Well, once again I am replacing my oil pressure switch...this time on my '88.
I have owned one or more Fiero's continuously since 1987 when I ordered a GT. During the years since I have replaced at least 10 oil pressure switches, and NEVER had a Fuel pump relay failure.
Now, as I understand it, the ONLY purpose for the switch in the oil pressure switch is to provide backup to the fuel pump relay, e.g., when the oil pressure at the switch equals or exceed 4 psi, the switch closes and provides a parallel path around the fuel pump relay providing fuel pump power even if the fuel pump relay fails and remains open. Now I could understand it if the fuel pump relay was in SERIES with the oil pressure switch since that configuration would shut off the motor if the oil pressure fell below 4 psi....but it does not.
So, what is the point? After all, if the fuel pump relay fails (rarer in my experience than the oil pressure switch failing) one only need swap relays with the A/C relay right next to it....or wrap a wire around a couple of terminals on the relay and short it out. So what is the point of the expense and failure rate of the oil pressure switch???
I bring this up because it would like to replace my oil pressure gauge with an after market unit which requires its own oil pressure sender. If I do this I either lose the oil pressure switch or must put a 'T' in the line to install both! If I did the latter I would still have the same failure rate of the oil pressure switch and ADD mechanical complexity and what ever the failure rate is for the after market sender.
So, my question (finally), is simply what is the down side of deleting the oil pressure switch?
Really looking forward to your opinions and discussion,
It also shuts off the fuel pump if oil pressure drops off, as in an accident, to prevent fuel from spilling if the key switch stays on. It was GM's way of avoiding the expense of adding an inertia switch to cut power to the pump.
------------------ 1986 SE Aero coupe.
3.4 DOHC swap is complete and running, now just have to finish the rest of the car...
Well, unless I am mistaken, the oil pressure switch is in parallel with the fuel pump relay, so even if the oil pressure switch shuts off power to the fuel pump, it does not affect power through the fuel pump relay.
What you describes makes perfect sense if it were in series with the relay.
It also shuts off the fuel pump if oil pressure drops off, as in an accident, to prevent fuel from spilling if the key switch stays on. It was GM's way of avoiding the expense of adding an inertia switch to cut power to the pump.
Elimination of the circuit will put the full amp draw of the pump on the relay, shortening contact life. If your relay goes bad on the road, the car dies. Not good in traffic. Even with a bad relay, the O/P circuit will allow the car to start after building oil pressure by engaging the starter.
You seem to go through a lot of OP senders. I would look for the cause of that, and correct it.
It also shuts off the fuel pump if oil pressure drops off, as in an accident, to prevent fuel from spilling if the key switch stays on. It was GM's way of avoiding the expense of adding an inertia switch to cut power to the pump.
FALSE
You can test this by unplugging the oil pressure switch while the engine is running. It will not kill the engine (assuming the fuel pump relay works). Also, look at the electrical schematics for the fuel pump. There are 2 parallel paths of power to the pump. Deleting one of those two paths will not break the circuit.
Edit to add: Pulling the electrical plug from the oil pressure sender while the engine is running can actually be used as a quick easy test to see if the fuel pump relay is working. If it's not, the engine will sputter and die.
As for why the fuel pump relay has a backup switch... not sure. Maybe GM was afraid of being sued by people when their cars konk out in the middle of traffic. Maybe the engineers didn't trust a relay to be the sole power source for the pump. Your guess is as good as mine.
That said, if you're tired of replacing oil pressure senders, and trust your fuel pump relay enough to not have a backup, then I guess just leave it be. I'm surprised you're having that many oil pressure switch failures on an '88 Fiero. They have the "good" oil pressure senders. Are you buying the cheapest ones, or are you getting name brand ones?
[This message has been edited by Blacktree (edited 07-07-2015).]
Olejoedad and Blacktree....you seem to have a small conflict in what you say about the OP switch.
My understanding was that the ECM turns on the pump by engaging the relay while the engine is being started (i.e. key turned on), yet while the oil pressure hasn't built up sufficiently. Once the oil pressure is sufficient, the ECM disengages the relay while the oil pressure switch keeps the fuel pump running. Should the engine stop, the oil pressure will drop and the fuel pump will automatically shut down. I don't believe it has anything to having a parallel path to allow for more current to the fuel pump. That would just be bad engineering design to underrate either or both of the current paths to the fuel pump.
