[s-cars] Lost a Cylinder: Who knows Electronics?

[Paul Gailus]

You may want to check and make sure that there are no other
components tied to ECU pin 21 besides this resistor.
If you are lucky, there is a protection diode or similar device
tied to a 5 volt or other supply voltage that is defective,
and it's pulling up the output voltage and turning on the POS.
There could also be another protection diode to ground,
but it wouldn't cause the symptoms you're describing.

If pin 21 connects only to the 1K resistor and nothing else,
then your measurements indicate that something is wrong with
this resistor. But even if this resistor is bad, it still sounds
unlikely that it would causing the POS to turn on like you
describe. However, it shouldn't hurt anything to try replacing it.

Paul


----- Original Message ----- 
From: Brian Powell <powellb at gmail.com>
To: <s-car-list at audifans.com>
Sent: Wednesday, December 08, 2004 10:35 PM
Subject: [s-cars] Lost a Cylinder: Who knows Electronics?


> Greetings, driving home last night, I suddenly lost power and the
> engine ran very rough.
> 
> Got home, pulled injectors and found it was cylinder #5. Swapped
> POS's, still #5. Swapped plugs, still #5. Swapped injectors, still #5.
> Tested the coil with a gap tester, and when you would turn on the
> ignition (not even turn to start), it would spark, then, turn to start
> engine, and #5 would not spark again. Bad coil, I think to myself. I
> replace that coil with a spare I had. Same symptoms.
> 
> Okay, turn to Bentley page 28-09 and test the ECU to POS leads with an
> LED tester. Cylinders 1, 2, 3, and 4 all check out. Go to pin #3 on
> the second 4-pin POS connector (corresponding to cylinder 5). The LED
> lights up when you just turn on ignition (engine off). When you turn
> over the engine, the LED doesn't blink for each firing signal like
> every other cylinder. Check for short between ECU harness and POS
> connector: checks out. M***er F*****!!!!
> 
> ECU seems to have a short on cylinder 5. UGHHH!
> 
> Take apart ECU. Start tracing back from pin 21 to see what I can find.
> All of the cylinder Pins feed to a bank of 5 resistors which then
> connect to some custom Seimens chip. Okay, let me test the resistors.
> They are BROWN-BLACK-BLACK-BROWN-BROWN which corresponds to 1kOhm with
> 1% tolerance. I test each resistor and every cylinder comes in at 997
> Ohms EXCEPT the resistor for cylinder 5.
> 
> Cylinder 5 measures 977 Ohms when I measure it in one direction and
> 999 Ohms when I measure in the other direction (i.e. common on left
> side then common on right side). This makes no sense as the resistor
> shouldn't care. All of the other resistors measure precisely 997 no
> matter how I measure them (I have checked this 15 times now).
> 
> My question is thus: This resistor is outside the specs on the
> resistor sheet for the given color code. Those who are electronically
> inclined, could it being roughly 3% out of spec (for some reason in
> one direction) cause this short that I am getting? Essentially coil 5
> is always on except when cranking the engine. If so, I can simply
> unsolder this resistor and solder in another 1kOhm resistor.
> 
> I can't afford another ECU right now unless someone wants to sell me a
> busted ECU cheap and I can try to make one good one out of two.
> 
> Any help and suggestions are greatly appreciated.
> 
> -- 
> Cheers,
> Brian
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