Okay I am stumped! 90 745 Turbo:
no ground from ECU to fuel pump relay, and no signal from ECU to fuel injectors (getting steady 12 volts from ECU to gray injector terminals, I think should be a ground signal pulse).

CPS(rpm sensor) is good. Fuel pump relay is good. Suppression relay good, getting 12 volts to green injector terminal. Throttle switch good. Air mass meter gets 12 volts power and ground is good, don't know how to check AMM further on no start engine.Fuel pumps are good.

If I ground terminals 18 and 20 on ECU it fires up for a few seconds as it burns whatever fuel I squirted in by grounding 18.

Bought a used, supposedly good ECU, same results with it and my original.

Diagnostic unit gives constantly lit LED, no flashing code, could this be a clue, or due to that I removed negative term from battery before checking the diagnostic unit. (I guess looking for codes should have been my first step)

SO, Any ideas on what is keeping ECU from grounding terminal 20 for fuel pump and terminal 18 for injector signal?

Haven't been through cold start stuff thinking there's no reason they would inhibit basic injector signal or fuel pump relay.


Car had been run low on gas lately as my 17 yr old has been scrounging quarters for gas, but then this condition arose suddenly, with seemingly no warning

Thanks for any advice,
Fradog

 

Tell us what is going on with your car so we can know why you are testing what you are testing.

What ignition/ fuel system does your car have?

For starters, make sure the battery is at the correct voltage and clean and secure the intake manifold grounds and the battery ground.

Put the diagnostic pin in #2 on the diagnostic box, turn the key to on and hold down the button for 6 seconds, see if light goes out.
Then try on #6 of not out.

 

what about the 'radio suppression relay'? this is under the hood, probably on the left fender near the power steering reservoir. it provides the power to the fuel injectors. quite easy to test, really, unplug one of the injectors, turn the ignition on, and ONE injector pin should be a sold 12.x volts (battery voltage) relative to chassis ground (EFI rail would be a good ground). the other pin will be 'floating' so give indeterminate voltage.

 

Okay I'll back up a bit, the car is a 90 740 turbo with 150 k miles my dad bought new and has maintained exceptionally well, my 17 yr old has now been driving. I had the timing belt replaced when we got it and the mechanics put in a new rpm sensor for good measure. It's been running fine although it broke a starter a few hundred miles ago. See earlier post.
It has the LH Jetronic fuel injection system, and the EZK ignition. I do have a good wiring diagram.
Car died on my son while driving: loss of power, running rough, died. I showed up and determined no fuel pump operation and it had a spark. (Checked with timing light). It was pretty low on gas, so I added 5 gallons to see if it would help. Got it started but it ran really rough , and when I put it in drive it died. Was delivering no power! Had it towed home 2 miles.
Checked battery and grounds, then before checking diagnostic unit (didn't know it was there until reading up that night) I disconnected neg terminal and put the battery on a trickle overnight just to top it off after all the cranking.
Checked diagnostic unit, (see original post).
I have checked the radio suppression relay, and get 12 (battery) volts to the green wire of connector to injectors.
This doesn't change when I swap to the other, used ECU.
Problem is I also get 12 volts at the grey wire side going back to the ECU.
So either there is a short somewhere between the grey wires and battery voltage, or it is designed that way and the ECU delivers intermittent 0 volts to pulse the injector. I guess I need to makes sure that the wires back to the ECU are good. Will do that next.
It did run rough for a while where it died, so at that time the injectors were doing something, I guess, unless it was just running on leakage or something.
Only time I can get any combustion now is if I manually ground the pin to the gray wires of the injectors.
What I'd really like to know is the logic for the ECU and what conditions, other than no rpm, cause it to not supply ground to the fuel pump relay, and no control voltage to the injectors. From what I understand older cars had a charge air over pressure switch that would cut out the fuel pumps, but I can't find that or the equivalent in the car or on the diagram.
The throttle switch I guess could do it, but I have checked it at the ECU connector: ground at closed, infinity at throttle open, and I hear it click.
I suspect the AMM but thought it unlikely to cause a no start.

Sorry for the novel, but I hope this adds any needed info to help the diagnosis along. Thanks for the interest and help,
Fradog

 

the ECU pulses the injectors to ground to fire them. its whats called an 'open collector' circuit in electornics. the other side of the injectors is tied to power via teh radio suppression relay, so when the ECU grounds its side, that turns the injector on, width of that ground pulse determines how much gas.

the fuel pump relay is actually 2 relays in one. one half of it powers the ECU and stuff, and tells the radio suppresion relay to turn on too, that half should come on with the ignition and stay on til you switch the ignition off. the OTHER half of the fuel pump relay is the actual fuel pump relay, it powers the fuel pumps, and is only turned on when the motor is turning over per the pulses from the crank position sensor.

 

what he said^^

From what I understand, ignition "on" closes one switch inside of the fuel pump relay to allow the constant 12V at terminal 30 to flow over to the other switch in the relay through junctions 87-1 and 85.

The second switch does not close until there is a signal of engine rpm from in your case, the crank sensor. When this signal is received, it completes the circuit through the ECU and/ or the air overpressure switch and this closes the second relay switch which in turn powers the main fuel pump through junction 87-2.

So, if you take the fuel pump relay plastic cap off and reinsert it the relay and turn the ignition to "start", both switches should be closed. If the second one is open, then the crank sensor is likely bad.

I can't speak to the overpressure switch or what if anything would have superseded that.

 

I dont think there is a turbo overpressure switch on lh2.4 cars, but I don't have 1990 schematics, only 1991+