What I'm realy trying to say is that, if the fuel pump relay is working and the engine is turning, the ECM is receiving distributor reference pulses, the fuel pump relay will be energized, feeding power to the fuel pump on an alternate path from the oil pressure switch, so the fuel pump would still run and so would the engine. From what I've been able to determine, this is not a fail safe as is the 4+3 OD pressure switch you correctly mentioned. This is a backup to prevent the engine from quiting if, while running you were to lose either the oil pressure switch or the fuel pump relay. I'd really like to collect a few more data points from people that can verify that their cars will start and run fine with the oil pressure switch disconnected. But you have to have a good fuel pump relay or it invalidates the test. People that have long crank times, but have not troubleshot and found that when first switching the ignition to on, not start, that they can't hear the fuel pump run, could have a bad fuel pump relay or associated electronics and will not be able to help in this quest. Everyone knows that you can remove the fuel pump relay entirely and the engine will run fine, but you will have longer crank times because you won't get a fuel line prime as the fuel pump won't run until the oil pressure switch closes, but my belief is that the same is true for the oil pressure switch except you won't have the long crank times so you wouldn't even notice. I'd like to be able to verify this on more than 1 '86 and 1 '87 though.

 

Ray-I'll look into this a little when I get a chance. I blew the driver in the ECM and had to rig up a momentary switch to 'prime' the pump. Once the car was running, it ran fine. Again, with the blown FP driver in the ECM, I had no problems once running. It adds another twist to the scenario.

 

Add me to the "no" list. I have no clue how it would even work. Your car has no oil pressure when it isn't running. So don't you think it wouldn't even let the fuel pump run to start the car? Plus when my pickup tube got moved many times at full throttle I'd hit single digits on the fuel pressure (when I'd immediately let off after seeing it). The red light was even on and the engine kept running.

 

Thanks for the pics Scorp, i can see it now.
Does anyone have the correct P/N? When i went to Advance they pulled up 4 different switches, the one i have in my hand right now looks alot like the piece sitting on the end of the sender. It has one prong and bolt threads, but no fuel-filter-looking thing attached to it.

The guys there were asking me if mine has a 'light' or 'gauge' for oil pressure, and that confused the hell out of me. I thought all digidashes had a gauge and a red warning light, or are they talking about something else?

What tools/socket do you need to get it out?

 

Vader for my 86, from advanced auto "OPS68 oil pressure switch". works great.

Good luck Larry :seeya :seeya :seeya

Life is not measured in the breaths you take, but
in what takes your breath away.

 

Your car uses both types.

"lights" is the on/off type, its the two-pronged one. The correct name for it is the "oil pressure switch."

"guages" is the variable one, (one prong). This is the "oil pressure guage sending unit."

 

I think this switch sends a signal to the fuel pump relay. The fuel pump relay ignores the no-pressure situation on startup and powers the pump for 5 seconds no matter what. Makes sense to me. After that, the oil pressure switch must be closed for the relay to send 12v to the fuel pump.

 

I had the same thing when I got mine from the local Autozone.. I todl them it is for a GAUGE, which shows the Presuure itself. they gave me what I needed, and Im using it. Worked fine..

By the way if the FP sender does not work the car runs FINE...
I drove around 1/2year with it always stuck to 80PSI and had no performance issues.

 

No, the Switch doesn't send a signal to the Fuel Pump Relay. The Fuel Pump Relay, is energized solely be the ECM. What you have is two parallel paths for Fuel Pump power that both come from the same Fuseible Link and both join up at the Fuel Pump Fuse. One path goes Fusible Link - Oil Pressure Switch - Fuel Pump Fuse, the other goes Fusible Link - Fuel Pump Relay - Fuel Pump Fuse. We know about the Oil Pressure Switch. It closes @ 4psi. This path is easy to understand, when the switch closes, you'll have 12v through the Oil Pressure Switch to the Fuel Pump Fuse. The other path is the one that causes the confusion as the Fuel Pump Relay is controlled by the ECM. We know that the ECM will energize the Fuel Pump Relay when you initially switch to ON for 2 seconds to prime the Fuel Rail. What I'm saying is that the ECM also energizes the Fuel Pump Relay whenever the ECM is receiving Reference Pulses from the Distributor. The Distributor outputs Reference Pulses whenever it is turning, so if the engine is turning, the ECM is receiving Reference Pulses from the Distributor and it energizes the Fuel Pump Relay. As the engine continues to turn on the Starter or on its own if it started immediately, the oil pressure reaches @ 4psi and the Oil Pressure Switch closes, you now have two sources for Fuel Pump power. The Fuel Pump will continue to run with either.

