Blowing fuel pump fuses on your S-4? Check this out!
09-28-2005, 10:53 PM
#1
Drifting
Thread Starter
Blowing fuel pump fuses on your S-4? Check this out!
Are you blowing fuel pump fuses on your S-4 and don't know why? Try this.....unplug your O2 sensor and then start it up! That's right....the O2 sensor is tied in with the fuel pump fuse! And if your O2 sensor insulation is as nasty and crumbly as mine, it will short and blow the fuel pump fuse when you turn the key. Hope this saves some 
one hours of troubleshooting and fuel pump changing!
one hours of troubleshooting and fuel pump changing!
08-16-2006, 08:29 AM
#6
Race Car
08-16-2006, 08:45 AM
#7
Drifting
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Yeah, if you haven't checked your O2 sensor wire routing lately, good idea. If the wires fry on the exhaust, there could be more trouble from them than that.
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08-17-2006, 06:24 PM
#8
Rennlist Member
yes
check them VERY CAREFULLY. Mine shorted with the expected fuse blows... but it looks like it may have taken the LH brain with it. 
I'm hoping Rich can rebuild it and its death was a coincidence - and it isn't full of charcoal.
I'm hoping Rich can rebuild it and its death was a coincidence - and it isn't full of charcoal.
08-18-2006, 07:26 PM
#9
Rennlist Member
OK I just got an email back from Rich. My LH is fine.
What say the guru's on this one? Here's the background:
1987 S4 - 5 Speed
113k miles.
Replaced O2 sensor 3 months ago
Blue a fuel pump/sensor fuse two weeks ago. All the O2 sensor wires were bare (!). Buttoned back up, no more blown fuses.
Spent probably an hour or two cranking & testing before that to figure out it was the fuse. Relays jumpered and checked. 12V at the injectors, but no apparent signal. Figured the brain was zotzed, but it is OK.
Have spark. Fuel pump runs fine. Have fuel.
I'm beyond frustrated here. Last time I spent 20+ hours diagnosing and replacing the crank angle sensor. It's fine now! (didn't have spark that time)
Not sure what to check next. I've gone through most of the EKZ/LH troubleshooting/training manual from Porsche (awesome book).
I've got an o-scope and was using it, but can't produce the nice display/waves seen in the manual for ANY of the tests. I'm betting it is user (me) rather than the scope, because I can't get the triggering right. Any suggestions on setup here?
What say the guru's on this one? Here's the background:
1987 S4 - 5 Speed
113k miles.
Replaced O2 sensor 3 months ago
Blue a fuel pump/sensor fuse two weeks ago. All the O2 sensor wires were bare (!). Buttoned back up, no more blown fuses.
Spent probably an hour or two cranking & testing before that to figure out it was the fuse. Relays jumpered and checked. 12V at the injectors, but no apparent signal. Figured the brain was zotzed, but it is OK.
Have spark. Fuel pump runs fine. Have fuel.
I'm beyond frustrated here. Last time I spent 20+ hours diagnosing and replacing the crank angle sensor. It's fine now! (didn't have spark that time)
Not sure what to check next. I've gone through most of the EKZ/LH troubleshooting/training manual from Porsche (awesome book).
I've got an o-scope and was using it, but can't produce the nice display/waves seen in the manual for ANY of the tests. I'm betting it is user (me) rather than the scope, because I can't get the triggering right. Any suggestions on setup here?
08-18-2006, 08:11 PM
#10
Under the Lift
Lifetime Rennlist
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You say you have spark.
You say you have fuel.
BUT, will the car fire up on starter fluid shot down into the MAF?
If so, you may have fuel in the rails, but no fuel going into the cylinders.
Next test would be to check injectors with noid light. If they aren't firing with a good LH brain, then possible short in one of the injector leads. A single shorted lead will take out all the injectors.
You have evidence already - 12V to the injectors but "no signal." I.e., they are not being taken to ground due to a short.
You say you have fuel.
BUT, will the car fire up on starter fluid shot down into the MAF?
If so, you may have fuel in the rails, but no fuel going into the cylinders.