Olejoedad and Blacktree....you seem to have a small conflict in what you say about the OP switch.
My understanding was that the ECM turns on the pump by engaging the relay while the engine is being started (i.e. key turned on), yet while the oil pressure hasn't built up sufficiently. Once the oil pressure is sufficient, the ECM disengages the relay while the oil pressure switch keeps the fuel pump running. Should the engine stop, the oil pressure will drop and the fuel pump will automatically shut down. I don't believe it has anything to having a parallel path to allow for more current to the fuel pump. That would just be bad engineering design to underrate either or both of the current paths to the fuel pump.
Please correct me if you disagree...
I don't know anything about this, but that sure seems right to me. It's how electric fuel pumps have been installed in old cars for years - use the oil pressure switch to hold the relay down. No pressure, no pump, no fire. A fuel pump takes virtually no power - maybe 6 or 8 amps. A "big" Walbro 255lph pump takes less than 12a. Small automotive relays are 20a - 12a isn't even a dent.
I spent a lot of time researching these types of circuits when putting an electric pump in my Falcon - I like the safety aspect of the oil pressure switch but there isn't an elegant way to handle the "handoff" the way a fuel injection ECM handles the cranking and an oil pressure switch handles the running. I ran into a lot of confusion over the way these circuits work, and in fact some documentation that suggests the way GM implemented it is prone to failure. Perhaps that is why in the Fiero community the consensus is they work differently than they should?
Go to *any* other GM forum, whether truck or car, and ask how the oil pressure switch interfaces with the fuel pump. You will get this answer:
quote
Here’s how it works: When you turn the key on to start the engine, the ECM activates the fuel pump relay (providing power to the fuel pump) for about 3 seconds and then releases the relay. This is to prime the system. Once the engine begins turning over and building oil pressure the oil pressure switch activates and takes over providing power to the fuel pump.
It's only in Fieroland that the concept of a "shared load" exists. From a technical or safety standpoint, that makes zero sense.
since Blacktree's test (that is, unplugging the oil pressure switch and the car continues to run) is valid, then the fuel pump relay CANNOT be shutting off after 3 seconds....or any other amount of programmed time unless another source of fuel pump power exists....a third power source.
The schematics are clear, the oil pressure switch is not a particularly useful redundancy....
Olejoedad and Blacktree....you seem to have a small conflict in what you say about the OP switch.
My understanding was that the ECM turns on the pump by engaging the relay while the engine is being started (i.e. key turned on), yet while the oil pressure hasn't built up sufficiently. Once the oil pressure is sufficient, the ECM disengages the relay while the oil pressure switch keeps the fuel pump running. Should the engine stop, the oil pressure will drop and the fuel pump will automatically shut down. I don't believe it has anything to having a parallel path to allow for more current to the fuel pump. That would just be bad engineering design to underrate either or both of the current paths to the fuel pump.
Please correct me if you disagree...
The ECM has no input for oil pressure. However, once the ECM begins to receive ignition pulses it re-energizes the fuel pump relay. Pull the OP sender plug when the engine is running. That will answer your question.
thesameguy said: Go to *any* other GM forum, whether truck or car, and ask how the oil pressure switch interfaces with the fuel pump. You will get this answer:
"Here’s how it works: When you turn the key on to start the engine, the ECM activates the fuel pump relay (providing power to the fuel pump) for about 3 seconds and then releases the relay. This is to prime the system. Once the engine begins turning over and building oil pressure the oil pressure switch activates and takes over providing power to the fuel pump."
As I understand it, there are 2 conditions where the ECM will activate the fuel pump relay:
1) The first ~3 seconds after the ignition switch is turned on. 2) Whenever the ECM is receiving reference pulses from the ICM (i.e. when the engine is running).
The engine will run just fine with the oil pressure switch disconnected, because the ECM keeps the relay on while the engine is running. It does NOT hand over fuel pump control to the oil pressure switch. Not only does the Fiero ECM work this way, but the GM '7730 ECM that I swapped in (from a Chevy Beretta) does as well. So it isn't just "Fieroland" where it works this way.
But don't take my word for it. Go check it out for yourself.
[This message has been edited by Blacktree (edited 07-07-2015).]
I'm not refuting that it works as you describe (because obviously it does), only that the concept of the circuit existing as it does is to share the fuel pump load is in error. I've never seen that suggested anywhere else than here. Splitting the fuel pump load in half to save a relay of 4a just doesn't make technical sense. 4a is nothing. That would be like having a two relays for the parking lights or four for the headlights.
