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Old 09-03-2019, 12:12 AM   #1
thessejway
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Default Blowing my fuel pump fuse

As the title says, my car it just blowing the #1 fuse like it's nothin.

My 93 945t has been blowing the fuel pump fuse the moment it's given a bit of throttle or sometimes just after a few seconds of idling. It happened the first time 2 minutes from my house after a ~150km drive and required a tow.

I have a brand new relay and pump in there and as far as I can tell the wiring from the fuse/relay to the pump is all good. I've jumped the bottom left to middle right pin on the relay to see if the pump is working, and it runs without blowing the fuse.

And idea or suggestions would be greatly appreciated

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Old 09-03-2019, 12:52 PM   #2
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Sounds like some electrons leaking to ground in the ignition switch circuit. Or possibly from the ignition coil, I believe the red/wire sends a signal to the relay that the car is running.

I just have to say you're supposed to suck not blow, common misconception....
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Old 09-03-2019, 01:46 PM   #3
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Thanks for the reply
I just put in a new bosch ignition coil but I will have to chase down that wire. Very possible that things got a little over heated in the engine bay on the long trip and turned a little melty next to the turbo.

Thanks for the advise, might be why my fuses aren't happy with me
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Old 09-03-2019, 01:48 PM   #4
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Could a clogged fuel filter or struggling in-tank pump be causing the main pump to work harder than it should and cause issues?
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Old 09-03-2019, 02:28 PM   #5
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Besides the fuel pump relay, fuse #1 also supplies power to the fuel injection and ignition ecu's as well as the oxygen sensor heater. Of those the most likely culprit is the latter. Check that the wiring to the sensor hasn't melted to the exhaust. If it looks okay try unplugging the heater as the sensor may have an internal short.
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Old 09-07-2019, 05:03 PM   #6
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Thanks for the replies everyone

Been super busy, so haven't had any time to look at it further until today.Took a quick visual inspection of all the wiring around the ignition coil and O2 sensor and didn't notice anything. Tried starting the car with the o2 sensor unplugged, but it still instantly blew the fuse when the engine cranked. Hopefully tomorrow I will have time to more to dig in a little deeper with a multi meter.

In the mean time, Is there anything else people think might be worth checking out?

I'm not great with electrical stuff so all tips and advice is appreciated.
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Old 09-14-2019, 12:23 PM   #7
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Still haven't solved my issue.

But some progress... I just figured out that it doesn't blow the fuse if I unplug the ecu. Not sure if that would give more of a hint to what it could be. Plugged in my stock untuned ecu and it still blows the fuse.
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Old 09-19-2019, 08:31 PM   #8
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Turns out all I had to do was clean the Ground for the o2 sensor. We'll see if that holds up
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Old 09-30-2019, 09:29 PM   #9
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Still having the issue

I thought I had fixed it, replaced the oxygen sensor, and all was well. Just went out for a drive, and my car started blowing the fuse every time I made a left turn or went over a bump. Definitely thinking its a short somewhere.

Is fuse 1 blowing on left turns a bumps a sign of anything in particular that anyone else has come across? Any help is appreciated. Thanks!

For clarification, the fuse doesn't blow when the car is stationary and I turn the wheel to the left. Only when it is in motion.

I am completely stumped and don't know where to look anymore.
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Old 09-30-2019, 10:15 PM   #10
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A wiring diagram.

Disable legs of the circuit to isolate the short.
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Old 10-01-2019, 07:45 AM   #11
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Quote:
Originally Posted by thessejway View Post
Still having the issue

I thought I had fixed it, replaced the oxygen sensor, and all was well. Just went out for a drive, and my car started blowing the fuse every time I made a left turn or went over a bump. Definitely thinking its a short somewhere.
You replaced the sensor. Did you replace the entire sensor with the car-specific wiring, or did you use a universal with splice kit? Have you tried disconnecting the sensor's heater plug under the hood before a test drive?

If you've done that, I have no more bright ideas past close physical inspection of the wiring, because I realize you can't isolate the wire that is shorting and drive the car as well, except for the O2 heater load. Yes, definitely a short somewhere.
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Old 10-01-2019, 11:49 AM   #12
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If it's a circuit that's needed, one can add a fuse to the suspect leg of the circuit, then see if it blows.

