Fuel Pump Power Q's on '75 S

[Hezath]

Hi guys, I was driving my 1975 S Targa a fortnight ago and the car stalled going over a speedhump and wouldn't start again until I turned off the headlights (increasing the system voltage slightly).

I then parked it, went and did what I had to do, returned a few hours later and the car wouldn't start at all.

My fuel pump has always turned on with the key in 'Acc' rather than 'Ign' on, so it's really distinct (loud when everything else is silent) when it doesn't turn on. I couldn't get it to start at all and trailered the car home.

If I turn the ignition on, there is no voltage across the terminals that are supposed to feed the pump. Today I put a 12V car battery across the terminals of the pump and it fired into life no problem. So I think my initial problem is not the pump itself.



In the front compartment of the car, there are bays for fuel pump fuses and fuel pump relays, but they are empty - no electrical equipment there.

In the back there isn't much either, three fuse bays with two fuses. Two relay bays with one relay (pictured below) and one hard-wired connection which clearly isn't how it came from the factory! hahaha





I traced the fuel pump wires up into the engine bay and they went through this plug into the loom that runs down the LHS of the engine bay. Not actually sure whether they go forwards or backwards when they reach that loom.



I would like to get the car running again, maybe correct the wiring problem with the fuel pump switching with accessories also? Keen to hear what you pelicans have to say about that one - how it should be, or what works for you guys. I wouldn't rule out a separate circuit with its own switch, but maybe that's a bit sacrelidgeous on a Porsche.

The car is somewhat of a bitser, 3.0L SC motor etc, so I may need some gurus to tell me what is from what.

Appreciate any feedback!

[ossiblue]

Your 75S does not have a fuel pump relay, that's why it comes on with the ignition switch although it should not start until the switch is "on", not "accessory" as you have pointed out. IIRC, the pump is powered from fuse #7 in the upper fuse block in the front of the car. If you look closely, you should see the red/green striped wired going to the fuse. Check to see the fuse is making contact across the terminals. If so, check to see that you get power from both sides of the fuse. The power to fuse 7 comes from terminal 15 of the ignition switch.

Edit: Ed is correct, see his post below, that the power to the pump is not fused--it comes from the line side of the block and is powered directly by the red/black wire from the ignition switch. However, since your problem happened after going over a speed bump, it's highly suspect that a connection was shaken loose somewhere. It could be the battery cable or the ground at the battery, check that they are secure. Make sure the plug into the back of the ignition switch is seated. Make sure the plug from the ignition harness through the bulkhead into the trunk is seated (look under the dash). Make sure the connection of the red/green striped wire to the fuse block is secure. The wild card here is you have some non-factory wiring that may change these connections. If you can post a picture of the fuse block in the front, where the fuel pump wire is located, that could help greatly.

[E Sully]

It sounds like a problem with the charging or battery system. If you can get it running with a booster battery, put a volt meter on the battery. If it is not around 13.8V you need to check the alternator and regulator. Look for a Red/Green wire on the fuse panel in the front trunk of the car, this is the feed to the pump in the rear. Normaly if is connected to the unprotected side of the fuse, and the fuse does not protect it. The power is normaly from a Red/Black wire under the same terminal that comes from the ignition switch.
The fact that you were able to get it to run with a boost leads back to a charging problem. It is difficult to say exactly since the previous owner change out the engine and did some home wiring. The SC used a relay, and the fuel pump in that car used a switch in the air flow plate to activate it, but from your description it seems these were not hooke up in the swap.

[manbridge 74]

Good advice above.

When you get it figured out you might consider wiring up a hidden switch using a relay if just for theft protection. Thieves always look for these sorts of switches but they may be forced to leave if it takes to long to find it.

[RWebb]

put an addl. temp. wire to get voltage at the terminals for the pump; make sure it works; good time to clean the filter too

But you will have to check each point from battery thru switch to pump terminals - I might bet on the switch...

disconnect battery for safety and wire a small 9V "transistor" batter in to check for voltage - it has so little amperage you cannot do much harm

I see a lot of gunk in the last pic in that connector so take them apart & clean carefully

[Hezath]

I don't think there are any voltage problems. I very recently changed the voltage reg in the alternator, and it has a huge brand new battery; cranks very quickly etc.

When I had a multimeter across the pump terminals, I did what I normally do to turn it on and there were 0 volts displayed. The spare battery I used was fully charged ~12.9V.

I will go back to the front of the car and locate that fuse, check voltages etc.

[Hezath]

Well, indeed the red and green wire at the front of the car had been burnt and was no longer making good electrical contact.


I cut it short and re-attached it, fuel pump fired into life no problem. Any ideas regarding what caused the problem?

Any comments about other things you see in the photos that need attention (i.e. those other wires that have been burnt since I bought the car) are welcome.

