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Fuel Relay fuse keeps blowing

 
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kylekev  



Joined: 02 Jan 2012
Posts: 49
Location: Evansville Indiana

PostPosted: Fri Nov 15, 2013 3:16 am    Post subject: Fuel Relay fuse keeps blowing Reply with quote

hey all,

My 78 924 just recently been having some problems with my fuel relay fuse blowing on me. Every time I go to turn the ignition the fuse blows. I haven't had this problem up until a couple days ago. I do know the fuel pump relay is possibly shorted out not giving power to my AAV, Thermo time switch, etc. but still gave power to the fuel pump. Honestly not a hundred percent sure if the relay is broken but it does have some burn marks on the bottom of it and after trying to figure out why i'm not getting power to my AAV, thermo time switch, etc I was think that could be the problem.
I ended up finding one online and bought it after looking for one for some time now. The one thing that bothers me now is that the power going into the relay goes through the fuse before entering the relay and originally i was thinking the ground for the relay went bad. Which I changed and made new ground points and still the fuse broke.
So the question is, why am I getting to much power to my fuel relay? New relay will be in on Tuesday, which will hopefully fix my power problems to my AAV, TTS, etc. But I am pretty sure when i put the new relay in the fuse will pop.

Thanks again everyone
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fiat22turbo  



Joined: 18 Jan 2006
Posts: 4040
Location: Portland, OR

PostPosted: Fri Nov 15, 2013 3:50 am    Post subject: Reply with quote

Excessively dirty/corroded connections can cause fuses to burn.

Time to go make sure your wiring from the fuse panel through the firewall isn't damaged or shorting out.

Of course check out the various grounds and connections both under the dash and under the hood.

The fuse panels themselves can fail internally due to corrosion and age. That requires a bunch more work to deal with, so eliminate the low hanging fruit first.
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Stefan
1979 924 Carrera GTS (clone-ish)
1988 944 Turbo S (Silver Rose)
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larchie  



Joined: 19 Jun 2003
Posts: 297

PostPosted: Fri Nov 15, 2013 11:33 am    Post subject: Reply with quote

Here's a thorough and helpful PDF troubleshooting procedure for a relay which think should be similar to or the same as '78 924 fuel pump relay incorporating some specific electrical checks covered by fiat22turbo's diagnosis: Troubleshooting a Nonfunctional Fuel Pump.
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kylekev  



Joined: 02 Jan 2012
Posts: 49
Location: Evansville Indiana

PostPosted: Mon Nov 18, 2013 4:04 am    Post subject: So far Reply with quote

Thanks everyone for your help so far.

Everything looks pretty good behind the fuse box. The big red wire (Power for the pump and everything) looks to be in good shape and the connections look clean as well as the connections for everything else for the relay. That red wire runs into another connection hooked into the fuse box where a cluster of power wires are going to and then runs out to the engine bay.

Question is where do I look under the hood for this power wire for fuel pump. I'm hoping under the hood is where the problem is.

Also. Could I just run a new wire from the battery to this connections behind the fuse box and then to the fuel relay?
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Ian89C4  



Joined: 01 Apr 2011
Posts: 544
Location: North Carolina

PostPosted: Mon Nov 18, 2013 7:03 am    Post subject: Reply with quote

The power for the fuel pump runs in a harness from the back of the fuse panel down the inside drivers door sill and up under the rear deck covering. From there is goes down in front of the fuel tank through the rear deck lid through a grommet. There is splits three ways, one line goes to the fuel level sender, the other to the reverse switch, and then down to the in tank and external fuel pumps. The internal pump is powered and grounded from the same line that the main is powered from.

I would imagine that those would get corroded and nasty first. disconnect the battery, then pull off the leads from the pumps and clean them up. Also check the ground for them that is located at the very back of the rear compartment by the rear light housings.

Hope it all works out.

Also, do not run a wire from the fuse block to the pump directly, IIRC the power lines from the 25 amp fuse run to the relay which is then distributed out to the pump.

There are other things run off of the 25 amp fuse, I would not really mess with it.

Cheers,

Ian
_________________
Ian Edgerly
Spiveys Corner, North Carolina

Current:
1981 924 Weissach Race Car ("Serenity")
1980ish 931 ("Alice")
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kylekev  



Joined: 02 Jan 2012
Posts: 49
Location: Evansville Indiana

PostPosted: Fri Nov 22, 2013 1:42 am    Post subject: Moving forward Reply with quote

So i checked under the back end and found were that strand of wire splits three ways. One to the back up switch, one to the tank (at least what I believe) and the other to the fuel pump.

Long ago I was thinking my ground wasn't good enough for the fuel pump so I cut it and made a new ground for the fuel pump. So the original isn't grounded anymore. Could this be my problem? I have had this not grounded for some time now and I have drove it many times this way.

Other then that everything looks clean, or at least what i can see. Back up switch is above transmission and I could feel the boot still on the plug. For the fuel tank sensor, it runs into the tank and i cant see that obviously.

Thanks for the help so far everyone.
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kylekev  



Joined: 02 Jan 2012
Posts: 49
Location: Evansville Indiana

PostPosted: Fri Nov 22, 2013 2:05 am    Post subject: Tried with no success Reply with quote

I went ahead and tried grounding that one ground I had cut to the fuel pump and still i had a blown fuse when i went to try to start the car. I knew it was a long shot but i had t try.

