Fuel Pump Fuse Keeps Blowing
03-11-2008, 02:21 AM
#1
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Fuel Pump Fuse Keeps Blowing
....about 20 - 30 seconds after I turn it over. 
- the car is a 1990 euro S4 with 118000km -
Unfortunately I have virtually no service history.
As some of you may have read in my intro thread I just bought this car and the fuel has recently been drained from it for shipping, the battery has been boosted and I am sure the car has been flooded a few times in the last week.
She fires right up when I turn the key...idle starts to settle and smooth out....then the fuse goes and the car stalls. I bought fuses with the little led that lights as soon as it fails...I can start it and watch the fuse go from the drivers seat.
I have swapped the relays from horn to fuel pump...no change
I try to listen for the pump when I turn the key over..I hear the relay click but nothing else...however one of the two pumps must be operational to have the car start. I was of the understanding I should hear the pump pressurize as I have clearly in VW's I have owned.
After it stalls the smell of fuel is very heavy.
I placed my first order of maintenance parts and want the car gone over top to bottom....
If any of you can point me in the right direction please do.
Brad
- the car is a 1990 euro S4 with 118000km -
Unfortunately I have virtually no service history.
As some of you may have read in my intro thread I just bought this car and the fuel has recently been drained from it for shipping, the battery has been boosted and I am sure the car has been flooded a few times in the last week.
She fires right up when I turn the key...idle starts to settle and smooth out....then the fuse goes and the car stalls. I bought fuses with the little led that lights as soon as it fails...I can start it and watch the fuse go from the drivers seat.
I have swapped the relays from horn to fuel pump...no change
I try to listen for the pump when I turn the key over..I hear the relay click but nothing else...however one of the two pumps must be operational to have the car start. I was of the understanding I should hear the pump pressurize as I have clearly in VW's I have owned.
After it stalls the smell of fuel is very heavy.
I placed my first order of maintenance parts and want the car gone over top to bottom....
If any of you can point me in the right direction please do.
Brad
03-11-2008, 07:39 AM
#2
Brad,
The fuse should be No 38 and is rated at 15 amps and supplies the 2 fuel pumps and the O2Sensor-Heating. For reference you can get a fuse and relay chart from http://928gt.com/928specs/90relay.htm . This fuse is located down stream of the fuel pump relay XXVI. Better still you should purchase Jim Moorehouse Technical CDs, they are invaluable and have the electrical wiring diagrams that you will require as well as nearly all the information to maintain your 928 including the WSM and PET6. Jim is a member of Rennlist and you can contact him through this forum.
The indication of a fuse that continuously blows is that the current draw is exceeding the rated capacity of the fuse, however, with the delay it does not appear to be a full short otherwise you would get an blown fuse almost immediately.
As you have advised the fuel was drained out of the fuel tank, so do you know whether the car was turned over (cranked) without fuel. The 2 fuel pumps are fully submerged and rely on the fuel for cooling and lubrication. If they have been run dry there may have internal damage to the electrical wiring, which will be required to be checked out.
The first pump is an intank pump located at the bottom of the fuel tank just upstream of the second pmp. The second pump is an external pump and is located just under the RHS rear of the vehicle. There are photographs and a description of how to change out the external fuel pump on the web site of www.928gt.com, so if you down load a copy of this procedure you will be able to recognise where the pump is located and how to get to it.
Fault Finding.
First I would check the electrical connections to the fuel pumps to ensure that you have no shorts or bad connections. Before you check this ensure that you disconnect the negitative lead from your battery as a safety precaution, as you are working near fuel.
Second. With the fuse removed and the electrical connections to the fuel pumps dsiconnected check for continuity of the wiring to each fuel pump by using a Digital Multi Meter (DMM) in series with a jumper lead from the Central Electrical Panel fuse 38 outlet to the pump inlet connection ( while it is disconnected). You should get a small resistance reading (ohm). If you get an infinity reading you have a broken wire.