There is some other component involved here, some design intention that hasn't been sussed out. (And I don't think it's redundancy, either, because the fuel pump relay isn't under any more stress than the headlight relay and I'd *much* rather lose power than headlights. Cars stall from all sorts of reasons that aren't fuel related. Say, ignition modules, and we don't have two of those for safety. But lights failing at the wrong moment? Easily catastrophic.)
What I read somewhere a long time ago was that the circuit was conceived to operate as I suggested, and for some reason that was changed. I just can't find that info. It's probably moot anyway.
[This message has been edited by thesameguy (edited 07-07-2015).]
Electricity takes the path of least resistance. Perhaps some of the current goes thru the OPS, perhaps not. This - "Elimination of the circuit will put the full amp draw of the pump on the relay, shortening contact life." - was conjecture on my part. Sometime when I'm really bored, I may take the time to check resistance on each leg of the circuit.
Thank you all. This has been one of the most polite yet informative disagreement postings (dare I call it 'arguments'?) I can recall on this forum. You all have been informative and I thank you for it.
Originally posted by olejoedad: Pull the OP sender plug when the engine is running. That will answer your question.
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Originally posted by Blacktree: But don't take my word for it. Go check it out for yourself.
I appreciate you both being eager for me to run out and check it myself, but firstly, I have a 4.9 and secondly the car is completely torn apart for my Aventador build. Testing will have to wait for about 4 years....
In some GM vehicles they do drop the secondary oil pump switch and just depend on the relay.
My thoughts - Redundancy. Seeing that it isn't totally uncommon for a Fiero era relay to fail it makes sense. Later totally sealed relays are much more robust in their construction but the relays in Fieros seem to me to be prone to failure. It is also not uncommon to see the driver transistor of the ECM to fail. 85 when the 7170 ECM came out GM didn't have much data on reliability of ECMs and relays so they played it safe.
Too bad they didn't design a redundant distributor pick up and ICM.
I have also had several 88 senders/switches to fail, including a brand new one. The "new" one actually turned on the pump after I had shut off the engine and turned to go upstairs. My solution was to go in behind the fuse block, and move the fuel pump circuit to the "IGN" circuit instead of the "BAT" circuit. A quick splice/solder/heatshrink, and I haven't had a problem in 2-3 years. Some people will claim that this causes a voltage drop to the pump, but I haven't experienced that. Works fine. I also have a 4.9, btw...
------------------ Raydar 88 Formula IMSA Fastback. 4.9, NVG T550 Praise the Lowered!
Just out of curiosity - what kind of symptom was presented to make you check if the oil pressure switch was bad if the relay is working (since it is a backup)?
If Relay is Dead then the engine starts and runs on OP switch. The engine does take longer time to start because needs to build Oil Pressure to close OP Switch. In Fact, when you have long cranking time then GM says check FP relay in many models, Not just Fiero.
EITHER contacts can have problems causing higher resistance. High Contact Resistance can cause a local low voltage to the pump and that can fry the pump itself. If the relay is a bit burned and OP switch is good then power goes to low resistance contact and saving the pump. Unlike most other electric motors... Replace an in tank FP is a very nasty job and can't be done on the side of the road. You can drive w/o A/C, Heater blower, and most cases even Radiator fan, working. See my Cave, Electric Motors
Safety? No Oil Pressure feature to save the engine? Some GM cars and light trucks might use OP switch as a safety item but I've never seen this on any OBD1 setups.
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Originally posted by Raydar: I have also had several 88 senders/switches to fail, including a brand new one. The "new" one actually turned on the pump after I had shut off the engine and turned to go upstairs.
88 fails electrically is better then old style that leaks oil when engine runs. Every bad old style I've seen most failed diaphragm and leak oil. They don't like wet area mounting, like Fiero V6 w/ A/C, and have a nasty habit losing contact for water damage.
Repeatedly frying contacts likely means you have others problems. Example: Bad/iffy FP grounds can cause low volts and FP draws more Amps. That can burn contacts and/or fry the pump. See link above.
------------------ Dr. Ian Malcolm: Yeah, but your scientists were so preoccupied with whether or not they could, they didn't stop to think if they should. (Jurassic Park)
Just out of curiosity - what kind of symptom was presented to make you check if the oil pressure switch was bad if the relay is working (since it is a backup)?
