1988 928 S4 No Injector Pulse
09-30-2022, 07:39 PM
#1
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1988 928 S4 No Injector Pulse
After finally thinking she would start, she isn't.
I initially had a seized fuel pump which got replaced, fuel is now reaching the injector rails, but the injectors arent firing as the plugs are bone dry. I have spark and the tach moves fine. After some ideas, I have scoured the forums too.
I have a scope, when testing the injectors there is a constant 12v with igntion on and whilst cranking on both wires. Am I wrong in thinking these should be being switched to ground? Thanks guys!
I initially had a seized fuel pump which got replaced, fuel is now reaching the injector rails, but the injectors arent firing as the plugs are bone dry. I have spark and the tach moves fine. After some ideas, I have scoured the forums too.
I have a scope, when testing the injectors there is a constant 12v with igntion on and whilst cranking on both wires. Am I wrong in thinking these should be being switched to ground? Thanks guys!
09-30-2022, 11:48 PM
#2
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Update, still not running. Rechecked crank signal with scope, no crank signal. I must be doing something wrong as I don't understand how I can have spark but no crank signal when checked with the scope... I checked it at the sensor itself as well as the ECU.
10-01-2022, 12:04 AM
#3
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The injectors get 12 volts with the key on. And because they are a coil of wire inside, they will show voltage on both sides, with key on.
The ground signal comes from the LH computer.
It's pretty common for the two wires to deteriorate under the boot and end up with two "bare sections". If any of the 8 locations (under the boots) touch wires together, the ground will be shorted out and the injectors will not work.
While I don't know what you have been doing, this frequently occurs when the loom is moved around.
You can confirm if this is the issue by unplugging all 8 injectors and then seeing if you have 12 volts on both sides (you should not.)
If you do, pull back boots until you find the "crossed" wires.
Some "brush on" electrical tape is your friend, to do a repair, for now, if this is your issue.
10-01-2022, 04:54 PM
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Correct. If you have spark, you have a crank trigger signal.
The injectors get 12 volts with the key on. And because they are a coil of wire inside, they will show voltage on both sides, with key on.
The ground signal comes from the LH computer.
It's pretty common for the two wires to deteriorate under the boot and end up with two "bare sections". If any of the 8 locations (under the boots) touch wires together, the ground will be shorted out and the injectors will not work.
While I don't know what you have been doing, this frequently occurs when the loom is moved around.
You can confirm if this is the issue by unplugging all 8 injectors and then seeing if you have 12 volts on both sides (you should not.)
If you do, pull back boots until you find the "crossed" wires.
Some "brush on" electrical tape is your friend, to do a repair, for now, if this is your issue.
The injectors get 12 volts with the key on. And because they are a coil of wire inside, they will show voltage on both sides, with key on.
The ground signal comes from the LH computer.
It's pretty common for the two wires to deteriorate under the boot and end up with two "bare sections". If any of the 8 locations (under the boots) touch wires together, the ground will be shorted out and the injectors will not work.
While I don't know what you have been doing, this frequently occurs when the loom is moved around.
You can confirm if this is the issue by unplugging all 8 injectors and then seeing if you have 12 volts on both sides (you should not.)
If you do, pull back boots until you find the "crossed" wires.
Some "brush on" electrical tape is your friend, to do a repair, for now, if this is your issue.
I had various shorts throughout the injectors, I went through and re-insulated all the wires in the engine bay as 90% of them were exposed under the boots. I will do the injector thing just in case. Any other ideas if this doesn't work?
10-01-2022, 08:07 PM
#5
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Correct. If you have spark, you have a crank trigger signal.
The injectors get 12 volts with the key on. And because they are a coil of wire inside, they will show voltage on both sides, with key on.
The ground signal comes from the LH computer.
It's pretty common for the two wires to deteriorate under the boot and end up with two "bare sections". If any of the 8 locations (under the boots) touch wires together, the ground will be shorted out and the injectors will not work.
While I don't know what you have been doing, this frequently occurs when the loom is moved around.
You can confirm if this is the issue by unplugging all 8 injectors and then seeing if you have 12 volts on both sides (you should not.)
If you do, pull back boots until you find the "crossed" wires.
Some "brush on" electrical tape is your friend, to do a repair, for now, if this is your issue.