MrNuke,
I assume you mean "OP sender" and not FP sender, but we are talking about the Switch not the Sender. The sender sends an analog signal to the Cluster for Pressure Display and in no way is involved with the Fuel Pump. The Switch has two pins the sender one.

 

Just a note -

The small sender has a ceramic compound (the black part in Timspeeds photo; may be different colors in other brands) which deteriorates over time (heat, vibration, petro compounds) and *will* leak. If you are getting oil dripping on the floor from the bellhousing/oil filter area, this *may* be the cause.

The two senders on my '86 are for the oil idiot light and the oil pressure digital display.

 

Pull the w/w motor. Three small bolts; one nut; one elect connector (and the leaf guard). I'd replace both senders; they're cheap, and if one is bad, the other is ready to go, too.

 

ADVANCED AUTO PARTS, tell them you have a digital guage
GP Sorensen Part #OPS68 OIL PRESSURE SWITCH $15.39


You will just need a small adjustable if you take off the distrbutor otherwise its just a standard socket, I wanna say 5/8 but I could be wrong


[Modified by ld85, 7:42 AM 7/12/2003]

 

:bs No they aren't! One actually does both the things you mention. The other is for something completely different.

The single pin unit in everyone's pics is the Sender. It is an analog device (meaning its output varies, in this case from 0 Ohms to around 90 Ohms at 80psi) that connects to the Cluster. The Cluster then displays the Oil Pressure. The Cluster also determines whether the pressure is too low and lights the red "idiot light" when Oil Pressure is < 25psi when rpm is > 4K or < 4psi when rpm < 800.

The two pin unit is the Switch. On most cars, and this is why the parts houses ask about guage or light so they know if you want an analog device or a digital device, this Switch feeds an idiot light. It is a digital device, being either open or closed. On our Corvettes this Switch feeds 12v to the Fuel Pump when closed.


[Modified by Ray Quayle, 1:42 PM 7/12/2003]

 

For what it's worth, on page 8A-20-6,7 of my 1987 GM Electrical Diagnosis Service Manual, the ONLY thing that energizes the fuel pump relay is the ECM and it does this ONLY on start.(It says so right on the page) After ignition, the oil pressure SWITCH closes(I believe it's 4 lbs.) and powers the fuel pump fuse block and also sends an input signal to the ECM that says "fuel pump on." Nowhere does it show any reference pulses from the distributor. Hence, once the car is running and the oil pressure switch were to be disconnected, the fuel pump looses it's power and the ECM no longer has it's "fuel pump on" input.

 

arbee,
The first part of your first sentence is the most important. "For what it's worth"; in this case not much. In school, Gordon goes on and on about how bad the early C4 manuals were and how many mistakes they contain. That's why he got involved in the writing/editing of the manuals. In my '86 manual on that page it says "FUEL PUMP RELAY CONTROL DURING STARTING". It doesn't say there is no voltage there during running. I'm still not saying you guys are wrong, I'm just saying that I've verified what I'm saying on two '86s and two '87s. Now I know this is not a cross section or anywhere near enough samples to confirm it as fact, but if one or two of you would try this test to see for yourselves instead of repeating what you have heard or what the manual says or doesn't say, we'd have the truth instead of conjecture. I'm not trying to spout heresy here, I'm asking for help to prove or disprove a theory.

Since you have the book open to this page and have a pretty good idea what is going on there, answer this for me:
What will you read between ALDL terminal G and ground when the Fuel Pump Relay is energized and de-energized with the engine running?
Now start your car and make the measurement and let me know what you get.

