[Gmecm] Ignition off fan

[Scott Peitzsch]

> So, since my fan goes straight off when I turn the ignition off, would I 
> be
> right in deducing that the $58 code goes through all the motions of
> controlling 2 fans, but the code for the 2nd (high) one achieves nothing 
> on
> the 749 h/w, but _does_ work on my 727 h/w? :-)

That is how I understand it.  You also need to be sure that the high side
power source for the fan relay is on battery power, not ignition switched
power in order for it to be able to stay active after ignition-off.

> If this is right, then it's probably worth me trying to find the #1 fan 
> pin,
> to get the ignition off running. Shame there seems to be no cross 
> reference
> for this! I guess I just check the quad driver pins...

This is the cross reference I use:

http://www.cruzers.com/~ludis/triplugecm.html

You can also see the '727 pinouts for fan control here:

ftp://ftp.diy-efi.org/pub/gmecm/manuals/1227730/90-7727V8TPI-6.jpg

> BTW: I'm also observing my fan flicking on then straight off a few times 
> as
> the engine temperature rises to the high fan ON temperature that I've set.
> The CTS is a little noisy (mostly +/- 1 C, with occasional dips of 6C !);
> but, looking at the plots of CTS vs. fan ON, there isn't a 100% 
> correlation
> between the CTS spikes and the fan state transition. I'm still trying to
> understand the code (not helped by not knowing what half of the zero page
> locations, particularly statuses, mean), but I've been wondering whether 
> the
> $58's high fan control logic isn't as sophisticated as that for the 
> control
> of the low fan (i.e. high fan has no delayed start up - which might reduce
> the likelihood of the 'flicking'). If the $58 only ran on h/w that didn't
> use the high fan logic, then GM may never have noticed this behaviour, or
> didn't care(?)
>
> Having said this, I'd expect the natural hysteresis of the ON/OFF 
> thresholds
> to avoid this kind of 'flicking' problem...
>
> I also plan to look at my CTS wiring. I 'cheated' by wiring the GND side 
> to
> chassis. This massively added to the CTS noise when I had had bad engine
> grounding (now fixed), but still may be adding some. I'd be interested to
> see others's CTS plots to compare noise.

This is a very bad practice.  Sensor grounds should always observe proper
"star" or "single point" grounding technique.  The correct sensor ground pin 
to
use on a '727 is C10, as shown here:

ftp://ftp.diy-efi.org/pub/gmecm/manuals/1227730/90-7727V8TPI-2.jpg

-Scott