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hoppy
Join Date: Jul 2009
Location: PA
Posts: 4
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relay fuel pump control circuit
Hi new user
I have a 1979 911 SC and in the process of restoring it. I am at a point were the electrical system is giving me some problems. The wiring is now connected and all lights wipers etc seem OK. The trouble I am having is the fuel relay terminals for the pump are confusing. When I turn on the ignition switch the relay pulls closed and energizes post #87 this deactivates #87a and stops the fuel pump. If I jumper terminal #87 to #30 the pump runs so pump is OK. It seems as the the relay coil is energized with ign switch in the on position it interrupts fuel relay voltage. The pin #87 only has 12 volts in start position and if jumper #87a to 87 starter cranks. I do not have a manual that details fuel components or correlation between devices. I am not getting fuel to fuel distributor. The engine runs when use starter fluid but can not get pump circuit figured out. |
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07-21-2009, 07:51 AM
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Registered
Join Date: Apr 2003
Location: Maitland, Florida
Posts: 892
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Turn the ignition on and see if the fuel pump actuates when you lift the sensor plate in the airpbox. If not the switch in the fuel distibutor is grounded.
Regards, Jerry Kroeger
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82 911SC Targa (05 Boxster S ) gone, but not forgotten 87 Suzuki GSXR-1100 1953 MG TD Mk II |
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07-21-2009, 08:27 AM
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Registered
Join Date: Oct 2006
Location: MYR S.C.
Posts: 17,321
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the FP is not suppose to run until air is comming into the engine. the air lifts the air sensor plate, removing the ground from the relay.
you can pull an injector and place in a jar. with the key on, lift the plate and check for fuel. all injectors will be spraying fuel, so do not lift it too long.
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86 930 94kmiles [_  _] RUNNING:[__] NOT RUNNING: ____77 911S widebody: SOLD88 BMW 325is 200K+ SOLD 03 BMW 330CI 220K:: [_ _] RUNNING: [__] NOT RUNNING:01 suburban 330K:: [_ _] RUNNING: [__] NOT RUNNING:RACE CAR:: sold |
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07-21-2009, 09:36 AM
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Registered
Join Date: Oct 2008
Location: Nash County, NC.
Posts: 8,471
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The fuel pump relay controls the fuel pump, the airflow sensor and the cold start.
jump 20 and 87a and the pump runs, start the car with that. Bruce |
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07-21-2009, 10:08 AM
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Registered
Join Date: Sep 2008
Location: Rio Rancho, New Mexico
Posts: 200
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Jalopyhoppy,
First welcome to the Pelican Parts technical forum. There is lots of help available here to help troubleshoot 911 problems. The issue with fuel pump relays used in CIS models has been a subject on this forum for many years. You may be able to find more detailed info by using the search function under relay or fuel. The circuitry is somewhat counter intuitive. When you first turn on the ignition, the fuel pump does not start. This could flood your engine if the fuel flowed for an appreciable time before it started. To solve this the Porsche engineers installed a switch in the air flow meter. As you crank the engine the flapper in the air flow meter rises and opens the switch, yes the switch opens. Opening this air flow switch causes the fuel pump relay to de-energize, drop out, whatever term you prefer. By dropping out, the relay sends power to the fuel pump, the fuel pump sends fuel through the injectors and if the plugs are firing the engine starts. So the key piece of info here is the fuel pump runs when the fuel pump relay is not energized. Most of us would expect the fuel pump to run when the fuel pump relay is energized. Wrong. Turn the ignition to the run position and the relay clicks on. No fuel pump. Crank the engine and the relay clicks off. The fuel pump comes on. When the fuel pump relay clicks off, the fuel pump gets power and pumps fuel to the injectors. As has been pointed out, with the ignition switch in the run position, lifting the air flow arm will cause the fuel pump to energize and fuel to flow through the injectors. Pull one injector and test for fuel flow. Typically fuel flow failure is caused by fuel pump fuse corroded, loose or blown, old worn out fuel pump, blocked fuel filter. The red fuel pump relay can be checked by replacing with a black relay. The red relay has a diode in the coil circuit. The black relay does not. The black relay can be used in the fuel pump circuit.
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Doug Was 2.7racer. '76, 2.7 w/Webers, JE pistons, Solex cams. Elephant bushings front & rear, 23mm & 28mm torsion bars, big brakes front & rear, Pertronix. Track car. '85 3.2 stock, Orient red, comfy street car. |
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07-21-2009, 03:53 PM
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Registered
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CIS troubkleshooting for '79 SC's........
Quote:
The CSV is not in the same circuit with the FP relay. The FPR controls the operation of the following: FP, WUR, AAR, TV ('79 SC). And has no effect on the performance of AFS and CSV. This is how I understand and interpret the schematic diagrams for '79 SC's. I've been an ardent student of CIS troubleshooting topics and would appreciate further education. Please advise. Thanks. Tony |
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07-21-2009, 05:41 PM
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hoppy
Join Date: Jul 2009
Location: PA
Posts: 4
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Thanks for the info it makes sense and now I can try and figure out all the other hard start problems. This car has been sitting for about 13 years so I am sure it is going to be a pain.
I know I will be begging for more advice and appreciate all views or sugestions |
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07-22-2009, 09:46 AM
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