The injectors get 12 volts with the key on. And because they are a coil of wire inside, they will show voltage on both sides, with key on.
The ground signal comes from the LH computer.
It's pretty common for the two wires to deteriorate under the boot and end up with two "bare sections". If any of the 8 locations (under the boots) touch wires together, the ground will be shorted out and the injectors will not work.
While I don't know what you have been doing, this frequently occurs when the loom is moved around.
You can confirm if this is the issue by unplugging all 8 injectors and then seeing if you have 12 volts on both sides (you should not.)
If you do, pull back boots until you find the "crossed" wires.
Some "brush on" electrical tape is your friend, to do a repair, for now, if this is your issue.
So just tested, 12V with key on, only one one side of the injectors when unplugged. So all good short wise?
10-01-2022, 09:53 PM
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Correct. If you have spark, you have a crank trigger signal.
The injectors get 12 volts with the key on. And because they are a coil of wire inside, they will show voltage on both sides, with key on.
The ground signal comes from the LH computer.
It's pretty common for the two wires to deteriorate under the boot and end up with two "bare sections". If any of the 8 locations (under the boots) touch wires together, the ground will be shorted out and the injectors will not work.
While I don't know what you have been doing, this frequently occurs when the loom is moved around.
You can confirm if this is the issue by unplugging all 8 injectors and then seeing if you have 12 volts on both sides (you should not.)
If you do, pull back boots until you find the "crossed" wires.
Some "brush on" electrical tape is your friend, to do a repair, for now, if this is your issue.
The injectors get 12 volts with the key on. And because they are a coil of wire inside, they will show voltage on both sides, with key on.
The ground signal comes from the LH computer.
It's pretty common for the two wires to deteriorate under the boot and end up with two "bare sections". If any of the 8 locations (under the boots) touch wires together, the ground will be shorted out and the injectors will not work.
While I don't know what you have been doing, this frequently occurs when the loom is moved around.
You can confirm if this is the issue by unplugging all 8 injectors and then seeing if you have 12 volts on both sides (you should not.)
If you do, pull back boots until you find the "crossed" wires.
Some "brush on" electrical tape is your friend, to do a repair, for now, if this is your issue.
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10-01-2022, 10:44 PM
#8
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Correct. If you have spark, you have a crank trigger signal.
The injectors get 12 volts with the key on. And because they are a coil of wire inside, they will show voltage on both sides, with key on.
The ground signal comes from the LH computer.
It's pretty common for the two wires to deteriorate under the boot and end up with two "bare sections". If any of the 8 locations (under the boots) touch wires together, the ground will be shorted out and the injectors will not work.
While I don't know what you have been doing, this frequently occurs when the loom is moved around.
You can confirm if this is the issue by unplugging all 8 injectors and then seeing if you have 12 volts on both sides (you should not.)
If you do, pull back boots until you find the "crossed" wires.
Some "brush on" electrical tape is your friend, to do a repair, for now, if this is your issue.
The injectors get 12 volts with the key on. And because they are a coil of wire inside, they will show voltage on both sides, with key on.
The ground signal comes from the LH computer.
It's pretty common for the two wires to deteriorate under the boot and end up with two "bare sections". If any of the 8 locations (under the boots) touch wires together, the ground will be shorted out and the injectors will not work.
While I don't know what you have been doing, this frequently occurs when the loom is moved around.
You can confirm if this is the issue by unplugging all 8 injectors and then seeing if you have 12 volts on both sides (you should not.)
If you do, pull back boots until you find the "crossed" wires.
Some "brush on" electrical tape is your friend, to do a repair, for now, if this is your issue.
There is a 2 pin, grey plug with red yellow and red grey wire colours. I can't for the life of me see where it goes. I just checked the rear grounds which are fine, knock sensors, MAF and crank are the rest of the wires that come out of the BOE wiring harness, but they are already plugged in... I hope this plug isn't related to the F.I circuit...
10-01-2022, 10:46 PM
#9
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If there was a short between the two injector wires at some point, then it is likely that the driver transistor in the LH is blown, trying to switch +12V to ground with no resistance in the path.
If the LH has never been rebuilt then do that, there is a ceramic hybrid circuit that needs to go away. a blown driver is much simpler.
