Re: 4000 RPM Limiter

[Robert Houk - SMCC Bos Desktop Hardware <rdh@UrQ.East.Sun.COM>]

   I figured everyone had gone through this cutout at least once by now.

   >      Stalking a new gremlin on my TQQC. It runs quite well until we get to
   >      4000revs. Then fuel pump switches off. If I keep foot in it it
   ...
   >   There are only three things that can cause the early
   >   mac-02 style computers to limit at 4000 rpms.  This is
   ...

   You can add excessive boost to that list.  According to the Service

Although probably not "just at 4000", that's a key RPM point. Overboost 
can nail ya at just about any RPM (put in an "S4" turbo, and watch the
ECU cut out at 2000RPM!)

   Training manual, the temp sensor isn't included, but I can vouch that it
   can cause the problem.  In addition, I've heard that bad grounds, in
   particular the cluster ground near the coil, can cause this.  In fact,
   anything in the wiring path to those sensors or the ECU ground can cause
   the problem.

Grounds grounds and more bloody grounds! there otta be a law . . .

   >I kinda thought the only 4000RPM issue was the manifold air temp, but...
   >
   ---snip---

    which results is a "signal voltage" that varies from
   >roughly 70mv (Yes, 0.070 volts) to around 140mv. Just wiggling the connector
   >to the ECU will use up about a third to a half of this signal range!

   Strictly speaking, its low current that causes conduction problems, though
   the two are obviously related.  I forget the details, but I vaguely recall
   that below about 10-15ma, contacts really need gold to be reliable.

Side-stepping the obvious rathole here, suffice it to say that the Audi
UrQ manifold air temp sensor design/wiring is da pits, and can cause *far*
more than its fair share of grief and aggravation!

   >I drilled a small hole in the ECU case, and ran a small "Hi Fi" cable with
   >gold-plated phono plug/jack into the ECU, and soldered the wire to the ECU
   >pins (make sure you get the ground right...),

   Do you mean you soldered to the gold tip???  In a recent tech tip, Ned

No, sorry, I'll try to be more clear. I took an audio "extension cord",
in this case, male phono on one end, female on the other, both gold plated,
and cut the cord in half. I then inserted one cut end through a small hole
in the ECU from panel near pins 18/19, stripped the ends and soldered them
to pins 18/19. I hallucinated a "strain relief" by affixing small cable
ties on both sides of the ECU faceplate to lock the cable into place. I
then took the other severed half of the cable and finagled it into the
wiring harness plug assembly, stripping and soldering the ends to the
manifold air temp "pins". I think I put the female end on the ECU-side,
but won't swear to it. It shouldn't matter what sort of "connectors" you
use (DIN, Phone, miniature phone, BNC, whatever is convenient to hand [and
won't easily get plugged into anything else "within reach" by service
droids beyond your control...]), just so long as it had high-quality con-
tacts (which can be taken to mean "gold" here for the "low signal levels").

   mentioned using something called Stabilant or some similar name.  Has
   anyone tried the stuff?  It sounds as if it would be perfect to prevent
   this kind of intermittent contact.  I think he said it was available from
   Audi dealers, and was recommended by Audi.

I'm highly dubious of most of these miracle cures, especially when applied
in an automotive environment where the curee is regularly subjected to temps
that range from damn cold to bloody hot with all sorts of humidity levels
(not to mention vibration) to help muck up the issues . . . especially since
all the ones I've seen urge you to "repeat regularly"!

					-RDH