[E Sully]

From a quick look, the Red fuse on the left looks a bit twisted.
Wires get hot when there is a poor connection, or excessive draw from a failed component. Contacts in relays can pit and corrode from age causing arcing and increased current draw. Corrosion from moisture, dissimilar metals reacting. Expansion and contraction of metals can cause loose terminations and poor conduction. There were quite a few electrical fires caused by this when aluminum wiring was allowed in homes, which is no longer done. Homes that have aluminum wiring needed to have the terminations redone by having a copper tail spliced to the aluminum with an anti oxidant compound, and then the copper was connected to the device. These cars are old and few people check the wiring connections. Occasional cleaning of contacts at terminations and connectors and the use of anti-oxidant on the terminals helps. Checking the screw terminations at the fuse block so they are snug, but not so tight they crush the wires.

[ossiblue]

The side of the fuse block where the evidence of overheating occurs is the supply or line side meaning those wire originate at the power source. The red wires at the end of the block probably overheated due to increased resistance at the terminal. I don't have a diagram so I don't know what those two fuses feed, but increased resistance from the load (bottom side of the fuses) will cause heating and it may not be enough to blow the fuse. Add to that a poor contact at the upper terminal and, over a period of time, enough heat can be generated to melt the insulation.

One thing I think I notice. The two terminal screws on the melted wires appear to be zinc coated steel instead of cadium plate--notice the rust on the edge of the slots? They may not be original screws or have lost their plating and, as a result, there could be more corrosion in the terminal that normal. Can someone else chime in on those two screws? Do they look non-factory?

[manbridge 74]

You are by the sea. More care is needed regarding clean connections using anti-oxidant, etc. Some screws do appear to be the wrong type or have lost their coating.

[Hezath]

Thanks for the great feedback guys, impressive observation skills. I will source some oxidization preventative product and clean up all of these connections with one of those tiny abrasive brushes. I wonder if I should try and source some brass screws to replace the zinc-coated ones?

[Hezath]

Well today I cleaned up a few connections. Tried to source some original porsche fuse box screws without any luck, wrecker said they would only let any go if they had a damaged fuse box. Hmmmmm... sourcing parts in Australia is a pain. There is definitely more that I can do further down the track but what I did today did not hurt.

While playing around with my friend tonight, we noticed a bleedingly obvious lack of power to the fuel pump during cranking. I've had starting problems that have necessitated 'priming' the k-jet virtually every time I drive it and I suspected the fuel pump wasn't working during cranking, but never knew a good way to investigate it as I found this car honestly a bit daunting.

Originally Posted by E Sully

Look for a Red/Green wire on the fuse panel in the front trunk of the car, this is the feed to the pump in the rear. Normaly if is connected to the unprotected side of the fuse, and the fuse does not protect it. The power is normaly from a Red/Black wire under the same terminal that comes from the ignition switch.

Yep, the red-green wire does the fuel pump. If I disconnect it with the fuel pump running, the fuel pump shuts off. If I give it 12V at any time, it starts up. But if I REMOVE the fuse, the fuel pump will continue running. Is the power coming from behind that terminal of the fuse box?

Originally Posted by ossiblue

Your 75S does not have a fuel pump relay

Awesome, thankyou.

Originally Posted by ossiblue

that's why it comes on with the ignition switch although it should not start until the switch is "on", not "accessory" as you have pointed out. IIRC, the pump is powered from fuse #7 in the upper fuse block in the front of the car. If you look closely, you should see the red/green striped wired going to the fuse.

The red-green wire is there, but it goes into what I would call #6. It gets power during 'Acc' and 'On'
A red-black wire is in the same position of what I would call #7. It gets power during 'On' and 'Start', which is when my fuel pump should be getting power, but is not. It is either getting nothing or suffering a huge voltage drop during cranking. This red-black wire seems to go to the ignition system.

Just to clarify:
If you put 12V from the battery onto the red-green wire, the fuel pump runs. If you remove the wire from the fusebox with the fuel pump on (Acc on or Ign on), the fuel pump stops. But when you remove the fuse from the fusebox, the pump keeps going! So it definitely is not receiving its power from the fuse, there is another connection behind the fuse box into that terminal I imagine?

[T77911S]

pull each wire out individually. clean the ends of the wires. cut them if they are too bad to get some clean wire.
clean the bottom of the screw that clamps the wires and if you can, try to clean the hole the wire goes in. '
also clean the clips that hold the fuses.

resistance = heat. unfortunately, the current draw does not go up to pop the fuse. everything just gets hot and melts.

you could install a relay to run the FP.
you could put an inline fuse at the battery then run a wire to terminal 30 of a relay. then use the normal feed to the FP from the fuse block to operate the realy. then connect the wire to the FP to terminal 87.
i am not a fan of modifying or hacking the factory wiring, but if keeps the car from burning to the ground, thats a different story.

