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fuel pump relay confusion

 
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924guy  



Joined: 29 Dec 2003
Posts: 2088
Location: Port St. Lucie, FL

PostPosted: Fri Jul 04, 2008 10:27 am    Post subject: fuel pump relay confusion Reply with quote

hey all,

so ive started to trouble shoot my 78' , and ofcourse the first thing i found was my fuel pump wasnt activating (priming) when i turned the key. quick check confirmed the fuel pump relay was bad (odd, part had no corrosion, and has less than a years use, though it sat for a long time)
So, for testing purposes i pull out the FP relay from my 931, and the pump kicks in as normal. the 78 frp also has an additional pin (50) which i think is for ignition , so the later relay will run the pump, but not bridge the ignition for starting if im correct, and could be why i have a no start problem.

so i do a search for a replacement relay, and find some conflicting info:

1. 477.906.059: 12v/13a with a 16a fuse on board, 6 pin. lists for 1978 only, and is the one i pulled out , bosch p/n 896444000
2. 433.906.059: 12v/10a lists for 79-82 n/a according to the faq. bosch p/n 896321, dont have one so not sure if it also has an on board fuse.
3. 931615113: 12v/16a list for 931 80-82, bosch p/n 896481, 5 pin. no onboard fuse.


pretty basic stuff , but here where the confusion strikes. my 78 is a single fuel pump yet its fpr is rated for 13 amps. the 79+ is rated for only 10 amps, and they use two pumps.. and the 931 fpr uses two pumps and is rated for 16 amps (which seems about right) so what is different in the n/a two pump system that makes it require a lower amp fpr? they ALL use the same external pump to my knowledge, not sure of the draw, but for some reason the ratings are different..

also, the relays all apparently (making an assumption here, i havent checked) have 16 amp fuses, either on board or in the panel. this tells me that the relay will fry before the fuse blows, a costly situation, except for the 931 unit, maybe, and that looks more like a simultaneous meltdown to me...

Sooo, if my logic is correct, there is a built in flaw in the fpr circuit, and this is the reason we all go through fuel pump relays so often(under rated fpr amperage, or over rated fuse.) seems to me, we really want a lower rated fuse in the circuit, so it blows before the relay fries, and it needs to be located between the 87-30 bridge, like on the early relays, but then will deliver enough to reliable run the pump? Or if theres a fpr that will work with a 20a rating, that would be an ideal solution.
these relays should almost never go, unless the fuel pump itself malfunctions and starts drawing crazy amperage(after all its the pump that pulls the amps, not the relay itself.), yet they do fail an often and i think this is why. am i onto something here???

then there the ignition circuit tie in at the fpr, where i believe my 78 has an issue, but im still decoding that one..

whats your take on this?
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78 924
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1303RS  



Joined: 22 Aug 2007
Posts: 48
Location: Chandler, AZ

PostPosted: Fri Jul 04, 2008 2:05 pm    Post subject: Reply with quote

Ummm sure. Im lost with it. No but really if what you have found out is true, do we change the fuse to solve our problems? I am having the same problem with my 78. I installed a new relay, and nothing, was told to jump the fuel pump poles, and it starts. Did I burn up the new relay? Now I have to find out why it happened so I dont do it again. Such fun tortuous things we put ourselves through.
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!tom  



Joined: 28 Aug 2006
Posts: 1880
Location: Victoria, BC Canada

PostPosted: Fri Jul 04, 2008 2:30 pm    Post subject: Reply with quote

The older cars (I know the 78 is in this category) do NOT have a fuel pump "prime" pulse (as you put it) when you turn the key to "on."

There are different relays for different years because the relay is controlled in different ways.

The older cars (such as the '78) have a switch on the air metering plate that controls the fuel pump. So, the pump will not turn on until after the engine cranks, causing the air metering plate to deflect.

This is why the accumulator is so important, so that pressure is available for the engine to start.

Later models have a different relay which is activated through the signal to the coil, and I believe it is this style relay which will prime the system when the key is turned to "on."

You won't get this behaviour in a properly working '78.
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Smoothie  



Joined: 01 Jan 2003
Posts: 8030
Location: DE (the one near MD, PA, NJ)

PostPosted: Fri Jul 04, 2008 3:51 pm    Post subject: Reply with quote

We found here - http://www.924board.org/viewtopic.php?t=10360 that both the 931.___ and later 924NA 433.___ relays are 16A, and the earlier 924NA 477.___ is 12A.
If you do indeed have a 10A 433.___, maybe there's a whole batch of improperly made 433.'s floating around out there..?
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924guy  



Joined: 29 Dec 2003
Posts: 2088
Location: Port St. Lucie, FL

PostPosted: Sat Jul 05, 2008 7:01 am    Post subject: Reply with quote

well, this is interesting!

If the 77.5-78 cars dont have coil lead tied into the fpr, and that is what drives the fuel pump pulse , then putting a later relay in should not result is a pulse, yet in my car, it did. i also remember having that pulse with the 906059 relay, though i could be mistaken, its been a long time since ive driven this one. i suppose my next step is to trace out the fpr 50 pin, and see where that leads me. And to check for a line to the air metering housing.
i don't have an exact build date for my car, the sticker is long gone, but i have noticed it doesn't quite match the wiring schematics , perhaps it was a transitional model. makes things even more interesting. i am convinced that i have either a loose contact or intermittent connection that is essential for the starting sequence. before retiring the car, i often had to rattle the fuse box to get her to start. now that i think about it, it could have been a failing fpr that needed a good jolt to get the plate to fully swing, or a loose wire.
in the meantime, ill pony up for a new 6 pin fpr , and continue to trace lines as i have time.. porsche certainly built allot of tech into a simple function...
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78 924
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Mike924  



Joined: 12 Aug 2004
Posts: 2601
Location: IoW UK

PostPosted: Sun Jul 06, 2008 3:03 am    Post subject: Reply with quote

924guy wrote:
porsche certainly built allot of tech into a simple function...

Germans!
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!tom  



Joined: 28 Aug 2006
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Location: Victoria, BC Canada

PostPosted: Tue Apr 20, 2010 2:15 pm    Post subject: Reply with quote

A little more detail about the fuel pump relay 896444000.
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!tom  



Joined: 28 Aug 2006
Posts: 1880
Location: Victoria, BC Canada

PostPosted: Wed Jun 16, 2010 4:18 pm    Post subject: Reply with quote

I've been driving around the last few months using a jumper wire instead of a fuel pump relay.

See my previous post for more details of this; I'm posting here because my previous details are in the 931 section.

I'm not proud of this fact, but finding time to work on this project has been difficult.

I've determined the fuel pump relay is just fine, from some testing I did a couple months ago. The issue I'm having is pin #15, or the pin that is supposed to get power when the key is turned to "on" or "start" to power the relay coil is always dead.

I pulled the blue C connector, and the pin looks fine. I tried to clean the connector, and I smeared some dielectric grease on the pins on the fuse/relay panel, and plugged it back in, and there's still no juice on pin #15 in the relay socket, according to my test light.

My next step will be to test the continuity of the wire between the pin on connector C and the fuel pump relay socket; I just need to get my DVM back (I lent it out a couple weeks ago).

What are my chances there is a failure internal to the fuse relay panel? Remember what started my wiring saga.
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