuel pump power is controlled by the rev limiter and air flow sensor plate inputs, so I'm guessing there are similar control inputs for the DME. If the control inputs to the DME are speed sensor and TDC, then it seems to me the fuel pump control (pin 20) should work since the motor runs fine.

I know I must be missing something here, just can figure it out.

Thanks for your help,
Erik

[Steve Grosekemper]

Erik,
Rev control comes from the DME to the injector output and has nothing to do with the fuel pump switching.

If the engine runs, the DME relay should be powering the fuel pump.
Have you tried a new DME relay?

#20 pin from the DME powers the relay, if the computer runs the pump should run.

You are checking power at the red/green 2.5mm wire about 2.5 feet from the end of the harness near the battery connection?

[Dan Chambers]

Steve,
Right now I'm using the old relay circuit (relay #1 on your SC) for fuel pump power and it works fine, but I'd like get fuel pump power from the DME relay. The reason for this is that I think the DME provides a rev limiter (right?) and it just seems like the right thing to do.

On the SC, fuel pump power is controlled by the rev limiter and air flow sensor plate inputs, so I'm guessing there are similar control inputs for the DME.
Erik

Erik:

I'm not sure about SC engines, but on most (if not all) Motronic - DME systems, the rev-limiter is electronic and is spark, not fuel, limiting.

So, if the car is running fine, and the rev limiter on the Motronic system is functioning and matches the "safe" high-rpm of your engine, you should be good to go. I.E. if the rev limiter is set to 6500 RPM, and your engine's red-line is 6400 to 7000 rpm (like my 944), it should all be okay.

Just my observations from a 944 with Bosch Motronic DME.

[Steve Grosekemper]

Steve,
Right now I'm using the old relay circuit (relay #1 on your SC) for fuel pump power and it works fine, but I'd like get fuel pump power from the DME relay. The reason for this is that I think the DME provides a rev limiter (right?) and it just seems like the right thing to do.

On the SC, fuel pump power is controlled by the rev limiter and air flow sensor plate inputs, so I'm guessing there are similar control inputs for the DME.
Erik

Erik:

I'm not sure about SC engines, but on most (if not all) Motronic - DME systems, the rev-limiter is electronic and is spark, not fuel, limiting.

So, if the car is running fine, and the rev limiter on the Motronic system is functioning and matches the "safe" high-rpm of your engine, you should be good to go. I.E. if the rev limiter is set to 6500 RPM, and your engine's red-line is 6400 to 7000 rpm (like my 944), it should all be okay.

Just my observations from a 944 with Bosch Motronic DME.

Actually any Porsche originally equipped with a catalytic converter shuts the fuel off. CIS injection turns off the pump, and all the rest turn off the injector signal.

Cars without a converter pre-77 Porsche use a rev-limiting rotor to short the ignition system.

You don't want to dump raw fuel on top of a 500-600 degree converter

[Jad]

You don't want to dump raw fuel on top of a 500-600 degree converter

Cool fire

[Dan Chambers]

Actually any Porsche originally equipped with a catalytic converter shuts the fuel off. CIS injection turns off the pump, and all the rest turn off the injector signal.

Cars without a converter pre-77 Porsche use a rev-limiting rotor to short the ignition system.

You don't want to dump raw fuel on top of a 500-600 degree converter

Ah; I stand corrected. Sorry for the mis-information.

[helgeseg]

Steve, Dan,
Sounds like I should just continue to use the old fuel pump relay and not worry about it.

Thanks for your insights, as always a good educational opportunity for me.

Erik