[E Sully]

On early cars the fuel pump power is normally from the non-fused side of the terminal block. The fuse actually protects the wire on the opposite side of that terminal. Porsche did not choose to fuse the fuel pump and a relay till later model years. The fuse blocks have straps on the back that connect fuses together, so adjoining terminals are sometimes connected. I don't have a good or complete diagram for your year, but it seems to show fuse position #12 for your car.
It should get power from the ignition switch terminal #15 which is power in run and start, not acc.

[ossiblue]

Ed has clarified your situation as to why the fuse has no influence on the operation of the pump. I also did not have access to a correct wiring diagram so my fuse position was not correct for your model year (I had to use a 73 diagram), but Ed has outlined what you should try to do--find the fuse terminal that becomes hot when the ignition switch is in the "run" and "start" positons, not the acc. Try terminal 15 first and if that is cold in the off and acc positions but becomes hot when the switch is in the "on", go ahead and use it. Your noticeable drop in voltage to the fuel pump is likely related to the wire being connected to an acc. terminal as, in some cars (not sure of Porsche), the acc power is shut down during cranking and restored once the switch is returned from the "start" position. Finding a terminal that is powered only when the ignition is in "run" and "start" should fix that problem as well.

[Hezath]

I just noticed that the fuse panel has embossed numbers behind each fuse, starting at 1 at the back and going towards the front, but you seem to be counting from the front? Has mine been installed upside-down?

Originally Posted by T77911S

i am not a fan of modifying or hacking the factory wiring, but if keeps the car from burning to the ground, thats a different story.

I would opt for keeping the wiring original also, but what I have here clearly is not factory - things have been stuffed with before i got the car. I'd love to keep it as simple and close to the original diagrams as possible, but I haven't found a black-and-white solution yet.

Originally Posted by ossiblue

Try terminal 15 first and if that is cold in the off and acc positions but becomes hot when the switch is in the "on", go ahead and use it. Your noticeable drop in voltage to the fuel pump is likely related to the wire being connected to an acc. terminal as, in some cars (not sure of Porsche), the acc power is shut down during cranking and restored once the switch is returned from the "start" position. Finding a terminal that is powered only when the ignition is in "run" and "start" should fix that problem as well.

Yes, the terminal next to the one the fuel pump wire is connected (red-black) behaves like we want (On and Start), but the fuel pump draws 6 amps so it would be unwise to power the pump from that terminal.

I think a relay will be necessary, and the signal wire will come from the terminal where the red-black wire is connected.

Thanks for all the assistance so far guys!

[Hezath]

Well tonight the car received a relay powering the red-green wire, with the signal-wire connecting to the red-black wire's terminal. The car now starts under its own cranking which it didn't do previously, and the fuel pump works consistently when the car is driving. Could probably still do with a 'tune-up' of the K-Jet, but now I can just jump in the car and start it, hassle free.

I installed a junction box to gracefully simplify some of the ugly aftermarket stereo wiring also, and ran the wire for the fuel pump power from that junction box also. It doesn't look factory but it is a bit neater. Maybe I could have just run it off of one of the bridged terminals on the fuse box, but I was hesitant to put another 6amps through a system that I know nothing about, particularly on a model of car where electrical fires seem to be commonplace.

I still can't figure out how the hell it ever worked before the problem developed, it seems that it was simply wired up incorrectly, but it isn't something that I have ever changed.

I really want to say thanks for all of the feedback that you guys provided. Working through problems like this can be a challenge so I hope I didn't come off as unappreciative.

[ossiblue]

Originally Posted by Hezath

Well tonight the car received a relay powering the red-green wire, with the signal-wire connecting to the red-black wire's terminal. The car now starts under its own cranking which it didn't do previously, and the fuel pump works consistently when the car is driving. Could probably still do with a 'tune-up' of the K-Jet, but now I can just jump in the car and start it, hassle free.

I installed a junction box to gracefully simplify some of the ugly aftermarket stereo wiring also, and ran the wire for the fuel pump power from that junction box also. It doesn't look factory but it is a bit neater. Maybe I could have just run it off of one of the bridged terminals on the fuse box, but I was hesitant to put another 6amps through a system that I know nothing about, particularly on a model of car where electrical fires seem to be commonplace.

I still can't figure out how the hell it ever worked before the problem developed, it seems that it was simply wired up incorrectly, but it isn't something that I have ever changed.

I really want to say thanks for all of the feedback that you guys provided. Working through problems like this can be a challenge so I hope I didn't come off as unappreciative.

Glad you got it working.

Now that you have installed a relay, was it a 5 pin relay? If so, I might suggest you go one step further and wire the relay to the air flow sensor switch that is on you 3.0 liter engine. This is a safety precaution that will shut off the pump if the engine dies while the ignition switch is in the "run" position.

If you are interested in adding this safety feature, let us know and we can explain how to do it as it is a relatively easy addition since your engine already has the sensor switch.