10-01-2022, 11:06 PM
#10
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There is a 2 pin, grey plug with red yellow and red grey wire colours. I can't for the life of me see where it goes. I just checked the rear grounds which are fine, knock sensors, MAF and crank are the rest of the wires that come out of the BOE wiring harness, but they are already plugged in... I hope this plug isn't related to the F.I circuit...
10-01-2022, 11:19 PM
#11
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There is a possibility that the wire colors are discolored and you could be right. Would this ISV be visible without removing the manifold? Would it be likely to cause a no start or is this something I shouldn't worry about right now. I'm going to try and get my hands on a noid light tomorrow so I don't have to use the oscilloscope.
In regards to rebuilding the LH, I am in New Zealand and my budget is tight so this could be a struggle. I have removed the ECU's and will have a look at them under a microscope for anything obvious. I can't help but think I am missing something, or just doing something wrong. Little bit of history, crank sensor and female connector (loom side) got replaced. All the boots on ALL the engine harness plugs got pulled back and inspected for shorts so I am 90% sure that isn't the issue. Is there anything else in the fuel injection circuit I should be checking before rebuilding the LH?
In regards to rebuilding the LH, I am in New Zealand and my budget is tight so this could be a struggle. I have removed the ECU's and will have a look at them under a microscope for anything obvious. I can't help but think I am missing something, or just doing something wrong. Little bit of history, crank sensor and female connector (loom side) got replaced. All the boots on ALL the engine harness plugs got pulled back and inspected for shorts so I am 90% sure that isn't the issue. Is there anything else in the fuel injection circuit I should be checking before rebuilding the LH?
10-02-2022, 02:13 AM
#12
Former Sponsor
Do you have power and grounds to LH?
Is the LH getting the signal from the crank trigger?
While you may have other posts about your vehicle have no idea of what you have been doing to this vehicle, before this thread. (I can barely keep up with the thirty cars at my shop, much less all the cars here.)
How about a summary of that, to help me?
Is the LH getting the signal from the crank trigger?
While you may have other posts about your vehicle have no idea of what you have been doing to this vehicle, before this thread. (I can barely keep up with the thirty cars at my shop, much less all the cars here.)
How about a summary of that, to help me?
10-02-2022, 06:40 PM
#13
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Do you have power and grounds to LH?
Is the LH getting the signal from the crank trigger?
While you may have other posts about your vehicle have no idea of what you have been doing to this vehicle, before this thread. (I can barely keep up with the thirty cars at my shop, much less all the cars here.)
How about a summary of that, to help me?
Is the LH getting the signal from the crank trigger?
While you may have other posts about your vehicle have no idea of what you have been doing to this vehicle, before this thread. (I can barely keep up with the thirty cars at my shop, much less all the cars here.)
How about a summary of that, to help me?
My aplogies, yes I have power and ground to the LH, next up is I'm going to test the EZK speed signal to the LH, then I know I can be sure the LH is rooted.
Engine has been out, headgaskets done, cambelt, w/pump etc. Engine back in. Replaced crank sensor, replaced seized fuel pump, replaced ALL igniton and fuel injection relays. Re-insulated various shorts in the engine bay wiring loom. There is a heap more but it's a pain to type on my phone so I'll write out more when I have PC access. Fuel reaches fuel rail, spark at ignition leads with spark tester. Fuel isn't being squirted in.
10-04-2022, 02:20 PM
#15
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After finally thinking she would start, she isn't.
I initially had a seized fuel pump which got replaced, fuel is now reaching the injector rails, but the injectors arent firing as the plugs are bone dry. I have spark and the tach moves fine. After some ideas, I have scoured the forums too.
I have a scope, when testing the injectors there is a constant 12v with igntion on and whilst cranking on both wires. Am I wrong in thinking these should be being switched to ground? Thanks guys!
I initially had a seized fuel pump which got replaced, fuel is now reaching the injector rails, but the injectors arent firing as the plugs are bone dry. I have spark and the tach moves fine. After some ideas, I have scoured the forums too.
I have a scope, when testing the injectors there is a constant 12v with igntion on and whilst cranking on both wires. Am I wrong in thinking these should be being switched to ground? Thanks guys!
if you can swap with a known good LH. it could prevent you from chasing your tail for a while.