[Gmecm] Re: Undignified death of a project :P

[David Allen]

  Congratulations on the award, nice ride...  Hard to beat that candy-apple 
red!
  One comment for you though.  I have 3 GM engines with the following ECM's, 
1228746, 1226869, and 1227137.  Two are TBI and one MPFI.  All are retrofits 
into different year cars (and a boat).  None have an oil pressure switch. 
The only control for the fuel pump is the ECM's "fuel pump relay drive" 
output.  On these ECM's that output goes high (+12V) breifly when the key is 
turned on; and stays high for as long as the ECM is seeing ignition 
reference pulses.
  The only thing keeping the pump on is the ECM fuel pump relay drive 
signal.  If the engine stalls with the key still ON, the pump stops about 3 
seconds after the engine stops turning. To keep from setting a code, the 
ECM's "fuel pump relay drive" and "fuel pump signal" lines are tied together 
and both go to the relay's coil.
  I've never had the ECM shut down the pump while the engine is running.
  All the schematics show the oil pressure switch as you describe, in 
parallel with the ECM-controlled relay.  Because it is a parallel circuit, 
either the ECM-controlled relay, or the oil pressure switch can keep the 
pump running. Since (at least on my 3 engines) the ECM keeps the relay 
pulled in as long as the engine is turning, the oil pressure switch can open 
and it will not make any difference. The fuel pump current will still have a 
complete circuit through the relay.
  I see the oil pressure switch would be a redundant feature to allow the 
engine to keep running if the relay failed but other than that it is just 
added parts and wiring that are not necessary.
  Boy I agree a 7 psi shutdown on an engine under load will not save the 
engine even if the ECM didn't keep the relay on!
  My expertise is older GM stuff and off-highway diesels (work for 
Caterpillar dealer).  I'm not challenging your expertise but I want to 
mention that not all ECM's turn off the fuel pump relay after the engine is 
running.
  In real-world experience I'd expect more no-run problems due to shorted/ 
grounded wiring to the oil pressure switch, than I would due to failed 
relays on an ecm-only setup.  IMHO, I would leave the oil pressure switch on 
a factory setup, but it's not worth the trouble to install on a retrofit.
  On the newer big diesels, there is a table in the ECM "oil PSI Vs temp Vs 
RPM."  It has a pressure transducer telling the ECM oil PSI, and an oil temp 
sensor.  The ECM will detect the engine loosing oil pressure at any RPM. It 
will derate the engine (reduce engine power), activate warnings, and 
eventually (depending on the safety/ liability concerns) shut the engine 
off.  Some of the calibrations (particularly on trucks) never shut off. 
They derate to the degree that you can only just creep in low gears; this is 
to keep from creating a traffic accident liability due to "intentional 
avoidable shutdown."
  Some older stationary engines had 2 oil pressure switches, one at 10 PSI 
and one at 40 or 50. The starter would begin turning the engine, and when 
the 10 PSI switch closed, the fuel solenoid would activate.  After the 
engine was running, a timer would change to the high-pressure oil switch 
several seconds after startup (giving the engine time to rev up to operating 
speed).  If the engine dropped below 40 or 50 at operating speed, an 
automatic shutdown would happen.
  Just my two cents worth!!
Thanks,
David




----- Original Message ----- 
From: "Andrew Gibson" <andrewsharyn at yahoo.com>
To: <gmecm at diy-efi.org>
Sent: Saturday, September 08, 2007 7:50 PM
Subject: [Gmecm] Re: Undignified death of a project :P


> Since this is the GM-ECM area I will add some info I hope everyone will 
> find useful. You can challenge me on it if you want but having been a 
> diagnostic tech for 10 years, I know. If you still don't think I know 
> anything check www.nadiauto.com , yes that's my new ride.
>
>  Low oil shutdown on GM products: Been there for years. Yes I had it on 
> the truck. It works by the ECM powering a fuel pump relay on initial key 
> on and cranking. Then the oil pressure switch takes over. The switch may 
> directly power the pump or drive the fuel pump relay. Either way you have 
> to have below 7 psi oil pressure for the amount of time it takes for the 
> engine to burn off the residual fuel pressure. Works good if you flip the 
> car upside down. Doesn't work so well to save the engine. :(
>
>  Bypassing the Jeep fuel pump drive strategy: Jeep and all Chrysler 
> products do not use a low oil pressure shut down. They use an Automatic 
> Shut Down relay (I like automatic self destruct myself) which powers the 
> fuel pump, ign coil, and injectors. The ECM turns the ASD relay on after 
> it sees the cam and crank signals and properly indexes the engine for 
> timing. After the indexing it commands the relay on so you still get a 
> delay in building fuel pressure. Which is why a Chrysler product has to 
> turn two full revolutions before starting when a GM only has to rotate 1/4 
> turn. I'll keep my GMC truck.