1989 porsche 928 will not start

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27 Jan 2010 10:13 #88 by 1989porsche928
Hello, I have narrowed the problem down to the fuel injectors not getting power. the LH ECU has been replaced with a rebuilt one from JDS porsche, internel and external fuel pump has been replaced same as fuel filter. the car sparks, when i put a noid light in the fuel injector harness it does not flash or turn on at all. any clues or steps to finding out where a short it?

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27 Jan 2010 10:59 #90 by adminVB
Replied by adminVB on topic 1989 porsche 928 will not start

the LH ECU has been replaced with a rebuilt one from JDS porsche


I would expect John (of JDS) would be very able to advise you, have you emailed him with your issue?

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28 Jan 2010 12:31 #95 by Kiln_Red

...when i put a noid light in the fuel injector harness it does not flash or turn on at all...


Then you're most definitely correct. No voltage. The first thing I would do is have a helper turn the key over while you stand at the back of the car and listen to see if the fuel pump is being energized. If not, then jumper the relay for the fuel injection ecu and try again. If you have success, then all you need is a replacement relay. If you still have nothing, check your grounds. If these simple checks don't get you anywhere then report back to this thread and we'll go further with some more detailed troubleshooting.

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30 Jan 2010 22:23 #97 by 1989porsche928
i replaced the pump after it didnt start so the pump was bad and now it still wont start, both internal and external where replaced, i can here the pump, i can here it run continuasly when i jump it the relay.

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05 Feb 2010 10:35 #101 by Kiln_Red
If we're talking about an '89 model then it has always been my understanding that there is just (1) external, in-line fuel pump. If you're absolutely positively sure that you're hearing the pump being energized then you certainly don't have a power supply issue to the computer. You must have a break in the injector circuit or there is a fault with the computer itself.

Let's start by doing a resistance check on the entire circuit. Measure resistance of plug "X" on the CE board. Measure between terminal 8 and ground with a functioning ohm meter. Report back with the figures ASAP.

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11 Feb 2010 14:08 #106 by 1989porsche928
Do you mean relay x to ground? like If I where to take relay x off and put a multi meter wire in one of the sockets and the other one to ground? do I need to disconnect the battery?

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13 Feb 2010 10:05 #108 by Kiln_Red
No. I mean at the actual plug connections seen at the lowest point of the CE board. Sorry, my choice of the word "terminal" was a very poor one as point "8" of the connection is not "terminal 8." But, nevertheless, be sure that your performing this test at point "x" of the plug connections. Disconnecting your battery is not necessary.

If your still a little unsure of what I mean, this test is not that significant because it won't tell us anything specifically about that circuit. Though, if the results were a higher value, it would prove a break. To be frank with you, I would be willing to bet that your ecu has failed. That being said, you need to get in contact with Rich from electronikrepair.com/

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02 Mar 2010 14:47 #120 by 1989porsche928
austin the ecu is good i go it replaced by jds porsche, it didnt even needed to be replaced

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19 Mar 2010 19:14 #133 by 1989porsche928
I took it to the mechanic this week i will post an update on the cause of the problem once it gets fixed and i get another extremly high bill

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20 Mar 2010 00:02 #134 by 928mac
OUCH!! that sucks

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04 Apr 2010 23:04 #160 by Rich928
The LH fuel injection module provides the switch to ground for the fuel injectors, it does not provide the "power" to the system.

The fuel injector system voltage is provided by the LH Relay, which is "turned on" by the LH when the ignition is turned on.

So, the proper diagnostics is to jumper the LH relay and check to see if there is battery voltage at the fuel injectors, and at the LH connector pin positions that correspond with 12V system power.

Note, on 1984 Euro, 1985 - 1986 Porsche 928 LH systems, the fuel injector voltage is provided by the fuel pump relay.

I can provide a detailed theory of spark and fuel injection operation white paper in PDF format, just drop a line to my This email address is being protected from spambots. You need JavaScript enabled to view it. email address.

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07 Apr 2010 09:39 #170 by Richter12x2
Replied by Richter12x2 on topic 1989 porsche 928 will not start
For what it's worth, I picked up my 1985 928S for next to nothing because the dealer kept looking for issues with wiring, computers, starters, etc on a no start situation.

After spending a couple hours with the multimeter, I found 4 bad relays. (LH Computer, EZF Computer, don't remember what else). Replaced them and it fired right up. I talked the seller down almost $1000 for what turned out to be $10 in parts.

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03 Jan 2014 12:33 #16516 by 1989porsche928
just to follow up on this very old post. for some reason the connectirs on the rebuilt ecu had corrorsion in it.

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03 Jan 2014 16:18 #16520 by spun

If we're talking about an '89 model then it has always been my understanding that there is just (1) external, in-line fuel pump..


The 89 has an internal and external pump.... just as a follow up.

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03 Jan 2014 16:44 #16522 by 1989porsche928
spun the problem was fixed already i just posted what the mechanic found was wrong in case someone had similar problems. like a over look or under look the obvious

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