Third. With the DMM connect one lead to one end of the wire and the other to an earth connection. If you get an infinity reading then the wire is OK. If you get a resistance reading you have a short, which you will have to locate and repair. If you believe that you have a short circuit you can make up an external jumper lead and connect it to the fuse outlet and to each pump inlet connection as the pumps are wired in parallel. Re connect the negative leads of the fuel pumps and try them out by reconnecting the negative terminal of the battery. I would suggest that you use a fuel pump relay jumper lead to do this test. I describe this lead below.
Forth
If the wiring is OK you need to check out each pump for its operation. As every 928 owner should know you should carry a jumper lead to jump terminal 30 and 87 of the fuel pump relay for checking out the operation of the fuel pumps (Do not, repeat Do not connect any jumper lead to terminal 85 or 86 otherwise you will destroy the LH computer). A more sophisticated jumper lead is one that has a switch included in the circuit and the lead should be long enough so that the switch can be operated from the back of the car near the fuel pumps. In this case if your DMM has a amphere measuring connection, preferably 15 amps, connect the DMM in series with the jumper lead, turn on the switch and read the amp draw. I should be well below 15 amps. Note the reading. If the reading is over 15 amps turn the switch off immediately otherwise you will "fry" your DMM. Do the same for the second pump and record the reading. It should be well below the 15 amps, as a matter of fact the total load of both pumps should be below 15 amps. If this is the case then the fault may be in the wiring to the O2 sensor and heater on the Cats.
Fifth.
Chech in continuity of the wiring to the O2 sensor and also whether there is a short circuit as described above. Unfortuately I not home at the moment and I do not have access to all my manuals etc., so I cannot advise on how to check out the O2 Sensor - Heater. In addition my car is not fitted with Cats.
By carring out the above you should be able to locate the fault which will require repair or replacement of the defective component.
Tails 1990 928 S4 Auto
The fuse should be No 38 and is rated at 15 amps and supplies the 2 fuel pumps and the O2Sensor-Heating. For reference you can get a fuse and relay chart from http://928gt.com/928specs/90relay.htm . This fuse is located down stream of the fuel pump relay XXVI. Better still you should purchase Jim Moorehouse Technical CDs, they are invaluable and have the electrical wiring diagrams that you will require as well as nearly all the information to maintain your 928 including the WSM and PET6. Jim is a member of Rennlist and you can contact him through this forum.
The indication of a fuse that continuously blows is that the current draw is exceeding the rated capacity of the fuse, however, with the delay it does not appear to be a full short otherwise you would get an blown fuse almost immediately.
As you have advised the fuel was drained out of the fuel tank, so do you know whether the car was turned over (cranked) without fuel. The 2 fuel pumps are fully submerged and rely on the fuel for cooling and lubrication. If they have been run dry there may have internal damage to the electrical wiring, which will be required to be checked out.
The first pump is an intank pump located at the bottom of the fuel tank just upstream of the second pmp. The second pump is an external pump and is located just under the RHS rear of the vehicle. There are photographs and a description of how to change out the external fuel pump on the web site of www.928gt.com, so if you down load a copy of this procedure you will be able to recognise where the pump is located and how to get to it.
Fault Finding.
First I would check the electrical connections to the fuel pumps to ensure that you have no shorts or bad connections. Before you check this ensure that you disconnect the negitative lead from your battery as a safety precaution, as you are working near fuel.
Second. With the fuse removed and the electrical connections to the fuel pumps dsiconnected check for continuity of the wiring to each fuel pump by using a Digital Multi Meter (DMM) in series with a jumper lead from the Central Electrical Panel fuse 38 outlet to the pump inlet connection ( while it is disconnected). You should get a small resistance reading (ohm). If you get an infinity reading you have a broken wire.
