Re: 4000 RPM Limiter

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

      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 cycles on/ 
      off. 
      Checked air temp sensor and it has 20 ohms resistance (within 
      tolerances). The throttle switch also operational. My manual doesn't 
      refer tp any other factors that would cause the fuel pump to kick off. 
      Any ideas?

   There are only three things that can cause the early
   mac-02 style computers to limit at 4000 rpms.  This is 
   the limp home mode.  It means you have a bad input to the
   computer.  The three are, 1.) air intake temperature
   2.) coolant temp (sic?) 3.) idle position switch (?).
   I don't have the doc';s right here in front of me
   so I may have it partially wrong.  But you've got some
   simple little electrical problem that can easily be
   identified with a DMM.

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

Anyways, is it solid, or erratic? And can you "gently" ease by the 4000RPM
point?

The man air temp is only applied when under boost, and only "fails" at
4000RPM (if you ease it to 5000, then floor it, will "work" OK).

I had an erratic 4000RPM cutout that was driving me crazy. Finally tracked
it down to poor cable-harness <=> ECU connection -- would unplug the ECU and
plug it back in and it'd be good for another week or two. POS! The thermistor
Audi uses for the man air temp ranges from around 18-19 Ohms to around 23-24
Ohms (cold to hot), 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! SFI!
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...), and the other end I ran into
the cable harness and soldered to the plug pins,  so now I have nice solid
low-resistance cable connection for the man air temp sensor that is still
easy to disconnect when I unplug the ECU. No problems for several months,
so I think I've finally fixed that bug.

You can fairly easily check this out -- pins 18 and 19 are the man air temp
input to the ECU, and are on the "top" row of pins with the ECU cover re-
moved. Hook up a good high-impedance DVM across those two pins, and watch
the voltage; at 4000RPM full boost it should be somewhere in the low 100mv
range, well below 150mv.

					-RDH