control unit blues

[quk@isham-research.demon.co.uk (Phil Payne)]

In message <C735981EB58DD111921500A0C99D79892BDAA7@CMS1.co.albemarle.va.us> Mark Trank writes:

> 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.

It's unusual for this controller to fail.   Note a rather peculiar
characteristic - if the plug is pulled out of the ISV itself, the
controller will _not_ operate the valve again until the ignition has
been switched off and on again.  This behaviour often leads to
mis-diagnosis of the real problem.  I don't have the 5k documentation,
but the MB engine uses the same ISV and its controller tests are:

VDO ISV tests

          ==+==+==                    ==+==+==
    |16|    |13|    |10|        | 7|    | 4|    | 1|
    |17|    |14|    |11|        | 8|    | 5|    | 2|
    |18|    |15|    |12|        | 9|    | 6|    | 3|

(5 is chassis ground.)

Tests with a DC voltmeter:

14 + 5 - ignition on - Power                        Battery Voltage

14 + 1 - ignition on - Code input B                 Battery Voltage

8 + 1  - ignition on
         throttle closed
         idle switch closed                         Battery Voltage

Tests with a diode test lamp:

17 + 5 - engine running - RPM from ECU              Diode must flash

7 + 5  - ignition on    - road speed sensor
         left front wheel turned by hand            Diode must flash

Tests with an ohmmeter:

13 + 5 -                - thermoswitch              800 to 1000 ohms
                                                    at 20c, 70 to 150
                                                    ohms at 80c

11 + 4 - ISV attached   - ISV resistance            3 to 8 ohms

Tests for cars with air conditioning:

6 + 5  - ignition on    - E87
         air conditioning on                        Battery Voltage

2 + 5  - ignition on    - N25
         air conditioning on                        Battery Voltage


It's quite normal for Audi mechanics to pull the plug off, hook in a 
meter, see no current and assume the controller is bad.  Do the above
checks to see if the controller is getting all that it needs, and then
hook up a ammeter in the ISV circuit - you're looking for 430ma.  A
value of 470ma is the controller's 'panic mode'.  The ISV is a current-
driven device - don't just hook it up to 12v to see if it works.  It
can fry it.  6v to 9v (couple of flashlight batteries) should make
it click a little.  If not - carb cleaner.

Don't forget to turn the ignition off and on each time you disconnect
the ISV.

--
 Phil Payne
 Phone: 0385 302803   Fax: 01536 723021
 (The contents of this post will _NOT_ appear in the UK Newsletter.)