Wiring to fuel pump problem

John Rogers · 11:54 PM

On our 1974 D-Jet car the fuel pump quit running. I checked and cleaned the fuses on the relay board and they have 12.5 volts. I could hear the relay click so I jumpered it (pins 30 to 87) and had 12.5 volts there. At the pump connection in the front there was only 7.5 volts? I put on new connectors but voltage only came to about 10 volts. I pulled the wire on the large relay board connector that feeds the voltage forward and at the pin there was 12.5 volts. I resoldered the pin connector but voltage stayed at 10 volts in front and the pump would not run. I removed the pump and with 12.5 volts to it, the pump runs fine, as it should. So as a temp fix I ran a switched 12v line to the fuel pump and the car runs fine. I am thinking it is a bad or rubbed section in the wire and I am not sure if I want to go to all the trouble to run a new wire, external to the harness, to the front? anyone have any ideas or other things to check?

bowlsby · 04-25-2004, 06:02 PM

Bad relay board? Did you locate the circuit on the 14 pin connector and see what voltage you get there? Clean up the 14 pin connector?

Maybe swap relay boards? If its a bad wire in the harness you have no choice but to run a new wire to the pump, or find th wore problema in the harness and repair it, but it sounds like a voltage drop at a corroded connection.

John Rogers · 04-25-2004, 08:12 PM

Thanks for the idea, but I checked the voltage at pin 14 and it was 12.5 volts when I was jumpering the relay. I also went to 12.5 with the relay when first turning on the key, when you normally get the little "burrrp" from the pump. I am thinking a wire problem?

914GT · 04-25-2004, 08:42 PM

Did you measure the voltage at the fuel pump between the pins in the connector? With the connector pulled off you should measure your full 12.5 V between the terminals. You could have a bad ground and miss it if you've been hooking your voltmeter neg. lead to the car body. Check your ground by measuring the resistance between the brown wire at the pump connector and the car body, should be less than 1 ohm.

John Rogers · 04-25-2004, 09:37 PM

Thanks for the suggestion, but I checked both between connectors and from + to another ground. That was one of the last things I thought of before I ran a temp wire. The voltage was the same?? The car was running yesterday afternoonwhen my daughter got home with no troubles. She has been away for 9 months and the car was only driven about once a week by me to keep things lubed and losened up. I guess it was po'd at me maybe??!!

Dave at Pelican Parts · Dave at Pelican Parts's Garage

Unplug the AAR and make sure that its connector can't touch a ground. Its lead is on the same circuit as the fuel pump. See if that helps... It could be shorted at the wire, or in the AAR.

--DD

John Rogers · 04-26-2004, 07:59 AM

I can't remember if I unplugged the AAR or not? I think it stays unplugged but I'll check it. When I looked at the electrical diagram I noted the AAR is connected "upstream" of the pin on the 14 pin connector but I imagine it could still be a possibility? The voltage at the pin on the 14 plug connector was approx 12.5 volts.

Dave at Pelican Parts · Dave at Pelican Parts's Garage

The AAR is connected to the 12-pin connector. Power comes in through the 14-pin one, then through the relay board, then out through the 14-pin one again to the pump. Are you seeing 12.5V on the pin going to the pump itself?

If you are, and if you are seeing significantly less than that on the plug itself, then you have a problem in the wire somewhere.

Or the ground... Were you checking voltage from the "hot" wire of the connector to the ground wire? If so, re-check against a known ground, like the battery (-) terminal. That would show up a ground fault.