Next test would be to check injectors with noid light. If they aren't firing with a good LH brain, then possible short in one of the injector leads. A single shorted lead will take out all the injectors.
You have evidence already - 12V to the injectors but "no signal." I.e., they are not being taken to ground due to a short.
08-22-2006, 01:04 AM
#11
Rennlist Member
Haven't gotten my brain back yet so I haven't been able to do any testing. Hopefully I'll be able to hit it tomorrow.
I'd find it quite surprising that something not related to the fuel fuse would die at the same time, but wierder things have happened.
I'd find it quite surprising that something not related to the fuel fuse would die at the same time, but wierder things have happened.
08-22-2006, 07:31 PM
#12
Rennlist Member
OK.
Car WILL run on starter fluid - we've got spark.
Re-checked everything - Fuel pump relay NOT energizing. I am not getting voltage at the pump, nor the O2 sensor. Pump works when I bridge 87/30 terminals. I'm also getting voltage at the 20pin on the LH brain plug (through relay XX)
Relay is fine, works in the horn slot.
LH Brain fresh back from Rich - worked great in his car.
It sounds like the crank sensor issue; but since I'm getting spark I have my doubts. I'm getting a waveform out of the EZK for the speed signal to the LH but I can't duplicate what's in the test manual. Since I have spark I assumed it was due to to operator error?? I'm going to dig into my crank angle sensor wiring again next but I needed a break.
Could the EZK been damaged with my little O2 short episode??? I am totally lost.
Car WILL run on starter fluid - we've got spark.
Re-checked everything - Fuel pump relay NOT energizing. I am not getting voltage at the pump, nor the O2 sensor. Pump works when I bridge 87/30 terminals. I'm also getting voltage at the 20pin on the LH brain plug (through relay XX)
Relay is fine, works in the horn slot.
LH Brain fresh back from Rich - worked great in his car.
It sounds like the crank sensor issue; but since I'm getting spark I have my doubts. I'm getting a waveform out of the EZK for the speed signal to the LH but I can't duplicate what's in the test manual. Since I have spark I assumed it was due to to operator error?? I'm going to dig into my crank angle sensor wiring again next but I needed a break.
Could the EZK been damaged with my little O2 short episode??? I am totally lost.
08-22-2006, 07:55 PM
#13
Rennlist Member
Here's the "output" of pin 13 on my o-scope - at least close to it.
There's no scale on the output diagram in the manual. Very irritating, so I'm not even sure if the period/magnitude is close??
There's no scale on the output diagram in the manual. Very irritating, so I'm not even sure if the period/magnitude is close??
08-25-2006, 10:59 PM
#14
Rennlist Member
bump...
I've purchased this unit yesterday to assist me with my diagnosis... and I'm a geek, so it should be a good addition to my laptop.
http://www.picotech.com/single-chann...illoscope.html
I've purchased this unit yesterday to assist me with my diagnosis... and I'm a geek, so it should be a good addition to my laptop.
http://www.picotech.com/single-chann...illoscope.html
08-26-2006, 09:55 AM
#15
Rennlist Member
Make sure that that signal also gets to pin 1 LH ECU. It should be 12v amplitude and 2 x engine rpm.
Does the tach flicker when you crank the engine, or show OK when you start the engine on lighter fluid ?
You say fuel pump is not beintg energised, so check you have 12v on on side of the FP relay pin 86 according to the manual, this is ignition switched 12. When the LH gets the rpm pulse from the EZK it grounds the other side of the FP relay coil.
Don't worry about those "wiggles" on the rpm waveform, it's the amplitude that is important.
When you bridge the FP contact, won't the car start ?
If not double check that the rpm pulses are getting to pin 1 LH.
Does the tach flicker when you crank the engine, or show OK when you start the engine on lighter fluid ?
You say fuel pump is not beintg energised, so check you have 12v on on side of the FP relay pin 86 according to the manual, this is ignition switched 12. When the LH gets the rpm pulse from the EZK it grounds the other side of the FP relay coil.
Don't worry about those "wiggles" on the rpm waveform, it's the amplitude that is important.
When you bridge the FP contact, won't the car start ?
If not double check that the rpm pulses are getting to pin 1 LH.