 

Sorry Ray, I'm out of here. You just want to be insulting and I won't Participate. However, to answer your question, with the car running and the relay de-energized, terminal G to ground would read 12V because you would read the backfeed from the oil pressure switch. If the relay were to be turned on somehow, the contacts would become open and G to ground would read 0. Terminals D,E, and A are simply contacts controlled by the coil(Terminals C and B)


[Modified by arbee, 4:11 PM 7/12/2003]

 

arbee,
I'll understand if want to exit the thread, but please understand I meant no insult to you or any other Forum member. I get really wired when one of the two or three subjects that I think there is a general misconception about come up. I re-read my post (as I should have before posting) and see what you mean and want to extend my apologies to all that were insulted. Gordon Killebrew really is wary of the accuracy of the early C4 manuals, so the "Not Much" reply to your "For what it's worth" was meant as a slam on the manuals and not you personally. My reference to people "repeating what you have heard or what the manual says or doesn't say" wasn't directed at you either, although after re-reading, it does sound like that. I've been going round and round with others about this before and vented when I shouldn't have. I really need you guys as I don't get the opportunity to see many early C4s here in England. You guys are the wealth of experience I have always drawn on. You have been since I joined the forum. Most of what little I know came from this Forum.

If this apology has meant any difference to you (and I hope it has), I want to say that I agree 100% with your answer to my question. Are you saying that you have measured ALDL terminal G (bottom left) to ground while the engine was running and saw 12V? Anybody else? Because that is exactly the info I'm looking for to finally put this right in my mind.

Again, sorry to all and thanks for everything, past, present and future.

 

Mr Quayle and all,

Good work brother Quayle.

The oil pressure/fuel pump power system on Corvettes is a typical GM design. This design is explained in a GM training manual- GM Fuel Injection Diagnosis #16009.10-6, page 4-28, published by Service Technology Group.

"When the ignition is turned ON, before the starter is enganged, the PCM energizes the fuel pump relay by providing system voltage. If the PCM does not recieve ignition reference pulses within 2 seconds, it will shut off the fuel pump relay. The PCM powers the relay circiuit as long as it receives ignition reference pulses."

"The oil pressure switch provides a backup circuit to the fuel pump relay. If the fuel pump relay fails to supply power to the fuel pump, it will receive power from this circuit. The oil pressure switch contacts close when pressure is over 4 psi. The may result in extended crank times."

Those are direct quotes from page 4-28 and should help in explaining this circuit. It must be one of the most misunderstood circuits on a Corvette.

Now, the "signal" sent for fuel pump power is used by the ECM when setting a Code 54(I think that's the number). The PCM/ECM DOES monitor electrical power to the fuel pump. I do not have a 85-87 manual but pretty sure this is the feed back circuit the computer uses to monitor fuel pump power and setting code 54. It is not used for spark control or fuel pump relay control.

I hope this helps.

Also, I would replace both units when working this area. They can both leak and a new one wouldn't hurt.

dlmeyers 90 coupe zf6 3 speed shocks

 

David,
Amen to that Brother! It's this confusion that I'm trying to sort out. I believe the training manual you reference wholeheartedly. This is what my limited experimentation has shown. I'd like to pick up a few more tested data points, just to say it's proven.
That is the correct code and it does come from the tie point of the Fuel Pump Relay and the Oil Pressure Switch on the '85-'87.

Thanks for your input.

 

OK then guys, why do I continue to get a Code 54. New fuel pump, new fuel pump relay, replaced coil so contacts for distributor wiring are new and actually bothered to reconnect the wiring. What does "low voltage at the fuel pump mean? Also get code 41 (distributor reference signal). Oh yeah, and a code 42 (EST). This is on an 86 model.
Now you are saying that these are not related and that code 54 originates at some connection. Please help me understand this so that I can finally fix this. Ray, we have discussed that it could be a bad ECM and I am still leaning to that conclusion, but want to eliminate any other cause so that I don't just blow another ECM if there is a short or other gremlin.
If you can help me with this, I will tell stories about you one day to my great grandchildren and your names will live on forever. And I would REALLY appreciate it.