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Wayah Road Warrior
 
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930 Fuel Pump Fuse Overload - The Solution

Are you tired of being stranded by the side of the road with a blown fuel pump fuse on your 930?

Are you tired of worrying if you will be starting the next 'Help ... My car won't start thread?

Well here you go. Illustrated step-by-step instructions for splitting out the electrical load on your fuel pump circuit.

This is what caused me to finally get around to doing the upgrade :



Scary? Here is a closer look. The wire was charred and brittle. The fuse holder had melted. God knows what was keeping the circuit complete, but I suspect I was running without benefit of a functioning fuse.



Here is how the circuit fit into the larger scheme of things. The OE bullet fuse (#6, the empty slot) had been replaced by a 25 amp blade fuse but still carried the load from both fuel pumps.



First thing ... and very important ... disconnect the battery. You are going to be messing with a plateload of spaghetti behind the fuse box ... nothing good can come from that if the system is carrying a charge .....



Ok, time to get access to the back of the fuse block. Remove the 10mm Hex screws in the upper right and left of the fuse block.



Now, remove the phillips screw in the lower left corner of the fuse block. Be careful you do not let the nylon spacer behind the panel get away from you. If it falls, you will not be able to retrieve it.





Remove the smaller phillips screw in the lower right. (I have a headlight relay modification installed, your fuse block may look a wee bit different).



Now give yourself a little more room to work by freeing up the wires from the chassis mounted wiring loom clip. It bends to free the wires.

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Last edited by Shadetree930; 11-15-2010 at 06:06 AM..
Old 11-13-2010, 12:14 PM
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Wayah Road Warrior
 
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Continued

Next thing I did was to clean up the previous wiring. I removed hot lead from the original fuel pump fuse circuit (Fuse #6) at the hot side (top) of fuse block. You may not need to do this in a OE environment.



I cut the fried blade fuse wiring off of the original factory wire and soldered the end. (In the factory wiring loom, this wire runs up to fuel pump relay #1 which then jumps to fuel pump relay #2).



I then reconnected this to the bottom of the fuse #6 holder. It will no longer have a purpose, but I did not want a loose wire floating around behind the fuse block.



Next I removed fuel pump relay #1(closest to the windshield) from the fuse block holder and pulled the socket out from the back of the fuse block. Pro-tip ... when re-installing these plugs, use a little lube. It helps with the tight fit.



In order to find the jumper wire between the two relays, I had to unwrap the OE sticky tape.



The wiring is really secured. I had to cut the string holding the bundles in place as well.



Repeat this process for fuel pump relay #2. Once you have things sufficiently unwrapped, you should be able to find the jumper wire that runs from pin #30 of fuel pump relay #1 and pin #30 of fuel pump relay #2. Hint ... it is the RED one.



The point of the project is to cut the jumper wire and run power from the battery to each fuel pump relay pin #30 thereby reducing the overall load (for the entire fuel pump circuit) running through a single wire (from fuse #6 to relay #1, and then to relay #2).

I am going to put a blade fuse in-line from the battery to each relay. This is the blade fuse assembly that will be used (requires two). Make sure the wiring is 12 Gauge.

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Last edited by Shadetree930; 11-15-2010 at 06:14 AM..
Old 11-13-2010, 12:38 PM
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Wayah Road Warrior
 
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Continued

I installed some crimp connectors on each end and also added a few feet of 12 guage wire to the battery end.



Time to cut the jumper.



Crimped the blade fuse onto the #1 relay wire.



Repeated for the #2 relay wire.

Then I had a great idea. Rather than have a ghetto install, I drilled holes into the top of the fuse block holder. I would run the wire up through the holes allowing quick access to the fuses. The magnet is there to catch filings and shrapnel from getting into places where it could cause me grief later on.



Four holes in all....



I had to redo my earlier crimps in order to get through the holes but the end result is much cleaner looking. A word of caution... be careful how you place these. The hood lands right where the fuse holder is. If you are not careful, you could crush the fuses. Make sure they lay flat, not standing up (like in this picture) and you will have no problem.

The location of this wiring mod as well as the drilling and holes were specifically located where they were so that the state of the fuses could been seen by me without removing the cover and also so that the next owner or any mechanic working on the car (if there ever is one) would clearly see this mod had been done. These are the two fuses you quickly want to be able to see on a dark and stormy night as you poke under the hood in the middle of the mountains.

Updated since post below .....

