928 GTS fuel relay problem

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08 May 2011 15:07 #4735 by Phoenix123
I have been going crazy for a week trying to figure this out.

The other day, I went to the store and when I came out, the car started, I put it into gear, moved about 3 feet, and the car stopped. I could not get it to start, so I had it towed home.

Once I got home, I cranked the car and applied starter fluid and the car ran, so I felt it was a fuel delivery issue. Upon checking, I found out that the fuel pump was not running, so I removed it and checked it with a 12 volt power supply...it ran fine. I re-installed it and swapped both the MFI and fuel pump relays with known good ones (tested in the horn position). The car still would not start.

Upon inspection, I noticed that the fuse socket for the #38 fuse was melted, so I removed the fuse panel and replaced the 5 position fuse block from an extra fuse panel I purchased on Ebay. I have thoroughly checked that the wires are in the correct position. Upon re-attaching the battery cable and trying to start the car, it still did not start and the flasher relay clicks rapidly. Also, I am getting an air bag failure alarm on the dash display.

I removed the cover from the fuel pump relay. When I start the car, and manually latch the relay, the car runs fine. This led me to believe that my LH brain had failed, so I removed the one from my 87 that runs fine. I connected the plug to it and it made no difference. I still would not latch the relay by itself, but if I manually close/latch the relay the car runs fine. I checked it twice. By the way, I always remove the ground battery cable when I am doing any work.

I have checked all the wires on the fuse panel for corrosion or shorts and have found nothing. I have checked for continuity between the fuse panel and the LH and EZK brains and both connections are at 0.6 ohms. I removed, sanded, and re-tightened the ground wires going to the fuse panel at ground position #5. I have rechecked, rechecked and I am at a loss.

Basically, I am not getting a 12 v positive signal to latch the fuel pump relay. Once the relay is closed, the car runs fine.

HELP PLEASE.

Thanks,
Scott

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08 May 2011 16:01 #4736 by 928mac
Replied by 928mac on topic 928 GTS fuel relay problem
It sounds to me that you may have a bad wire or connection.

They can be really hard to find because it looks like 12 volts at the pump but it may be only 2 or 3 strands of wire (or corroded connection)

The way I check for good current flow.

On heavier wire I will connect a head light in place of the (this time a pump)

For your purpose connect a horn to the fuel pump wires.
then start your checks and test.
When you get a good strong honk then it is good current flow.

BTW a strong head light hooked up like that may overload your wires
but I have watched for smoke to find the bad connection :eek:

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08 May 2011 16:07 #4737 by Phoenix123
Thanks. I will check this and use the idea in the future.

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08 May 2011 20:10 #4738 by SeanR
Replied by SeanR on topic 928 GTS fuel relay problem
Sounds to me like the Crank Position Sensor has crapped out.

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08 May 2011 20:48 #4741 by Phoenix123
Thanks, I found out that I am not getting 12V+ to any of the terminals (#86). Thus, the coil on the relay can't close because it has no power. There is a power wire that jumps from a number of relays, but I can't figure out where it gets its 12V+. In the wiring diagram it is labeled at #15.

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09 May 2011 09:01 #4749 by SeanR
Replied by SeanR on topic 928 GTS fuel relay problem
The signal comes from the computers when it detects the engine has rotation. The rotation signal is picked up from the crank sensor. Once that happens the fuel pumps are told to turn on, injectors fire etc.

The CPS is located under the air box on top of the bell housing. The issue will be the connector at the rear of the engine, in the V, under everything.

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10 May 2011 08:43 #4765 by the flying scotsman
The easiest way to get at the CPS is to remove the air filter box, upper and lower, remove the MAF, remove the rear fuel damper and fuel regulator assembly then you should see the connector for the CPS as it connector is on the bracket for the throttle cable.

Yes....its that buried.

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14 May 2011 14:38 #4798 by Kiln_Red
Replied by Kiln_Red on topic 928 GTS fuel relay problem
If I'm not mistaken, on your model 928, then both your ignition and fuel delivery would be shut down by a faulty CPS sensor. Byoassing the fuel pump relay would not run the car.

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14 May 2011 20:45 #4800 by Phoenix123
Thanks everyone, I have looked at a lot of things. My problem is that I can find no connecting wire between the ignition switch and fuse 39 on bus 15. There must be some indirect way that 12V+ power is reaching fuse 39.

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15 May 2011 13:49 #4808 by xstepson
Replied by xstepson on topic 928 GTS fuel relay problem
Did you merely substitute relays for the fuel pump relay? I recommend that you jumper the fuel pump relay between terminals 30 and 87. This removes the relay circuit form the equation.

My thoughts are that if the car runs with the jumper in place, your LH is indeed on it's way out. The fuel pump relay gets it's ground from within the LH. I have had 2 different cars with this issue. Both cars ran fine with a switchable jumper in place.

If the car does not run with a fuel pump jumper, then it is most likely the Crank Positon sensor. When the CPS is going bad, the car can do all kinds of crazy things and have innumerous weird symptons.

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16 May 2011 17:16 #4824 by pcar928fan
Bad ignition switch (where you put you key in to start the car) also makes VERY STRANGE things happen to our cars and is relatively easy to fix. The strangeness is usually weird electrical stuff that is intermittent and not usually related to how the car runs or drives.

James
78 Silver / Black-white #295
84 Ruby Red / Black AO84
88 Dark Blue / Linen-Black
92 Polar Silver / Dark Blue 92EURO
93 Arrow Blue / Black

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18 May 2011 21:49 #4846 by Phoenix123
I am thinking this sounds reasonable.

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18 May 2011 22:47 #4847 by 928mac
Replied by 928mac on topic 928 GTS fuel relay problem
Yes I have heard the the ign switch is a mind bender.

I think by moving the key a little back or forward the problem went away which confirmed the switch needs replaced.

Brad

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19 May 2011 00:53 #4850 by pcar928fan
Brad, there are things that go wrong when your ignition switch goes out that you would SWEAR can't be related... The strangest things start to happen! When really weird electrical things go wrong on these cars I just tell folks to start w/ the ignition switch. It is not to expensive to buy or replace! If that does not cure the problem, no harm no foul it is good insurance for the future!

James
78 Silver / Black-white #295
84 Ruby Red / Black AO84
88 Dark Blue / Linen-Black
92 Polar Silver / Dark Blue 92EURO
93 Arrow Blue / Black

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19 May 2011 07:41 #4852 by martinss
Replied by martinss on topic 928 GTS fuel relay problem

Brad, there are things that go wrong when your ignition switch goes out that you would SWEAR can't be related... The strangest things start to happen! When really weird electrical things go wrong on these cars I just tell folks to start w/ the ignition switch. It is not to expensive to buy or replace! If that does not cure the problem, no harm no foul it is good insurance for the future!


I just counted 10 terminals on the switch: www.mailordercentral.com/928intl/prodinf...964%20613%20012%2000 - so it is certainly doing a lot!

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20 May 2011 08:34 #4877 by SeanR
Replied by SeanR on topic 928 GTS fuel relay problem
Since we are talking about ignition switches, it's pretty important that you get a real one, and not one made in China. Had a few of those that failed just coming out of the wrapper. How do you know if you have a real one? Easy, it will have a 964 or 928 part number in yellow on it.

As for it failing in this case, it is possible and if you have access to a good new or used one, you can pop the electrical connector off the back of the installed switch, plug in the extra one and try to start it with a screw driver. If it runs great, if not, on to the next issue.

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