--DD

John Rogers · 04-26-2004, 09:16 PM

I checked the voltage at the pin and it was 12.5 volts. I checked at the fuel pump connector and it was low both with the ground at the connector and a ground on the frame. I talked to the shop where we bought it in 1998 and found that the wire from the 14 pin connector was also jumpered in the corner of the engine compt where the fuel pump originally was. I looked but did not see any connectors so I will have to crawl under there tomorrow and see if I can find some crimped wire connectors. I am betting that is where the voltage loss is taking place, otherwise I'll have to run a new wire up through the tunnel......

lmcchesney · lmcchesney's Garage

John, from you post I understand that your fuel pump was relocated from the back to the front. Do you know what method was use to wire the pump in the front. Usually, if a wire shorts out, then it draws excessive amps resulting in fuse link burning. You can check for this by checking the resistance (ohms) between the pump 12v pos source disconnected from the pump and the other pole of the meter to ground. If the wire sees a low resistance, that would point to a grounded supply wire. If the wire does not have minimal resistance, isolate the wire and check the resistance of the wire(long jumper from relay to meter connected to pump plug. That can identify breakage of the strands of the wire.
IF the power source was altered, the areas which shorting can occur would be front and rear fire walls and the shifter within the central tunnel. If a short is found here, the pump failure may be a symptom of injury to another wire in the tunnel.
L. McC

philinjax · 04-27-2004, 05:31 AM

A high resistance short to chassis ground from the BK/RD wire that feeds the fuel pump is a possibility but, I think more likey is a low resistance open in the BK/RD wire. A low resistance open suspect would be the wire connection to the relocated pump near the original location of the pump. Maybe John will find it today.

Phil

Dave at Pelican Parts · Dave at Pelican Parts's Garage

If there is a jumper at the original fuel pump location, it is very very likely that the connections there are dirty or corroded. That would result in high(er) resistance and therefore a voltage drop. This is most especially true if the jumpers simply plug into the old connector. Very much so if no steps were taken to protect that junction.

--DD

jellybellybug · 04-27-2004, 10:58 AM

Any possible way you guys can translate all that into new newbie terms?? What is the AAR and where are the 12 and 14 pin connectors? We are having the same problems (I think) with our car. (Had it 2 days) Seem to have everything right but the fuel pump is not pumping. How would I test it?

Thanks,

John Rogers · 04-27-2004, 11:39 AM

Well, the AAR is the auxiliary air valve that is open when cold so you have a fast idle and has power to it so it closes when it heats up and idle slows down. The 14 pin connector is the one at the front of the replay board (sets crossways) and the pin we are talking about is the one in the front-left most corner. It has a black/red wire to it. To check power you have to pull the cap off the connector (carefully) and either have someone crank the car or jumper the relay at pins 30 and 87 which gives power to the fuel pump all the time if the key is on. There are three other connectors and they are explained some in the Haynes manual. Good luck

Dave at Pelican Parts · Dave at Pelican Parts's Garage

The 12-pin connector is at the right-rear of the relay board in the engine bay.

Jellybelly, you really really need to get yourself a Haynes manual. Best $20 you'll spend on the car, if you plan on doing any of the work yourself! Pelican sells them, as do other places. But you want to buy from us to help support the board, right??

Fuel pump control circuit article:
http://www.914fan.net/fuelpump.html

Everything that is known about the type of fuel injection system used on your car:
http://members.rennlist.com/pbanders

--DD

jellybellybug · 06:00 PM

Got a Haynes and another by Lash Industries on the way but they wont be here til at least Friday(sorry bought before I found you guys but already got a "to buy" list going for here!)... I cant stand to wait that long! Its actually my hubbys car, but yes WE plan to do the work ourselves... we restored/customed my 72 bug (still in progress) all ourselves but this porsche is all new stuff to me. Thanks for the help!

Later: We bench tested the fuel pump after we finally got it off(thanks for the info) and it works fine.. so at least now I know that its somewhere in the wiring.... sounds like that'll be a blast...

John Rogers · 04-30-2004, 11:57 PM

As suspected, the problem turned out to be a wire connection. There were two in the 12 volt supply line that I repaired. One was just on top of the right side engine tin and the other one was under the car where the new wires ran forward and both were probably 10 to 12 year old crimp connections. I replaced a section of wire and soldered the connections and covered with heat shrink tubing so they should be good for another 10 to 12 years.