If I was working for NASA or Boeing, I would have put in grommets but really ... I am a SHADETREE mechanic ... we don't need no stinking grommets. (Edit) Truth is ... I subsequently used some 1/4 hose and made grommets/sleeves for the wires ... risk mitigated.



Repeat for relay #2 and once snaked through the holes in the fuse block holder, install a 3/8" ring terminal on the end of each wire. This end will install on the positive post of the battery.





Time to button things up. Just work backwards to re-install everything.

The finished product. I used a 15 amp fuse on relay #1 and a 25 amp fuse on relay #2.



BTW .... she fired right up afterwards. No drama.
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Last edited by Shadetree930; 08-06-2011 at 09:50 AM..
Old 11-13-2010, 12:59 PM
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OFF THE BOOST PIPE NOW...
 
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Great wriite up Paul. All I might add is to add grommets where the wires come out the top.

Last edited by A930Rocket; 11-13-2010 at 03:27 PM.. Reason: Fixed name error for Cole... :)
Old 11-13-2010, 01:10 PM
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Very nice.

Thanks for posting.

Jesper
Old 11-13-2010, 01:27 PM
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OFF THE BOOST PIPE NOW...
 
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Sorry Paul. I read the write up with out looking at the name and was thinking of Cole doing more electrical work...
Old 11-13-2010, 03:29 PM
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This is huge Paul - thanks for posting it!

With my recent issue of the fuse melting out of nowhere, is it possible there's nothing 'wrong' per se (i.e. fuel pump on way out or break in a wire)?

Did yours just start melting that fuse out of nowhere like mine?

Tx again - I'll endeavor this over the winter's slumber... she found her way into the garage finally last night after TR3 went to storage...
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Old 11-15-2010, 05:48 AM
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Quote:
Originally Posted by krasuskyp View Post
This is huge Paul - thanks for posting it!

With my recent issue of the fuse melting out of nowhere, is it possible there's nothing 'wrong' per se (i.e. fuel pump on way out or break in a wire)?

Did yours just start melting that fuse out of nowhere like mine?

Tx again - I'll endeavor this over the winter's slumber... she found her way into the garage finally last night after TR3 went to storage...
Yep, nothing wrong with my fuel system except for the as designed overload.

The circuit is so overloaded that ANY additional resistance sets up the 'fuse blowing syndrome'.

In my case, once the bullet fuse had been replaced, the overload went to a new point of resistance (anywhere there was a wire splice/join) and the heat went to work on the wires or whatever was in the area.

After a few blown fuses and finally upon seeing what could have turned into an electrical fire I decided it was time to set up shop under my shade tree and do some fix'n.

If I were a betting man, you have a crappy connection somewhere on the hot side of your fuel circuit. The result .... blown fuses. The crappier the connection .... the more often the failure.

Split the load and report back. I predict that you owe me a Bitburger bier. If on the other hand you end up needing a front fuel pump, call me. I have one rattling around in my bottomless TurboPalooza toolkit.
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Last edited by Shadetree930; 11-15-2010 at 06:31 AM..
Old 11-15-2010, 05:59 AM
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UDAMAN Paul - hugely appreciated. I'll find ?some? time this winter and run some diag's.

The rear fuse block is def corroded looking from being parked outside all summer. Damn I need a 3rd (4th!) bay...

Tx for the offer!!!
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past:

| '01 ///M5 | '96 993 C2 cab | '05 S600 Sport - biturbo V12@Just Not Right 495rwhp / 612rwtq |
| '58 TR3A | '01 //S8 | '95 //S6 6gang | '88 ///M5 | '87 190E 2.3-16 |
Old 11-15-2010, 06:54 AM
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Smart quod bastardus
 
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Sorry to resurrect this thread but I split the load on both fuel pump relays like described here and the issue of hot fuses has gone away.

Reason I am posting now is to find out how hot the relays feel after running the car for a short trip.

My car has developed a miss which I have been assuming all along is ignition related but am now wondering if it might be a rear fuel pump going bad.

The front pump was changed 5 months ago to a 044 pump when I split the relays circuit, but my relays were so hot last weekend after a 40 mile drive that i could not touch them without burning my fingers.

The front relay was hotter than the rear which was also very warm but I could touch it without being singed unlike the front pump relay.

Question 1):
Is this a sign that the front pump is doing all the work and the rear is failed or almost failed?

Question 2):
I pulled the rear pump relay and the car still idled...... how is this possible? Is the front pump is big enuff to run the car without a rear pump.

