Fuel pump electrical problem

CMXI-SC · 02-12-2002, 06:48 AM

Engine starts but won't run with red relay in (82 SC coupe). I jumpered between terms 87a and 30 on the fuel pump relay socket with the red relay out. The fuel pump runs and the engine starts and runs in this configuration. Can anyone out there give me some voltage readings at the socket with the key on and some resistance readings from the socket with the key off on a known good vehicle? Thanks for your help on this frustrating problem.
Jack

stormcrow · stormcrow's Garage

Take a ohm reading between terminals 85 - 86. This is the coil which should energize terminals 87a and 30.

There is a wiring diagram under 911 tech articles part 1, page 2.

If you don't get a reading between terminals 85 86 then the coil is bad and the relay needs replacing.

The wiring diagram shows that terminals 87a and 30 are norminally closed but that condition could exist with power on the circuit.

Terminals 87 and 30 are shown to be normally open on the wiring diagram.

Hope this info helps.

Steve

CMXI-SC · 02-13-2002, 07:49 AM

Steve,
I,ve Taken the readings on the red relay and got about 20 ohms one way and about 60 with the leads reversed (diode in ckt). Did this on two red relays so am reasonably confident they are good. Regarding the current flow prints, I believe Porsche shows their drawings with the ignition key in the run position. Contacts 87 & 30 are only closed when the ignition key is in the start position thus bypassing the air flow sensor contacts and the RPM limiter switch. It apears I am either missing the ground at term 85 in the run position or the battery at term 86 which holds the relay operated in the run position. Now if I can locate connectors T2e and /or T2a I can continue my journey. Appreciate your help.
Jack

Early_S_Man · 03-30-2002, 06:25 PM

Any update on the alarm system, Jack?

CMXI-SC · 03-31-2002, 06:16 AM

Warren,
Had just enough time to get into it up to my elbows. The one line statement in the factory manual, "2. Remove fresh air blower." does not come close to revealing the intensity of the project. Along with the fresh air blower is all the BEHR plumbing etc. for the AC. Am attacking as time permits. Will let you know as soon as I unbury the Alarm Module and perform tests as indicated in the factory manuel.
Jack

alan911 · 03-31-2002, 08:14 AM

Had the same problem. Relay in, did not start. Jumpered through the relay socket and ran fine. Found out that the female connectors inside the socket were slighly corroded and "expanded" and didn't make proper contact with the relay prongs. Its hard to see inside the little relay receptical. Problems occur when the relay prongs get "squished" and the female receptical expands over time. Thus no contact. BTW you can use any relay in the car to run or test the fuel pump circuit. Doesn't have to be the "RED" one. Try the horn relay. If another relay works then its that particular relay. If none of them work its the circut or receptical.

Early_S_Man · 03-31-2002, 10:16 AM

For the benefit of any SC owners, there are a few misconceptions out there about the operation of the red fuel pump relays that need to be cleared up ...

1. The fuel pump relay provides pwer to the fuel pump in two distinctly different ways during starting and 'normal' engine -running conditions. During the initial starting phase the fuel pump gets power from the Yellow #50 starter solenoid lead via the 'make' contacts #30 and #87. The fuel pump relay is operated by a set of 'break' contacts in the Air Flow Sensor Contact ... until ingested air flow is high enough to move the sensor plate ... then, power from Fuse #16 is supplied via the #30 and #87 contacts ... after the fuel pump relay releases due to the interrupted ground from the Air Flow Sensor Contact!

2. While the engine is running normally ... the fuel pump relay serves as an emergency stop device to cut off the fuel pump when a ground is provided to the relay coil terminal #85 from either the Air Flow Sensor Contact (meaning the engine has stalled -- but the ignition switch hasn't been turned off) or the Rpm Limiter Switch -- if max engine revs has been reached!

3. The factory alarm/theft-deterrent system has no schematic diagram in the factory service manual ... the one appearance being the Part I of the '82 USA 911 SC current flow diagram! In the last part of the electrical section of the manual there is a connector pinout and basic Voltage and test lamp checks ... AND an insignificant (to the writer, apparently) note saying that even without power, a failed Alarm Control Unit can DISABLE POWER TO THE FUEL PUMP, 'UNDER CERTAIN CIRCUMSTANCES!' MAYBE IT SHOULD BE CALLED THE OWNER DETERRENT SYSTEM? There is a workaround provided to disable the alarm system by jumpering both terminal #61 connectors at the alarm unit 8-pin socket ... after the unit has been disconnected!!!

Jack found that note while we were on the phone, and after I had suggested that the alarm unit was the source of the mystery ground that was keeping the fuel pump relay operated!

The following diagram may be instructive to interested SC owners ... who may want to remember this incident, and a few things about troubleshooting the fuel pump circuit!