Third. With the DMM connect one lead to one end of the wire and the other to an earth connection. If you get an infinity reading then the wire is OK. If you get a resistance reading you have a short, which you will have to locate and repair. If you believe that you have a short circuit you can make up an external jumper lead and connect it to the fuse outlet and to each pump inlet connection as the pumps are wired in parallel. Re connect the negative leads of the fuel pumps and try them out by reconnecting the negative terminal of the battery. I would suggest that you use a fuel pump relay jumper lead to do this test. I describe this lead below.
Forth
If the wiring is OK you need to check out each pump for its operation. As every 928 owner should know you should carry a jumper lead to jump terminal 30 and 87 of the fuel pump relay for checking out the operation of the fuel pumps (Do not, repeat Do not connect any jumper lead to terminal 85 or 86 otherwise you will destroy the LH computer). A more sophisticated jumper lead is one that has a switch included in the circuit and the lead should be long enough so that the switch can be operated from the back of the car near the fuel pumps. In this case if your DMM has a amphere measuring connection, preferably 15 amps, connect the DMM in series with the jumper lead, turn on the switch and read the amp draw. I should be well below 15 amps. Note the reading. If the reading is over 15 amps turn the switch off immediately otherwise you will "fry" your DMM. Do the same for the second pump and record the reading. It should be well below the 15 amps, as a matter of fact the total load of both pumps should be below 15 amps. If this is the case then the fault may be in the wiring to the O2 sensor and heater on the Cats.
Fifth.
Chech in continuity of the wiring to the O2 sensor and also whether there is a short circuit as described above. Unfortuately I not home at the moment and I do not have access to all my manuals etc., so I cannot advise on how to check out the O2 Sensor - Heater. In addition my car is not fitted with Cats.
By carring out the above you should be able to locate the fault which will require repair or replacement of the defective component.
Tails 1990 928 S4 Auto
03-11-2008, 08:48 AM
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This has been covered before. Sounds like your O2 sensor may be shorting out.
"Blowing fuel pump fuses on your S-4? Check this out!"
"Blowing fuel pump fuses on your S-4? Check this out!"
03-11-2008, 06:56 PM
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Shorted O2 wiring is much more likely. I have seen an S4 get its firewall area seriously burned by a fire started by somebody undoing the O2 sensor from the exhaust without disconnecting the wiring properly, thereby twisting the wires and straining them. Cost best part of $10k to fix properly. Disconnect O2 wires inside the car, and try fuel pump fuse again. My single pump pulled 7A last time I measured it.
I don' got no steenkin' O2 sensor here....
jp 83 Euro S AT 51k
I don' got no steenkin' O2 sensor here....
jp 83 Euro S AT 51k
03-11-2008, 09:56 PM
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O2 sensor sounded promising... Disconnected it, started her up and she blew just like before...
So much for the easy fix..
Oh well.
On to the rest of the diagnosis..
So much for the easy fix..
Oh well.
On to the rest of the diagnosis..
03-11-2008, 10:02 PM
#6
Drifting
where did you you un plug it from? Hopefully under the fuse panel. If it is not the O2 sensor, hooked up a Multimeter in series at the fuel pump fuse (with a fuse of coarse) and see how much current you get. Then do the same except disconnect the fuel pumps at the pumps. you may have a bad pump that is pulling too much current. This makes sense b/c it is blowing after 20-30 seconds or so rather than in a few seconds.
03-11-2008, 10:06 PM
#7
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Be aware that ALL 928s had the wiring fitted for two pumps, whether or not they were fitted. If only one pump is fitted the extra 2 wires are taped up and folded back on the loom . If these have been disturbed or corroded they could short. Do you know if you have 2 pumps or not ? Drop the pump cover (2 x 10mm nuts up under the bumper cover, look in horizontally fwd to where the pump hose goes into the tank - if there are wires going in there, you have 2 pumps. Many cars have the in tank pump fail....
jp
jp
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04-03-2008, 01:32 AM
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Problem solved...in tank fuel pump replaced and no more blown fuses...
Thanks guys.
She purrs....what a beautiful much anticipated sound.
....now onto the windows
Thanks guys.
She purrs....what a beautiful much anticipated sound.
....now onto the windows
