89, strange hard starting

[mouse]

Your manual diagram for the fuel pump appears to be for an S10. However, ALL gm stuff is wired up like this that vintage. (minus the hot fuel thing, it wasn't used on cars, I know that much)

According to CustomEFIs, this is the correct diagram for the 7747 computer (http://www.efitune.com/manuals/1227747/747ecm2.jpg). Not sure what the designations are at the bottom. After all we are talking about computer controls of the 7747, right?

 

[dyeager535]

That may be for a '7747, but the actual wiring COULD be different. No argument that it is wired up fairly similar to all other this vintage though, it looks about the same.

My interpretation of that diagram is thus:

440 OR feeds 12V to the ECM, oil pressure switch, and relay. Both pin B1 and C16. All agreed?

465(?hard to read) A1 on the ECM connector is the ECM control of the relay. I was under the impression fuel pump prime is handled by the ECM. Key is turned on, ECM commands two second pump prime, then off until it sees the distributor reference pulse. (DRP)

B2 on the ECM connector is indication to the ECM that power is being sent to the fuel pump. Thats proven by the statement at the ECM and by the fuel pump test connector, which if given 12V, runs the pump.

All wires leading to the fuel pump should be tan/wht.

Oil pressure switch is normally open, the only thing that can possibly close it in that diagram, is oil pressure. Since oil pressure takes time to build on startup, there is no way the switch is tied into startup with the relay and ECM working correctly. No argument, right?

I've seen it stated once (never backed up or proven though) that disconnecting the oil pressure switch on some application caused the engine to die. It is entirely possible that the fuel pump relay drive is timed in certain applications and just turns off after x seconds to let the oil pump switch take over, but you'd need to read the coding to see which ECM's operate that way. If it exists, it can't be too common, because the '165 and 730 ECM's both do not operate that way.

It would make little sense to do it that way in any application, but then again, so does not using an O2 sensor on certain EFI vehicles GM exported.

here's a post where JP86SS says the same thing, plus another diagram: http://www.thirdgen.org/techboard/tpi/354015-whats-sensor-called.html?highlight=fuel+pump+operation+pressure+oil

 

[mouse]

When I get extra time (I'm busy this week) I'll try a couple of experiments with the circuit to see if I can clear up some of this (at least for me!).

One last point...there are too many wiring schematics and diagrams out there, and we should be careful when using one.

Thanks Dorian.

 

[dyeager535]

Yes, you are right about the diagrams, best to stick with just one dealing with the trucks we are concerned about.

This (previously posted, but again for clarity) is what we should be using: http://s25.photobucket.com/albums/c59/daleearnhardt01/1990%20GM%20V%20Series%20Wiring/?action=view&current=111-1183_IMG.jpg Real GM manual, 1990, V-series, and the LO5 engine.

Should be very, very easy to test for those with a stock TBI truck, or at least a completely stock R/V series TBI setup. Pull the connector for the oil pressure switch, and see if it continues to run. If it was/is tied into the starting OR running portion as anything other than a failsafe, the truck will simply quit running. If it does anything other than keep running, then just need to verify the relay is working correctly, and is getting the ECM signal to switch the pump on. Pretty simple test if you've got the truck handy.

And I just noticed in the above diagram the wire color for the ground at the sending unit. Might as well have said "whatever we had lying around that day" lol.

 

[mouse]

My truck is currently under the knife, or I could do it today. Hopefully this weekend. BTW, the weather man is calling for 6-18 inches of new snow for us tonight...

 

[dyeager535]

Under the knife? Hope it's nothing serious so you can get to work on this ASAP!

Snowed here yesterday, all day. 60* last weekend, snow yesterday, freezing today.

 

[dyeager535]

Old post, but thanks to Randy (randy88k5) the question is solved. Straight from his PM, this is the result of the fuel pump relay/oil pressure test:

"OK, well the car starts with the relay disconnected, and it continues to run when the relay is disconnected while running. Since it is failry cold out, the oil pressure was already pretty high, so I really did not see a delay in starting with the fuel pump relay disconnected."

So the oil pressure switch is nothing more than a backup for the fuel pump relay. Nothing more, nothing less. Case closed. Thanks Randy!

 

[randy88k5]

No problem Dorian. Glad I could help...

 

[Thunder]

Yeah and i was wrong about the ECM controling the FP relay thru the OP switch. Sorry bout that. Must have ben thinking OBD2 ? Or old age is just setting in . The OP switch is just a back up for the FP relay. It allows you to get your vehicle to home or a shop if the FP relay fails. If you have to grind on the starter and turn your engine over till the OP switch sees oil pressure (4psi) then the engine starts. That is a sign of a bad FP relay.