Was directed here to get an answer to my question. Can anyone explain to me what actually happens and in what order when the car is first started. A GM mechanic and good friend told me this when my 89 rock was taking several cranks to start. He told me that for 2 seconds the fuel pump relay is energized to pressurize the system, and when the car starts the 12 source for the operation of the pump came from the pressure switch on the oil sender assembly. I believed him because my car will run without the relay being plugged in. Made perfect sense to me. I have been told by many people that this is wrong, that both the relay and the sender switch are in parallel to provide power in the event either device fails. After spending some time in the garage I found that my fuel relay turns on for the 2 sec., then it turns off and then it turns on again. I noticed a 12 volt input to the ECM at terminal B2 that comes from the relay after it closes and also the sender. Does the ECM look for something in a timed order. Appreciate an explaination of how things are timed an executed. TNX, Danno

 

The oil pressure switch IS part of a safety backup arrangement in case the ECM or relay fails. Says that right in the factory service manual. The relay and switch are in parallel.

At key on, the ECM runs the pump two seconds. Once the engine is cranking and the ECM gets distributor reference pulses the ECM starts the FP. This happens almost immediately. No reference pulses and the ECM won't command the FP to run.

If you disconnect the relay and try to start the car the engine will have to crank considerably longer until engough oil pressure builds to trigger the oil pressure switch.

Clear as mud?

 

YEP, I did the WHOLE sequence today. I was told that the oil sender 12 volt path was to protect the engine. BOY was I WRONG. ONE question, If the ECM has a catastrophic failure does it really matter if the fuel pump works? Thanks for the reply, have a feeling that I will get booted off sometime soon!

 

I think the oil pressure switch is there to protect you. Its cheap redundancy. It certainly isn't a catch all solution but its better than none.

Lets say you are pulling out into to heavy traffic and the FP relay or ECM fails. Without the oil pressure switch the engine would stop instantly potentially leaving you in the path of oncoming vehicles.

Catastrophic ECM failure? I've seen them fail badly enough that even limp home mode doesn't work and the engine stops. I'd think thats kinda rare though. More likely only a part of the ECM dies and you end up in limp home mode.

 

I put my foot in my mouth so BAD I am ashamed to log on. As a design engineer I would have put an oil press. switch backup to protect the motor. Honestly, I do not know what my ECM would do in the event of a massive failure. Being in electronics, I just look at the very simple failure of 5 volt regulator IC, hell it runs the whole damn thing. I cannot really say if the thing would EVEN limp home. I wish it would, but in my business I wish even half of my equipment would last until the ride up the mountain. Danno

 

My painless wiring harness has the fuel relay on it as well. There is also a red wire coming out of the fuel relay connector that I don't have hooked up...I'm beginning to wonder if this is the power lead for the fuel pump and it isn't being powered. So instead the power is going through the the oil switch. It does take it a little while to crank, longer than I would have thought...the Painless Needs to include schematics...

Does this sound right?

 

Could be, its hard to say without actually seeing it.

Disconnect the oil pressure switch while its running and see what happens.

 

Since I am going to eliminate the Distributor from my car when I get it all finished and go to coil packs, are there any suggestion on what to do with the relay to work around?

SB

 

The ECM/PCM you will be using should have a fuel pump output on it. I'd just connect this output to the existing relay.

 

Hi my name is Bob and Im a retard. Sorry, forgot to think before I asked that question. Im getting so wrapped up in trying to cover all the little features that the stock ECU does that I just missed the obvious.

SB

 

The oil presure set up is to shut the fuel pump off in a serious accident. The engine can stall in an accident, and if so you want the fuel pump to shut off. this idea goes back to the Vegas.