1990 20V no start when cold

[Cody Forbes]

I'm keeping the list in the loop here so *when* we get you fixed up the fix and the path to it will be documented. Hopefully I won't regret that :sigh:.

How much voltage did the sensors generate? Did you back-probe them while still connected to the ECU or unplug them? I've seen two in separate instances where the sensor cable was pinched and the signal wire shorted to the shield ground. The shield ground is only grounded when plugged in so unplugged they tested fine, but when plugged in and back-probes the signal was flat lined.


You should get injector pulse during cranking. It'll be quick and a small pulse, but it should exist. Since you have a scope check for a signal from the ECU to the tachometer.

To help lead you on the path to the fix I can provide some internal ECU theory. I can not verify that this data is 100% factual for your specific ECU, but based on my experience with similar ones it should be accurate. Basically speaking the ECU has two levels of data related to fueling. First it determines how fast the engine is turning to know when to inject fuel. Next it determines load, temperature, and some other things do it knows how much fuel to inject. As long as the ECU knows that the engine is turning and how quickly it will open the injectors. If the coolant temp, intake temp, pressure, and any of a few other inputs are wrong it may inject too little or too much, but still SOMETHING.

With that, if it's simply not injecting anything the ECU is either damaged, not being powered/grounded correctly, or it doesn't know that the engine is turning. If the ECU knows engine speed it will output a tachometer signal.


-Cody
(Sent from my phone, if a word doesn't fit blame Siri)

On Mar 27, 2012, at 6:38 PM, "Mark Rutherford" <mark.rutherford at emerytelcom.net> wrote:

> Looked at the waveforms on both sensors, the RPM sensor has a good sine
> wave, and the TDC has a single spike for each revolution.  I noticed that
> the RPM plug has 5 V DC on it with the key on, while the TDC does not.  Is
> this normal, is it dependent on where the flywheel stops.  I still have no
> fuel injected during the cranking cycle.  It seems like it should have a
> small amount, or am I wrong.  As a further test I inserted a 10K ohm
> resistor in place of the coolant temp sensor.  With this value it should be
> calling for a large amount of cold start enrichment.  Any ideas?
> 
> 
> 
> -----Original Message-----
> From: Cody Forbes [mailto:cody at 5000tq.com] 
> Sent: Sunday, March 25, 2012 6:08 PM
> To: Mark Rutherford
> Cc: <quattro at audifans.com>
> Subject: Re: 1990 20V no start when cold
> 
> Flywheel sensors can be bad and pass tests. Swap both from parts car or
> monitor them with an oscilloscope. Also don't get the plugs backwards, not
> sure if that's possible on your car but on 5000's, 944's and 911's I've
> fixed them many times when somebody confused the RPM and TDC sensor plugs. 
> 
> -Cody
> (Sent from my phone, if a word doesn't fit blame Siri)
> 
> On Mar 25, 2012, at 7:01 PM, "Mark Rutherford"
> <mark.rutherford at emerytelcom.net> wrote:
> 
>> I am looking for ideas on how to fix a no start condition on my 1990 90
> 20V
>> 7A.  
>> 
>> 
>> 
>> Here is a chronological history of work done so far.
>> 
>> 
>> 
>> Initially the car would not start when cold < 40F.  Would start with a
> shot
>> of ether, but would not rev well unless the throttle is eased into.  I
> have
>> replaced and checked everything I can think of( there is a list below of
>> items I have tried).  With the manifold removed and the fuel supply
>> connected I turned over the engine and noticed that no fuel in admitted
>> during starting.  Taking a injector to ground triggers the injector, but
>> they are not triggered from the ECU.  This does not seem right.  Shouldn't
> a
>> small amount of fuel be admitted during cranking?  Any experience, or
>> suggestions out there.
>> 
>> 
>> 
>> Pulled mass air sensor high voltage code.  Replaced code went away problem
>> stayed.
>> 
>> 
>> 
>> Pulled coolant temp sensor code, changed it, original was still in spec,
>> code stayed, problem stayed.
>> 
>> Checked wiring OK
>> 
>> Wired sensor directly into ECU still have code on 2 different ECU's.
>> 
>> 
>> 
>> Found throttle position sensor on bottom of throttle body had fallen off
> and
>> was cooked.  Replaced, still have problem.
>> 
>> 
>> 
>> Replaced the following from a parts car that will start at any temperature
>> and runs fine.
>> 
>> 
>> 
>> Mass air flow, idle stabilizer, throttle body and sensors, distributor,
>> computer and coil.  Still have problem.
>> 
>> 
>> 
>> Checked fuel pressure, 85 PSI, a little high, replaced pressure regulator,
>> holds pressure and meets specs from Bentley.
>> 
>> 
>> 
>> Checked fuel delivery volume, meets Bentley specs.
>> 
>> 
>> 
>> Replaced fuel injectors and rail from parts car, resistor pack, both the
>> original and parts car meet specs for resistance.
>> 
>> 
>> 
>> Check all wiring from plug A on the computer to source, replaced fuel pump
>> relay, RPM and timing sensor, all originals passed tests from Bentley.
>> 
>> 
>> 
>> Placed all parts back on the parts car.  Parts car starts and runs well.
>> 
>> 
>> 
>> Moved parts back to problem child.  
>> 
>> 
>> 
>> Checked pin in the fly wheel , found one about 15 degrees BTDC. 
>> 
>> 
>> 
>> Looked to ensure distributor, cam, and crank are all in sync.  No
> problems..
>> Condition remains.
>> 
>> 
>> 
>> 
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>> 
>> 
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