The latest failure was high but not pegged pressure at all times. The physical failure was corrosion within the socket. I replaced both socket and sender since sender had completely lost a pin into the socket and only the corrosion formed the electrical path. I packed the socket with dielectric grease this time!
GM only used the oil pressure switch to power the fuel pump circuit as a back-up in case the fuel pump relay failed. Fuel pump relay failures used to be fairly common back in the 80's when GM wasn't yet using weather sealed relays for the fuel pump circuit (which is the case for all Fieros). But with the advent and widespread use of weather sealed relays, failures became much less common so GM eventually dropped running the fuel pump power circuit thru the oil pressure switch altogether (which happened in the 90's) because it was unnecessary.
Concerning the debate as to the oil pressure switch being a necessary supplement to the fuel pump relay power circuit because of current draw of the fuel pump, I don't agree that the oil pressure switch is necessary. Look at the stock 88 Fiero switch - the terminals for the fuel pump wiring are so small in that sending unit they can't carry near as much current as the fuel pump relay can. And the fuel pump relay is going to take the entire electrical load of the fuel pump at key-on anyway - since the ECM powers up the fuel pump for 2 seconds at initial key on before the engine is started and oil pressure has to build before it can close the contacts of an oil pressure switch for the fuel pump circuit.
In every engine swap I do, I remove the fuel pump wiring from the oil pressure switch circuit completely and replace the OEM Fiero fuel pump relay with a newer GM weather-sealed relay. Never had any fuel pump relays since - not even in those swaps utilizing high performance fuel pumps that draw twice the current as the OEM Fiero pump.
If you want to keep the oil pressure switch for the fuel pump circuit so you have a back-up in case your fuel pump relay fails, you can certainly do that. However, I wouldn't recommend it if you are using a performance fuel pump that draws a lot more current than an OEM pump because the higher current draw will likely overload the contacts and terminals inside the oil pressure switch (none of which GM ever designed to operate a high current draw performance fuel pump).
-ryan
------------------ More is more. Less is not enough. Custom GM OBD1 & OBD2 Tuning | Engine Conversions & more | www.gmtuners.com
[This message has been edited by Darth Fiero (edited 07-09-2015).]
I agree with the above in that this is how the circuit is currently wired, but I wonder why it is not used as a Safety device to turn OFF the fuel pump if the Oil Pressure was too low. Cheap insurance against an oil pump failure and safety percaution for the moron that never checks their oil and drives around with only 2 quarts in the car.
Originally posted by edfiero: I agree with the above in that this is how the circuit is currently wired, but I wonder why it is not used as a Safety device to turn OFF the fuel pump if the Oil Pressure was too low. Cheap insurance against an oil pump failure and safety percaution for the moron that never checks their oil and drives around with only 2 quarts in the car.
Likely because when engine kill by switch for no oil pressure will happen in the middle of traffic at rush hour.
Engine w/o oil pressure (Oil light is on or gauge is low or 0.) can run long enough to pull over and safely stop, often w/ no or very little damage. Hell, People run Fiero V6 w/ very low oil pressure and think is normal and run for months to years. (V6 should be 30-45 PSIG at all engine temp and speeds.)
In vehicles, OP switch killing fuel sounds good on paper but is a bad idea in the real world. Many Static engines, like power generators, does use Oil Pressure or Oil Level switches to kill the engine because no-one watches the oil.
In every engine swap I do, I remove the fuel pump wiring from the oil pressure switch circuit completely and replace the OEM Fiero fuel pump relay with a newer GM weather-sealed relay. Never had any fuel pump relays since - not even in those swaps utilizing high performance fuel pumps that draw twice the current as the OEM Fiero pump. -ryan
Originally posted by edfiero: I wonder why it is not used as a Safety device to turn OFF the fuel pump if the Oil Pressure was too low. Cheap insurance against an oil pump failure and safety percaution for the moron that never checks their oil and drives around with only 2 quarts in the car.
I think the oil pressure switch for the fuel pump closes with 4-7 psi of oil pressure. It is very possible someone could run their engine low on oil but still have enough oil in it to produce 4-7 psi of oil pressure (enough to keep the fuel pump running via the oil pressure switch). But we all know 4-7psi isn't enough oil pressure for an engine turning high RPM, so it will still probably suffer damage with that low of oil pressure.
Based on this, what good would a fuel pump cut-off / oil pressure switch do for most people?
