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Old 01-25-2015, 05:39 PM   #1
Airfrix
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Exclamation No fuel - injectors does not work...

Hello,

Few days ago after long (half a year) period I started my car. Started right away, just with a touch to the key. After idling perfectly for ~5mins I drove it like 20meters and got the image, that few cylinders stopped working, the car died.

After testing found out that: fuses below radio good, fuel pump works, radio interference relay works, resistor pack good, no broken/burnt wires (ringed with tester, signal gets through), spark is fine.
Injectors should be getting +12v through one wire, and ECU sends ground signals through other wire.
Problem is, that I get +12v through both wires, even after I turn ignition on, even thou ECU signal wire should be reading like 0.4v with ignition on(tested on other 940T), seems like ECU does not send any signals to injectors. If I manually connect ECU signal wire to ground, injectors work, so wire is fine.

Tried changing ECU from other fully working car - nothing changed... still no signal to injectors...

Any ideas ? What commands ECU to stop injecting fuel ? Maybe some kind of relay, sender or other thing ?
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Old 01-25-2015, 06:07 PM   #2
2manyturbos
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The ECU WILL NOT fire the injectors without the RPM signal which it gets from the ignition computer. IOW, if you have a problem with the crank position sensor or the power stage, the RPM signal may be missing which will result in no injector pulses.
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Old 01-25-2015, 06:16 PM   #3
Airfrix
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Crank sensor is good, when I connect MS it reads rpm fine... Any advice on how to test power stage ?

edit: checked a bit about power stage failure. Looks like when its failed, there is no spark. In my situation spark is good ! :(

Last edited by Airfrix; 01-25-2015 at 06:25 PM..
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Old 01-25-2015, 08:25 PM   #4
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How are you measuring the ECU signal to the injectors? Injectors normally fire for only very brief periods, say 1/200th of a second, during cranking. This is way too fast to see on most multimeters.

You mentioned MS. There's an injector test menu in MS that allows you to fire the injectors without the engine running. Depending on wiring, you usually want the fuel pump fuse removed so the pump doesn't run, but the fuel pump turned on in the MS test menu so that the injectors get power. Listen for each injector clicking when in in test mode.
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Old 01-26-2015, 03:13 AM   #5
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Quote:
Any ideas ? What commands ECU to stop injecting fuel ? Maybe some kind of relay, sender or other thing ?
rsr, Radio Suppression Relay, is really the fuel injector relay. These are a known problem on these cars. On a T, it''s behind the power stage on the inner fender. I see you say it's working though, so maybe the grounds up behind the headlights on both inner fenders.
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Old 01-26-2015, 09:00 AM   #6
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Quote:
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How are you measuring the ECU signal to the injectors? Injectors normally fire for only very brief periods, say 1/200th of a second, during cranking. This is way too fast to see on most multimeters.

You mentioned MS. There's an injector test menu in MS that allows you to fire the injectors without the engine running. Depending on wiring, you usually want the fuel pump fuse removed so the pump doesn't run, but the fuel pump turned on in the MS test menu so that the injectors get power. Listen for each injector clicking when in in test mode.
MS controls injectors just fine, but the stock ECU dont :/ (i have quick connections so i can switch between MS and stock in a minute).

I measured injectors like this: multimeter check for continuity (checking if wire is not broken), then i checked if injectors are squirting by removing fuel rail and cranking (again, with MS its working, with stock ecu - not), otherwise i just connect spare injector and buddy holds it in hand while i crank - when its working its clicking very clearly.

So, in short - injectors are getting current from main, radio interference relays and there are no broken connections in between, only thing missing is that they are NOT getting "fire" signal from ECU.

So, why ECU does not give signal to fire injectors ? What broken parts or missing signals make ECU go "ye wont get any fuel" mode ?
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Old 01-26-2015, 10:08 AM   #7
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The reason you can measure 12V on both sides of the injector is simply that you are reading voltage through the injector.
The injector is operated by simple electro-magnet, which is a coil of wire wound around a iron core.
You are reading voltage at both ends of the coil of wire.
There is 12V potential there but the ECU must give it a closed ground reference for the electrical current to flow and open the injector.

