ECU repair, was No Start: Audi 100, 1990, I5 NF engine, non-turbo

Peter Orban orbanp1 at
Fri Sep 2 07:14:41 PDT 2011

Hi Marc,

Check that big main connector to the ECU. Are all the female pins tight? If they are not that could cause that 100 Ohm resistance.
If the connector checks out OK, take apart the ECU and see how are things inside. Check if there was any water intrusion into the box. There would be a thin O-ring around the perimeter the make the watertight seal. 
Track down that path that read 100 some Ohms instead of zero. Water intrusion could ruin the connection between the ground point and where you measured that. If there was water inside, there could be some corrosion of the traces, component leads. Follow through that trace visually. Most probably that ECU is only on a two-layer printed circuit. If there is some corrosion from water, chances are that it could be repaired (I did manage to repair corrosion damage in a TV that a tech had given up on it.)

See if there are any broken solder joints there, they would appear gray as opposed to shiny. Reflow them (with a soldering iron) if they look suspicious. Use a magnifier to check this. some of the ICs (memory, processor) would use sockets, that is also a source of unreliability. Remove and re-seat the integrated circuits, that would "clean" the connections.

Good luck, Peter

--- On Thu, 9/1/11, quattro-request at <quattro-request at> wrote:

Message: 3
Date: Thu, 1 Sep 2011 23:18:03 -0400
From: Marc Boucher <mboucher70 at>
To: <audi at>,    "Tony Hoffman" <auditony at>
Cc: quattro at
Subject: Re: No Start: Audi 100, 1990, I5 NF engine, non-turbo,
    non-quattro,    CIS-E-111 (California)
Message-ID: <SNT106-DS101EE6EC1DC80E66B2946C1180 at phx.gbl>
Content-Type: text/plain; format=flowed; charset="Windows-1252";

At this point, I've located a replacement ICU for a reasonable price.  I 
also believe that I've exhausted all reasonable tests that would point to it 
being something other than the ECU (ICU/Fuel-Injection CU combination).  So 
I'll probably order it.  Just wanted to first detail the additional tests 
that I've conducted, and information that I've gathered, and see if there 
were any final tests anyone has to recommend.

All three wires from hall sender to ICU were measured, once again, for 
resistance.  This time on the Rx1 scale.  There is no measureable resistance 
in any of these wires between the plug in the engine bay that connects to 
the hall sender and the plug that connects to the ICU.

Previously, I'd measured the voltage between the red/black wire and the 
brown/white wire of the hall sender, and got the expected 12 volts with the 
ignition on.  But then it occurred to to check the voltage between the 
red/black wire and the the car's ground...this is also 12 volts with the 
ignition on.  So then whats's the resistance between the brown wire and the 
car's ground with the ignition on?  Its circuit.  How can 
red be 12 volts to both the car's ground and 12 volts to the brown wire, but 
the brown wire has no connection to the car's ground?

Next I put the multiimeter between the car's ground and the hall sender's 
brown wire and cranked the engine.  It read about 5 volts.  Then I switched 
to a current scale which would allow current to pass and hence the car to 
start.  Car started, but I couldn't read the current, likely because its 
switched, not DC.

Just to be complete, here's a list of additional actions that I've taken:

Remove the ground wires attached to the intake manifold and test for 
continuity with ground at the ICU.  Even though it was good, I nonetheless 
cleaned the ring connectors and reattached them with a bit of dielectric 

Checked the continuity of every wire between the ICU and the fuel injection 
control unit !!!

Previously all grounds were checked successfully, but you'd raised the 
possibility of a wire showing a good connection using an ohmmeter but not 
being able to carry a sufficient current.  To eliminate this possibility I 
ran a second, parallel ground to each of the ICU and Fuel Injection unit's 
ground terminals.

Any last tests that anyone care to recommend before I get the replacement 

If I do order the replacement ICU, any other parts that I should probably 
pick up while I'm at it?  Not specifically for this problem, but since I 
plan on keeping this car a few more years, and since shipping parts 1 by 1 
gets pricey, I may as well stock up on a few things if I place the ICU order 
from Force 5, that needs to ship it.

Thanks again


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