Thanks everyone
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larchie  



Joined: 19 Jun 2003
Posts: 297

PostPosted: Sat Nov 23, 2013 4:59 am    Post subject: Reply with quote

So it looks like you have to systematically check for a short to ground. What follows are some main basic checks to find the short, and I apologize in advance if you already know how to do this basic circuit analysis.

The general idea of the procedure is by disconnecting the battery and the load and then placing one lead of an ohmmeter to load end wire and the other lead of the ohmmeter to ground, the circuit should be open, i.e. infinite resistance -- if not, then it's shorted to ground. (If you don't have an ohmmeter, then use a test light by following these steps but instead connect your test light between the disconnected anode of the battery and the disconnected positive wire which was disconnected from each load below -- if wire lights then you found the short).

You might start like this.
If you have not already done so, study the circuit for your 924 here on the Pelican site. I wrote these notes fairly quickly so be sure to check out the steps in case I've made typos or mental slips.

(1) Disconnect the anode terminal of the battery and remove the fuel pump relay and the seat belt warning relay.

(2) Disconnect the positive lead to the fuel pump and connect one lead of an ohmmeter to that green/black lead and the other end to ground, there should be infinite resistance. If not, then there is a short in this circuit. Next, find the one pole wire connector near the fuse-relay board which is connected to terminal 87 of the fuel pump relay and check for a short between the green/black wire at the connector and terminal 87.

(3) Disconnect the positive lead to the warm-up regulator and connect one lead of an ohm meter to that orange/white lead and the other end to ground, there should be infinite resistance. If not, then there is a short in this circuit. Next find the seven-pole connector in the engine compartment and check for a short between the orange/white wire there and the orange/white wire continuing to terminal 87 of the fuel pump relay.

(4) Disconnect the green/black positive lead to the auxiliary air valve and connect one lead of an ohmmeter to that green/black lead and the other end to ground, there should be infinite resistance. If not, then there is a short in this circuit. Next find the seven-pole connector in the engine compartment and check for a short between the green/black wire and the (same wire as this part of step (3)) orange/white wire from there and terminal 87 of the fuel pump relay.

(5) Disconnect the yellow/black positive lead to the air flow sensor contact and connect one lead of an ohm meter to that yellow/black lead and the other end to ground, there should be infinite resistance. If not, then there is a short in this circuit. Next find the seven-pole connector in the engine compartment and check for a short between the yelow/black wire from there to terminal 31b of the fuel pump relay.

(6) Insert one lead of your ohmmeter into terminal 15 of the seat belt warning relay and connect the other end to ground, there should be infinite resistance. If not, then there is a short between terminal 15 of the seat belt warning relay and the black wire from there to the fuel pump relay.

(7) Insert one lead of your ohmmeter into terminal 31 of the seat belt warning relay and connect the other end to ground, there should be infinite resistance. If not, then there is a short between terminal 31 of the seat belt warning relay and the brown wire from there to the fuel pump relay.

If these checks do not find the short, then the short must be somewhere either in the loads themselves or in the grounds to those loads.
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kylekev  



Joined: 02 Jan 2012
Posts: 49
Location: Evansville Indiana

PostPosted: Sun Nov 24, 2013 3:52 am    Post subject: Reply with quote

Huge thanks for all this good information on going through and finding a short. I've never done anything like this before (as in finding a short) so your info is appreciated.

I'll be working on this today to try to find this problem. Thanks so much again and I will keep you all posted.
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larchie  



Joined: 19 Jun 2003
Posts: 297

PostPosted: Sun Nov 24, 2013 6:29 am    Post subject: Reply with quote

Quote:
I've never done anything like this before (as in finding a short)...


Then, I should point out that I've left out details I don't know about either. For example, I think on the early '78s there was no in-tank pump and on the later ones there was. So when you disconnect the battery, remove the fuel pump relay, and disconnect the positive wire to the fuel pump to connect one lead of your ohmmeter and the other to ground, you might not have infinite resistance do to a short in several different wires depending on the particular circuit of your car.

For example, assuming you have a later '78 924 which it looks like you do based on your previous post, and if it is similar to the '79, then an infinite resistance cound indicate the short could be in wires to the fuel relay, the wires from the in-tank pump, the fuel sender unit or the reverse switch. These then might have to be tested separately.

Maybe someone who has a '78 like yours and knows the circuit precisely could clarify how to proceed if you do get infinite resistance on this test.

As far as reading the circuit diagram, there are Legends to the symbols on the Pelican Parts Porsche 924/944 Electrical Diagrams page a general link I did not post previously as they are among the most used diagrams on this discussion list.

Here's the Legend for the circuit diagram linked in my previous post:
J-17 Fuel Pump Relay
G-19 Air Flow Sensor Switch
G-6 Fuel Pump (so the diagram must be for the early '78s since no in-tank pump?)
N-9 Warmup Regulator
N-21 Auxiliary Air Valve

the names of other parts for your Fuel Pump Diagram Part 1 - Page 2 are listed for the '78 under Part 1 - Legend

To read the connectors as in T superscript 1 subscript 7a, the superscript is the terminal number and the subscript is a 7 pole connector.

If your '78 is like the '79 then the grounds are found as follows:
Terminal 31 ground of Fuel Pump Relay near the fuse/relay panel
Fuel Pump ground near left side luggage rear light
Warm Up Regulator near Distributor.
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