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Re: control unit blues


Mark Trank <MTrank@exch.co.albemarle.va.us> wrote:
As a follow up to my post yesterday (8/5), my 87 5K apparently has a bad
control unit, which is responsible for the rough idling problem
identified yesterday.  This part (I'm told) is manufactured only by VDO,
as the car is a turbo, and the equivalent Bosch part can't be
substituted.  As might be anticipated, dealer price is exorbitant
(approx. $375 US).

This one has struck me a while ago... on an 100 '87 5cyl.
Flaky idle came out to be the (famous) "dirty idle stabilization
valve (ISV)" rather than a faulty idle control module, which I
had in suspect before.

While digging into the electrics I discoverd that the idle control
module was indeed broken, but not in the idle part but in the part
which controls the fuel shut off valve... Dealer price was not much
better here in Germany for the part (at around DM 300,-) so I went for
"hey, it's broken anyway, I cannot break it..." and pried it open to
look what's ticking inside. Obviously VDO was not very helpful in
giving away schematics and so I had to use my DMM...

I was not able to re-engineer the whole thing but the following I
  - inside electronics worth ca. $30 containing a bunch of OpAmps,
    transistors and diodes. extrem robust circuit board.
  - transistor to control fuel shut off valve was a weak type for
    this application (and broken on mine..., replaced with a stronger
    type for 50c...) like a "fail-early-does-not-leave-the-car-stuck-
    but-can-be-replaced-for-much-bucks"-item. (*1)
  - transistor pair for control of the ISV are stronger and
    should rarely fail; if they do, the ISV will remain closed and 
    the car will stall on idle. (*2)
  - intermittent connection to either ISV or the temperature sensor
    causes a shut down of the module, car will stall on idle, off-on
    the ignition to reset.
  - wrong resistance of the ISV (too low or to high) will cause none
    operation of the ISV.
  - faulty solder joints should be very rare due to excessive coating
    of the board(s)

Conclusion to your problem:

  + Double check (by whom?) that its really the ISV control module and
    not dirty ISV or intermittent electrical connections (as suggested
    by other listers) to idle switch, compressor signal etc.
    * if your idle bounces at 1500, this indicates that the fuel shut
      off is working (*1)
    * if the idle is fluctuating anyway the control module actually
      _tries_ to stable the idle (*2)

  + short the temp sensor (front of engine block I assume, single wire 
    connection) does warm engine behave different? (you loose cold start
    assist with this setup)

  + disconnect ISV and turn the idle screw to get smooth ideling when
    engine warm, if this does NOT cure the calm the idle (somewhat)
    then there might be indeed a vacuum problem, clogged fuel filter,
    dying fuel punp etc, but no idle control module fault.

  + there might be other tips but for them I need to know the engine


'78  80 4cyl ??  (first car, did you ever built a tach from scratch?)
'82 100 4cyl DR  (yeah 75hp and 37mpg, sold at 240kmiles )
'87 100 5cyl KP  (wrecked (!) by a motorbike (!!) coming frontal...)
'94  80q4cyl AAB (my first q..., hoping winter comes soon...)

(Moron of the day: "...aspheric heated mirror glass $120. [+1min]" --
"Ok, forget it, I'll do it the after-market way [$10+20min+delivery]" --
"...and I tell You: You will have no heated mirror if you change the glass by
yourself" -- "I will" -- "No" -- "Yes" -- "You won't. I must know. [Period.
I'm the dealer, you know?]" -- "I will" -- "N.." [curtain].
One day later: "I have." -- "..?..")

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