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940 No fuel - injectors does not work...

A

Airfrix

New member
Joined
Jan 15, 2010
Location
Somewhere in Europe
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 ?
 
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.
 
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 ! :(
 
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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.
 
Any ideas ? What commands ECU to stop injecting fuel ? Maybe some kind of relay, sender or other thing ?
Click to expand...
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.
 
bobxyz said:
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.
Click to expand...

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 ?
 
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
 
Airfrix said:
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)
Click to expand...
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.

Airfrix said:
(i have quick connections so i can switch between MS and stock in a minute).
Click to expand...
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?
 
bobxyz said:
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.
Click to expand...

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.

bobxyz said:
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?
Click to expand...

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 :(
 
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.
 
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Airfrix said:
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.
(
Click to expand...

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.
 
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.
 
PromiseRing said:
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.
Click to expand...

Mine were full of corrosion, I removed the resistor pack, but it was pretty bad. No doubt that could cause problems
 
Ugh, thanks for reminder about my problem :D

Problem was SOLVED.

It was bad temp sensor. It was injecting SOOOO much fuel, the injectors barely closed, hence the lack of "clicking" from them. Spark plugs were wet as an ocean.
 
PromiseRing said:
Didn’t need a labscope at all. Sure they’re helpful, but unnecessary on a lot of old car repairs.
Click to expand...

Hey rookie, labscope would have shown huge injector pulsewidth.

Ever get that Regina car started?


OP: Nice!!! How did you diagnose it?
 
ZVOLV said:
Hey rookie, labscope would have shown huge injector pulsewidth.

Ever get that Regina car started?


OP: Nice!!! How did you diagnose it?
Click to expand...

Took me like 30 minutes to get it started. Once i knew it wasn’t getting fuel I simply listened for pump cycle. Heard that so assumed injectors weren’t firing (since there should’ve been fuel at the rail). So why won’t injectors fire? Hmm poor grounds on mani, bad crank sensor, bad ecu, gummed up ballast resistor.

Yes I got the Regina car running as best as I could. Put it in time, made sure it had fuel and spark etc. the kid basically took it through a lake so who knows what’s up with it
 
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