Starting problem

[texas tough]

my 89 wont start with the relay unplugged either...BUT the ECM, and engine were change,, its a 92 engine, .

 

[Mark Gilbert]

The wiring would still be from the 89 even if the PCM was changed. I would suspect. My next question though is if you start the truck with the relay in and unplug it once it is started....

I know the switch is supposed to be rated at closing the contacts at 4 psi, but I suspect it actually needs more.

 

[texas tough]

I think the computer primes thru the NC circuit in the relay, then SWITCHES to the NO side when cranking, and once the ECM sees a pulse in the module. AS LONG AS ITS SEEING A PULSE FROM IGNITION, THE ecm KEEPS THE RELAY CLOSED..

 

[Mark Gilbert]

That makes some sense. It just seems odd that there is a redundancy. What is the value of having the oil pressure switch and the relay maintaining fuel pressure? Obviously without ignition pulses the engine will no longer run and when the engine dies you will lose oil pressure, and that would shut down the fuel pump. Or the other case if you lose oil pressure then the relay, which will still be on because the ignition system is still functioning, will leave the fuel pump running and allow the engine to lock itself without oil...

It just seems like it would have made more sense for the PCM to prime the system and then when the engine fires up the oil pressure switch would be responsible to keep the fuel pump running.

I'm not disagreeing with anyone, just thinking out loud...

Now the ignition pulses maintaining the fuel pump relay makes sense on the newer (94 and newer) PCM's that don't use the oil pressure switch.

 

[texas tough]

I will go try and see what happens on the 89 BRB,,

ok unplugged it while running, died instantly.. 40PSI oil pressure

 

[texas tough]

GM has made ALOT OF STUFF, and changed wiring for many applications.. certain vehicles that were made may have had this oil pressure redundancy wiring, but both of my trucks dont. NO RELAY, NO RUN,.,
This is actually good for ease of troubleshooting.

 

[texas tough]

it would make more sense for the oil pressure redundency circuit to be used on vehicles with a more commercial application, like diesels. and 2500, 3500 series.

 

[texas tough]

later I will go back probe the fuel pump relay and see when it switches.

 

[dave s]

Well I received my new ALDL cable and cd the other day. Got it loaded on my laptop last night as well as downloading the TunerPro Rt program. I'll get the truck here tomorrow and plug this in and see what I can see. I'm fair on figuring out computer stuff. That said, from reading about this program I'm sure I'll have some questions. Can't wait to try it out.

 

[delta_p]

I know someone said it's not for safety but I still think the redundancy is if the relay is lost while driving and someone doesn't die because of the relay failure only.

The 4 psi always seemed low to me and I been meaning to bench test a spare oil sender switch I have.

40 psi and killing the engine by pulling the relay might be worth making sure the sender doesn't need replacing.