You could also do current measurements.
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Old 10-01-2019, 01:22 PM   #13
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IIRC the PCV heater on the rubber turbo intake boot is also powered through that fuse. You can just disconnect that and leave it off.
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Old 10-01-2019, 01:48 PM   #14
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Quote:
Originally Posted by ZVOLV View Post
If it's a circuit that's needed, one can add a fuse to the suspect leg of the circuit, then see if it blows.

You could also do current measurements.
+1
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Old 10-01-2019, 01:57 PM   #15
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How much fuel in the tank when you did the left turns/bumps? Still wondering if the in-tank pump is dead or dying, sloshing fuel could work the main pump harder and cause some problems?
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Old 10-01-2019, 03:07 PM   #16
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Quote:
Originally Posted by ZVOLV View Post
...one can add a fuse to the suspect leg of the circuit, then see if it blows..
It's a clever idea that was passed down to me that I wanted to share with the forum.
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Old 10-01-2019, 04:20 PM   #17
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Quote:
Originally Posted by ZVOLV View Post
It's a clever idea that was passed down to me that I wanted to share with the forum.
I might call that a pro tip.
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Old 10-01-2019, 05:21 PM   #18
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Thanks for the tips everyone! Gonna get right on this when the weather clears up.

Quote:
Originally Posted by ZVOLV View Post
A wiring diagram.

Disable legs of the circuit to isolate the short.
Quote:
Originally Posted by ZVOLV View Post
If it's a circuit that's needed, one can add a fuse to the suspect leg of the circuit, then see if it blows.

You could also do current measurements.
I do have a wiring diagram that I've been going off of, but I will certainly implement this brilliant technique

Quote:
Originally Posted by cleanflametrap View Post
You replaced the sensor. Did you replace the entire sensor with the car-specific wiring, or did you use a universal with splice kit? Have you tried disconnecting the sensor's heater plug under the hood before a test drive?

If you've done that, I have no more bright ideas past close physical inspection of the wiring, because I realize you can't isolate the wire that is shorting and drive the car as well, except for the O2 heater load. Yes, definitely a short somewhere.
I did have to splice the connectors, certainly an easy test that's worth a shot

Quote:
Originally Posted by PCH View Post
IIRC the PCV heater on the rubber turbo intake boot is also powered through that fuse. You can just disconnect that and leave it off.
Easy enough, so its worth a shot

Quote:
Originally Posted by Bricktothefuture View Post
How much fuel in the tank when you did the left turns/bumps? Still wondering if the in-tank pump is dead or dying, sloshing fuel could work the main pump harder and cause some problems?
This was a thought of mine too, but early in my testing I was still having the issue of the fuse blowing on startup when it was disconnected. So I ruled it out

Again thanks everyone for the help. I'm absolutely clueless and impatient when it comes to electrical issues. So this is all very appreciated!
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Old 10-05-2019, 01:42 PM   #19
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Working on the car now and noticing that the heated lamda sensor and in tank fuel pump are both of fuse 11. Whereas fuse 1 is the one blowing. Can they still cause issues on fuse 1 or can I cancel them out as being the cause?
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Old 10-05-2019, 02:08 PM   #20
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Quote:
Originally Posted by thessejway View Post
Working on the car now and noticing that the heated lamda sensor and in tank fuel pump are both of fuse 11. Whereas fuse 1 is the one blowing. Can they still cause issues on fuse 1 or can I cancel them out as being the cause?
I believe you can cancel them out as being the cause.
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Old 10-06-2019, 03:57 PM   #21
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Quote:
Originally Posted by thessejway View Post
Working on the car now and noticing that the heated lamda sensor and in tank fuel pump are both of fuse 11.
Actually, I think they're both being fed from the Fuel System Relay pin 5.

Following the diagram I have shows:

Battery > to Fuse 1

Fuse 1:
> to Ignition ECU pin 5
> to Fuel Injection ECU pin 4
> to Fuel system relay pin 3 brown (internally to pins 1 and 5 when triggered)

Fuel system relay pin 5:
> to fuse 11 pink (pumps)
> to O2 sensor heater pink

Fuel system relay pin 1:
> blue/yellow "jumper" to Fuel system relay pin 6, then to Radio Suppression Relay pin 2
> to large splice somewhere which feeds:
>> Idle valve blue/yellow
>> Mass Air meter pin 5 blue/yellow
>> PTC resistor (PCV heater) blue/yellow
>> Fuel Injection ECU pin 9 blue/yellow

A short in any of those wires could blow fuse #1.
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