Are you sure that the injectors are not firing?
Have you pulled a spark plug to see if they are dry or wet with fuel?
The reason that I ask is if the timing belt is broken or has slipped a few teeth, the injectors can fire and the engine will still not run.
I would at least check the timing belt.

hope this helps
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Old 01-27-2015, 10:31 PM   #8
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Originally Posted by Airfrix View Post
Problem is, that I get +12v through both wires, even after I turn ignition on, even thou ECU signal wire should be reading like 0.4v with ignition on(tested on other 940T)
As coalminer said, you're getting +12 on both wires because the ECU is normally not driving the injector line to ground - it's floating, and you read +12v going through the coil. Why do you say it should be reading 0.4v with ignition on? This means the injectors would be spraying continuously.

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(i have quick connections so i can switch between MS and stock in a minute).
Sounds like an interesting setup. Does your MS plug into the identical connector used by the factory ECU or a piggyback connector? Does it run with MS? If so, and if MS uses piggyback connector, I'd check the wiring between MS and factory connectors.

Any chance you have gummy old gas that plugged up the injectors?
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Old 01-28-2015, 06:08 AM   #9
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As coalminer said, you're getting +12 on both wires because the ECU is normally not driving the injector line to ground - it's floating, and you read +12v going through the coil. Why do you say it should be reading 0.4v with ignition on? This means the injectors would be spraying continuously.
Because I tested that on other 940T, which runs fine. Ignition on and it reads 0.4v on "ecu signal" wire (converts to negative when firing - plugged in light bulb and it lights a bit when injector gets signal to fire), other pin which is getting feed from radio relay just reads 12v.

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Originally Posted by bobxyz View Post
Sounds like an interesting setup. Does your MS plug into the identical connector used by the factory ECU or a piggyback connector? Does it run with MS? If so, and if MS uses piggyback connector, I'd check the wiring between MS and factory connectors.

Any chance you have gummy old gas that plugged up the injectors?
My MS wiring just takes few signals from stock ecu loom (spliced in), like +12v, idle valve, fuel injectors, temp sensor. Other things are wired separately. So.. Well, half things are using factory wires.

And no, thereis no chance I have old gas/plugged injecotrs, since with MS everything works just fine.

If i manually give ground to injectors signal wire (which goes from ECU to injectors) they squirt fuel like they should be. So... ECU for some reason dont give the damned ground signal :(
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Old 01-28-2015, 07:14 AM   #10
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One more oddball thought, maybe the ECU thinks the pedal is floored and has gone into flood clear mode, i.e. don't inject any fuel until engine starts or pedal is released? This seems unrelated to the initial failure.

Otherwise, check yet again that rpm signal is getting to ECU. If it doesn't see the engine cranking, it won't turn the fuel pump back on and won't pulse the injectors.

Last edited by bobxyz; 01-28-2015 at 07:24 AM..
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Old 02-01-2015, 04:13 PM   #11
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Originally Posted by Airfrix View Post
Because I tested that on other 940T, which runs fine. Ignition on and it reads 0.4v on "ecu signal" wire (converts to negative when firing - plugged in light bulb and it lights a bit when injector gets signal to fire), other pin which is getting feed from radio relay just reads 12v.
(
This is because ...when the ECU sends the ground signal to fire the injector, the 12V drops to almost to zero voltage as the magnetic field of the injector coil is energized. The magnetic field soaks up all of the current so the voltage drops (known as inrush current)....just for a nano second, which is about as long as the injector is open anyway.
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Old 05-12-2018, 08:47 PM   #12
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had a no start on a 1992 (?) 940 turbo today. Was baffled because I swapped ECUs, CPS, started on starting fluid etc. I knew there wasn’t fuel because it started on ether. I disconnected the ballast resistor connector, filed the spade connectors, and the car started right up.
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Old 05-12-2018, 10:35 PM   #13
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Labscope.
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Old 05-12-2018, 10:43 PM   #14
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Labscope.
Didn’t need a labscope at all. Sure they’re helpful, but unnecessary on a lot of old car repairs.
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Old 05-12-2018, 11:05 PM   #15
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Quote:
Originally Posted by PromiseRing View Post
had a no start on a 1992 (?) 940 turbo today. Was baffled because I swapped ECUs, CPS, started on starting fluid etc. I knew there wasn’t fuel because it started on ether. I disconnected the ballast resistor connector, filed the spade connectors, and the car started right up.
Mine were full of corrosion, I removed the resistor pack, but it was pretty bad. No doubt that could cause problems
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