dbrown928

After many weeks of scouring previous threads and trying multiple testing techniques I have come to this point, starting yet another no start thread.

Car: 87 S4 5 speed

What I know: Starter cranks fine, fuel pump runs with fuel pressure at the rails within specs, good firing spark plugs. Noid light in injector plug flashes once when key is turned to start position then nothing. Power at injectors with key on. Power on appropriate pins of EZK and LH. Tach does not move when cranking.

Things tested to this point:
Basics - checked and cleaned all fuses and relays, cleaned ground points
Checked all connectors at the bottom of the fuse relay panel

Bypassed alarm system at module behind glove box (30 to 87a)
drivers door handle is broken suggesting a possible alarm state

Swapped both computers with known good units and put this car's computers into another S4. They worked fine in the other car

Checked and cleaned computer connectors

Changed crank position sensor with known good unit (twice)

Checked wiring harness between crank sensor and computer - checked good

Checked LH pin 18 to injectors - good

Tested injector harness for shorts, manual inspection cutting boots back and disconnecting all injectors and computers then checking for continuity at plugs - all good

With all injectors disconnected tried noid light - no change

Swapped MAF and tried with it disconnected - no luck

Disconnected O2 sensor and knock sensors (one at a time) - no luck

Checked connections at hall sensor - good

Sacrificed one live chicken while doing a kabuki dance
(The chicken was dinner later that evening)

Any thoughts or suggestions would be greatly appreciated

davek9

Borrow a Scope or make one out of your Notebook PC using the Audio in connection and see if you have a square wave timing pulse as something could be holding down the overall peak.

Just for future reference I would not cut the injector boots unless I was looking for the source a known short or open.

I can help you w/building the scope if you need, it is really easy.

DaveK9

dbrown928

That would be helpful. The last scope went up in a garage fire and I have been looking for a cost effective replacement. A PC option would be handy.

davek9

Ok here are two sites one for the Scope Software and the other how to build a Voltage divider, everything you need can be had at Radio Shack.

http://www.zeitnitz.de/Christian/scope_en

You need to build a voltage divider else you will burn up your sound cars placing 12Volts DC into the Audio in  The range is usually only ±0.7V !!

http://hyperphysics.phy-astr.gsu.edu...ic/voldiv.html

edit: here are two pic's of one in action, this is a BAD Timing Pulse (it should be square), there is a pic of a good one in the manual, can't find the one I took at this moment.

Lizard928

If you have good spark while cranking that eliminates the CPS, and alarm system.

If you have no signal getting to the injectors while cranking then that is your problem. As you have confirmed that you have +12V to the injectors with the ign on and you have verified your brains are functional.
This to me leaves only 1 possibility, there is a fault in the wire(s) which supply the sginal to the injectors.

To test this I would disconnect the LH, turn the key on and verify that you have +12V to the inj with the ign in the Run position. Once you have done that have all the injectors plugged in, and test to see if you have voltage at pin 18. Due to the 14ohms of resistance through the injectors you should see less than 12V, but you should see a decent voltage. If you see 12V though, that means that there is a short in the wiring between the +12V line and the signal line from the LH.
This test will verify that all the wires are intact and that the signal can get through.

Andrew Olson had this same problem on another fellows car. I am pretty sure that they found a fault in the injector wiring. But do a search for topics started by him.

HTH

davek9

I think he checked the Wires already, and the one Andrew worked on turned out to be a bad Auto Kick Down Relay, it took a Scope to find it (ask me how I know). thats a pic of the '91's bad signal.

jcorenman

I am thinking that this is significant:

...which would lead to the crank sensor being a prime suspect, however...

And without a crank sensor signal there would be no spark, either.

However, the LH won't squirt fuel without a tach signal from the EZK, which is the same signal that goes to the tach on the instrument panel-- and the tach isn't moving while cranking. It goes from EZK pin-13 to LH pin-1, then thru the CE panel via W11 and J11 and the kickdown relay to the instrument cluster.

The EZK and LH tested good so it must be a wiring problem somewhere, an open circuit between EZK and LH or a short to ground somewhere??

This was a good move (except for the dinner part, that may have been a mistake).
Done properly, it should look something like this:

Lizard928

Thanks for that refresher.

And he stated he checked the wires, but how did he check them is the question.

davek9

This could help, someone had it posted in another thread.
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This is an excerpt of Porsche 928 Electrics - Wiring Diagram Primer ©Copyright Alan C. Moore 2006, 2007

See www.928-Electrics.com for further information, updates and other services - including the full Wiring Diagram Primer document this is excerpted from and details of the master document Porsche 928-Electrics of which the Wiring Diagram Primer is one section.
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dbrown928

All the wiring checks were done first with basic continuity then for specific resistance ranges.

The CE pass through between the EZK and LH is intriguing. I'll make that the next test.

dbrown928

Now that is one helpful graphic.
Has anyone ever produced something like that for the entire CE panel? I have the factory wiring diagrams but it takes a bit to track down those connections.

jcorenman

Interesting!! What about pulling the kickdown relay for a quick test, to see if it starts?

OOOPS, never mind-- it's a 5-speed!

Imo000

Have you checked the fuel injector relay?

jcorenman

Dave,

Nice picture, but I'm confused here. The sound-card input is AC-coupled, with a low-end frequency response of maybe 10Hz. So wouldn't a low-frequency square-wave look something like this?

dbrown928

Yes, I have tried replacing and jumpering the EZK relay XVI, Fuel pump relay XX and LH relay XXV.