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Fuel pump relay issue after rebuild
This is a problem that I knew I had but have been ignoring it until I had to deal with it. My 911SC will start fine, but it dies seconds after it starts. After scratching my head I found that it is a fuel pump/relay issue. I found that the fuel pump was only on when the ignition switch was in the "Start" position, but turned off as soon as the engine started and the ignition switch returned to the "Run" position. If I jumpered the fuel pump relay pins 87A and 30, the fuel pump whirs to life as soon as I turn the ignition on to the "Run" position, the engine starts and stays on. I've bench-tested the relay and it is functional. As soon as the key is turned on to the "Run" position, I can feel/hear the fuel pump relay click on, but the fuel pump does not turn on, which I suspect is normal. When the fuel pump relay is "off," the fuel pump is on - is this correct? It also appears that there are three things connected to the ground of the fuel pump relay coil: the alarm system, the airflow sensor and the RPM limiter switch. That if either of these acts as a ground source, the fuel pump relay will engage, shutting down the fuel pump. Does that sound right?
Ok, if that is the case, the it looks like it could be either the airflow sensor, alarm system or RPM limiter switch. I'm pretty sure I checked the airflow sensor action when I was rebuilding the CIS. That would leave the alarm system or RPM limiter. I don't have the key for the alarm system, but when I was re-assembling the doors, I checked that the alarm switch was in the "Off" position by measuring the resistance b/w contacts. So, that would leave the RPM limiter (not even sure where that is - behind the dash?). Has anyone experienced this issue and have any advice as to the most likely cause, or can correct/verify my understanding of the fuel pump relay electric system? One more thing - I bought the car dead (engine sitting in a snowbank beside the car), so I can't verify whether this circuit was functional before I got it. Presumably it was, though, as the PO indicated that he brought it to a mechanic b/c it was running rough (the mechanic pulled the engine and told him it needed a rebuild, PO didn't have the money hence the engine in the snowbank beside the car). Thanks. Geoff
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'82 911SC (Complete rebuild) Click here to go to the project website that chronicles the rebuild process |
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07-10-2009, 04:25 PM
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Location: Los Alamos, NM, USA
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07-10-2009, 04:28 PM
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Any other suggestions? When I put the ignition switch in the "Run" position, when I lift up the air plate the fuel pump should come on, right? It doesn't. I am suspecting the alarm system... Geoff
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'82 911SC (Complete rebuild) Click here to go to the project website that chronicles the rebuild process |
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07-10-2009, 04:59 PM
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Location: Maitland, Florida
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The switch in the air flow meter is isolated from the meter body bu a plastic sleeve over the mounting screw. If the sleeve is damaged or chafed, the switch is grounded and you get the cindition that you have described. Mine was intermeitent and drove me crazy.
Good luck, Jerry
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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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07-10-2009, 05:15 PM
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Disconnect the connector to the airflow switch and see if that solves the problem. If so, the airflow switch is likely malfunctioning.
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07-10-2009, 07:39 PM
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[EDIT] Ok, I just tried it and no difference. When I pull off the airflow switch connector, the fuel pump should come on when the ignition is in the "Run" position, and it doesn't. I didn't try to start it as the family is still asleep (my workshop is directly below our bedroom, and my wife is not an early riser like I am). But, if the fuel pump is not coming on with the airflow switch connector removed, I am thinking its probably the alarm system. I need to remove the fresh air fan anyway as it is squeaking at slow speeds, so I'll check the alarm unit that is tucked in behind it while I have it out. Thanks. [EDIT] Geoff
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'82 911SC (Complete rebuild) Click here to go to the project website that chronicles the rebuild process Last edited by squidmarks; 07-11-2009 at 03:45 AM.. |
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07-11-2009, 03:36 AM
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Yep, it was the alarm system. With the fresh air fan removed I was able to disconnect the alarm controller. When I turn the ignition switch to "Run" and lift the air plate, the fuel pump comes on.
Now, I don't have the key for the alarm system, but I am pretty sure the switch is in the "Alarm Off" position (I measured the resistance b/w the contacts on the switch and, based on info from another Pelican post that resistance indicated it was in the "off" position). I have no idea how the anti-theft system on the 911 works, but I am assuming that if the switch is in the proper position, the alarm controller should stop grounding the fuel pump relay. I guess that's a problem I'll work on another day. For now, I'll just leave the alarm system disconnected. Thanks for your help. Geoff
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'82 911SC (Complete rebuild) Click here to go to the project website that chronicles the rebuild process |
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07-11-2009, 04:32 AM
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