Thanks,
Fred
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Old 09-23-2013, 12:28 PM
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I'm under the impression that splitting the power is a solution the buys time for the invitable requirement of two new pumps, or to aid in safely delivering power to uprated pumps.

I'd say, with confidence, that overly hot relays are bad.
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Old 09-24-2013, 09:43 AM
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The fuel pump wiring and relay setup in these cars is just plain awful. The one fuse for two pumps is even worse. Enough has been posted about it.
It's not if, it's when will it have problems.

Probably all the red relays are made in China now so carry a bunch of spares in the car somewhere for the inevitable coasting in nuetral to the side of the road.
Old 09-24-2013, 10:49 AM
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As a 928 guy, I'm well versed in carrying spare relays!

I can definately see the need for some updates on the wiring, especially as the harness and other parts age, outside of the pumps. Great writeup here, I'm planning on following it pretty closely. I'll have to do some searching, it would be interesting to monitor the amperage on each pump channel over time to see how stable it is.
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Old 09-24-2013, 11:02 AM
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Any time relays are hot it means too many amps are being pulled through. So let's look at the causes.

1. Bad internal relay contacts. Burnt, damaged, etc. Not making full contact in order to pass full amp load. Relay too small for the load.

2. Bad relay sockets. These sockets melt when over-heated and get rubber between the terminal and pin. Make sure the terminals and pins are both clean and shiny, and the terminals are tight and make good contact to the pins.

3. Pumps pulling too many amps. CIS runs at 7 to 8 bar of pressure, and in a perfect world two pumps in series would share that pressure. I.E. each pump pushes 4 bar. An 044 pump pulls about 11.5 amps at 4 bar, but at 8 bar the 044 is close to stall and pulling 16 amps. If one pump is bad it can pull too many amps, if the second pump is having to make up for the pressure it will pull more amps.

Pump pressure/current chart:

ELECTRONIC CIS fueling & BOOST control

4. Wiring from relay to pump has a bad connection or partial short pulling more amps.

Just for fun, feel the temp of the pumps. Should be warm on a average day, but not too hot.
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Old 09-24-2013, 11:19 AM
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you can check the current on the 2 fuel pumps. might give you an insight to what is going on.

the thing that i dont like about this post is the wires going thru the drilled holes. thats a possible fire waiting to happen. those wires rubbing on the sharp edge is not good.

i am going to look into this more. there has got to be a better way to do this than to run a bunch of wires down to the battery and to make it look more like it was factory.
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Old 09-25-2013, 03:29 AM
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Quote:
Originally Posted by T77911S View Post

the thing that i dont like about this post is the wires going thru the drilled holes. thats a possible fire waiting to happen. those wires rubbing on the sharp edge is not good.
If you read the post carefully you will find that the rubbing issue was addressed (after the fact). Potential problem mitigated.
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Last edited by Shadetree930; 09-25-2013 at 05:32 AM..
Old 09-25-2013, 05:27 AM
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Hi guys--need some help

Blew a the #1-red round relay(porsche china-lasted less than 5+months just weekend driving) last week..almost wrecked limped home. Had a black-relay-(german) good for now.. :/ Can someone tell me what wires to connect if my #2 round relay(2nd clossest to the w/shield-no socket/missing-on 82sc-3.3t). My #6 holder is a 25amp fuse-fuel pump. An alternative setup..

Thnxs

Walt

Last edited by wjfk32; 09-25-2013 at 07:35 AM..
Old 09-25-2013, 07:25 AM
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Quote:
Originally Posted by JFairman View Post
The fuel pump wiring and relay setup in these cars is just plain awful. The one fuse for two pumps is even worse. Enough has been posted about it.
It's not if, it's when will it have problems.
I'd like to submit that pretty much the entire wiring-fuse-relay setup in these cars is just plain awful!
Wiring and HVAC controls = not Stuttgart's strong suits...
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Old 09-25-2013, 11:11 AM
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Agreed the fuseblock set up is terrible. It is fixable if your are not worried about maintaining absolute stock condition. I am currently going through my 87's wiring and correcting a lot of fires waiting to happen

Mike
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Old 09-25-2013, 03:52 PM
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Anyone consider the idea of re-wiring a complete standalone fuseable relay setup and bypass the factory wiring? Seems that would alleviate a lot of the problem.

Chris.
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Old 09-26-2013, 08:11 AM
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