Limp home mode on GM systems

[John Dammeyer]

Hi,

First I guess we have to decide what consitutes LHM.  For instance,  my water
temperature gauge input is used to determine when the engine is warm enough to
remove cold start enhancement.  At 150 degrees all enhancements are gone and up
to 150, the difference in engine temperature and 150 is used as a gain
enrichment factor.  If the gauge fails open it reports 240 degrees so I force it
to read 220 and remove enrichment.  If it fails short or stops responding I do
nothing.  I should probably add some sort of warmup timer that sets the
temperature to +150 if the temperature sender hasn't changed.

If the MAT fails open it reports -60 so the mixture is rich.  I haven't tried
the engine at -40 in winter so I do not know if I should consider -60 a problem.
In either case,  if it does fail this way,  it just enriches the mixture.  Not a
real problem.

TPS is only used for acceleration and WOT enrichment so if it fails the engine
stumbles under acceleration but does run.

O2 sensor only impacts the mixture a little bit so can be discarded if it fails.
ie: if it reports the wrong values the engine just runs a tinly bit rich or lean
but still runs fairly well.

MAP sensor This is the biggy.  If it fails such that it reports  low pressure
the mixture will be lean and the engine will have a major drivability problem.
If it fails with a high pressure reading the mixture will be excessively rich.
In all cases when the MAP sensor is disconnected,  the engine stalls.

The Barometer sensor is also somewhat critical if it fails with a low pressure
reading as the mixture will become very lean.  A failure with a high pressure
reading defaults to just below sea level (Amsterdam?) so becomes unimportant
other than enrichening the mixture somewhat.

I currently do not sense if the injectors are functioning or what the CD
capacitor voltage is so I can't determine a LHM from that.  If the RPM sensors
fail,  so does the timing and therefore so does the engine.  Fuel Pump is
determined from presense of pulses and I wouldn't subvert that because of fire
hazzard if the ECU thinks LHM is there when vehicle has crashed.

Cheers,
John Dammeyer

>Date: Sat, 30 Oct 1999 22:09:58 -0500 (EST)
>From: Mike Comai <comai at expert.cc.purdue.edu>
>Subject: Limp home mode on GM systems
>
>I'm putting a GM '747 system into a 1979 CJ5.  I was curious if anyone new
>what sensors/systems the ECM uses when it enters limp home mode.  I
>noticed that on the schematics it looks like the fuel pump is running all
>the time when the oil pressure switch is closed(i.e. bypasses the relay)
>and the only reason I can think for that would be if the ECM fails or
>enters L.H.M., or the relay fails.  I just wanted to make sure that I
>think of all the angles when I do the install, and I'd appreciate any
>more info on L.H.M. or any other odities that aren't easy to see.  Thanks,
>