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Old 08-12-2017, 02:58 PM
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RED928
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Default Intermittent Start

88 S4 Auto
117,000 miles

I've refreshed the fuse panel, checked all the grounds, and replaced the computers (LH/EZK). Sometimes she starts right up. Other times not. If I drive around the neighborhood I'm guaranteed to have the engine die at some point. When it does not start she cranks fine and all the gauges and other electrical seem to be in working order - other than ignition. I've hot wired the fuel pump and that occasionally will get her running but not always. I've tried starter fluid in the past and there's always spark. Is there a good "fault isolation" sequence I can try? Someone previously mentioned the "14 pin" connector under the hood - where's it located.

Thanks
Steve
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Old 08-12-2017, 03:06 PM
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polecat702
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Originally Posted by RED928 View Post
88 S4 Auto
117,000 miles

I've refreshed the fuse panel, checked all the grounds, and replaced the computers (LH/EZK). Sometimes she starts right up. Other times not. If I drive around the neighborhood I'm guaranteed to have the engine die at some point. When it does not start she cranks fine and all the gauges and other electrical seem to be in working order - other than ignition. I've hot wired the fuel pump and that occasionally will get her running but not always. I've tried starter fluid in the past and there's always spark. Is there a good "fault isolation" sequence I can try? Someone previously mentioned the "14 pin" connector under the hood - where's it located.

Thanks
Steve
At the jump post, passenger fender just behind the radiator. Should have a plastic cover, though most are missing the cover. Search is your friend.
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Old 08-12-2017, 04:14 PM
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Wisconsin Joe
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Did 'refreshing the fuse panel' include replacing all the relays?
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Old 08-12-2017, 04:46 PM
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Old 08-12-2017, 07:24 PM
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RED928
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Thanks... yes all new relays/fuses

Steve
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Old 08-13-2017, 11:37 AM
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Pulled the 14-pin connector... seems the wiring is a bit degraded/damaged. Also the insulation on one of the leads into the jump post has started to disintegrate. (See pics)

What's the best way to proceed?

Thanks,
Steve
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Old 08-13-2017, 11:46 AM
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You obviously need a new engine wiring harness. The cable going to the hot post is now a potential welding machine- the insulation [were it existing] should be red! This type of degradation is typical by the way.

Whilst you are at it check [and most likely] replace both the main earth strap and the engine earth strap as they will likely be shot as well.

Hopefully the two 10mm2 power cables from the other side of the hot post back to the central electrics are in somewhat better shape.
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Old 08-13-2017, 11:54 AM
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Thanks... they appear to be
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Old 09-12-2017, 11:15 AM
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Replaced the engine wiring harness with the harness Sean sells... still no start. I'm sure the harness needed replacing and the quality of the product was great.

Engine will crank just fine. I hot wired the fuel pump and that does not make any difference. Any recommendations on what should I check next?

Thanks,
Steve
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Old 09-12-2017, 11:23 AM
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Originally Posted by RED928 View Post
Engine will crank just fine. I hot wired the fuel pump and that does not make any difference. Any recommendations on what should I check next?
Did I miss replacement of the crank sensor? (aka Crank Position Sensor, impulse sender, ...)

Have you the Moorehouse CDs? If yes, have you looked at the LH/EZK troubleshooting procedure? If no, then get them.
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Old 09-12-2017, 11:38 AM
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You changed the relays, but could still have a bad one. Check / interchange the LH and the fuel pump are same with horn relay. The EZK is not similar. You say the spark is OK, so I'm guessing it is not EZK. Others can tell you what voltages to verify on the LH connector to LH computer. Or search threads here.

You can get relay/fuse chart at "928gt.com" if the chart is missing in your footplate that covers the relay board in foot well.

Hope this helps.
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Old 09-12-2017, 11:39 AM
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Did you check the earth straps? I doubt that would be your problem but these things contribute to difficulties and the condition often follows that of the engine wire harness.

Mine sort of looked OK until I looked under the insulation and saw half of the braids in a mess. Changing the main strap resulted in a huge increase of starting speed and immediate ignition returned.

If there is nothing at all then as Dave says above look at the CPS.
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Old 09-13-2017, 10:29 AM
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Is the crank position sensor (CPS) the same as the speed/reference sensor described in the manual?

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Steve
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Old 09-13-2017, 11:00 AM
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Originally Posted by RED928 View Post
Is the crank position sensor (CPS) the same as the speed/reference sensor described in the manual?
Yes. It's also called the impulse sender from time-to-time.

If the signal from that sender cannot have from it an RPM extracted (because it's dead, harness break, too noisy because of compromised shielding) by the EZK the engine will stop.

A wiring or connector issue can cause intermittent signal loss.

Impulse Sender/CPS/Speed-Reference Sensor uses the same connector material as the knock sensors. It crumbles to dust after 10-15 years. As it degrades it can cause intermittent issues.
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Old 09-13-2017, 11:05 AM
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Followup: the reason the engine stops with no speed signal is that the EZK needs that signal to coordinate spark. No speed signal no spark.

If you have a timing light, use it while cranking and see if you are getting spark.

Diagnosing no-starts always starts with figuring out what's missing: fuel or spark. On an LH 928 if you have no spark it is likely that you have no fuel because the EZF/EZK is what turns on the LH.
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