What's new
CK5
Register an account today to become a member! Once signed in, you'll be able to participate on this site by adding your own topics and posts, as well as connect with other members.

Do I really need the Fuel Pump Oil Pressure switch??

Eric M.

Eric M.

1/2 ton status
Joined
Dec 10, 2001
Posts
2,142
Reaction score
3
Location
Oakland, CA
Here's the deal. I know there has been some discussion here about what the Fuel Pump Oil Pressure switch does. Some say it shuts off the fuel pump when oil pressure drops, the fact is, it does the opposite. It is a back up to the fuel pump relay. As soon as the switch senses 4 psi of oil pressure it closes the fuel pump circuit - in parallel to the fuel pump relay.

Several years ago my battery would randomly die over night. It took a while to trouble shoot, but, I finally found that the Fuel Pump Oil Pressure Switch had a slight short in it. I replaced it.

Now, the ECMB fuse randomly blows and the truck just dies. Again, I trace it to a short circuit in the Fuel Pump Oil Pressure Switch that happens randomly. I disconnect the switch, put a new fuse in and it starts and runs fine, no SES light either.

My question - if this switch is suppose to be a back up to the fuel pump relay and twice now has failed and left me stranded, while the fuel pump relay is still original and working fine, would there be any issues with just leaving it unplugged? Maybe carry a spare fuel pump relay if I really felt I needed a back up?

Am I missing anything this unit does? I've got the shop wiring diagrams and all it seems to do is back up the fuel pump relay circuit.

Thanks,

Eric M.
 
I was having trouble with mine too so I bypassed it. Works fine, and I have a mechanical oil pressure gauge so I can keep an eye on it.
 
bowtieboy4life2000 said:
it is so if you pass out and crash if you loose oil pressure it will kill the fuel pump so you don't have a fire. it's a safty thing
Click to expand...

Better read again. It won't turn the fuel pump off. As long as the key is in start or run, the fuel pump relay will keep the fuel pump running. The switch can go open and the relay will keep sending power to the pump.

Both myself and Gibby are running with this switch disconnected (normaly open).

Eric M.
 
Eric, you are correct in it's function. I however suspect that you have a frayed wire leading to the sender that is grounding out and causing your fuse to blow. Depending on the year of your vehicle some of those senders are dual function and also is the pressure sender for your oil pressure guage. My 89 blazer is like that, a 3 wire switch that controls both oil pressure guage and also fuel pump relay back up. If yous doesn't also control your oil pressure guage you can ditch it altogether.
 
my system is designed without it...

it's just more chit to fail...
 
4X4HIGH said:
Eric, Depending on the year of your vehicle some of those senders are dual function and also is the pressure sender for your oil pressure guage.
Click to expand...

That's what the 3rd wire is for! Every page of wiring diagrams in the Service Manual showed the switch as having 2 wires. I have a mechanical oil pressure gauge installed, so I won't miss the somewhat accurate factory gauge.

I'm assuming it isn't a frayed wire. All I did was disconnect the switch and the short was gone. I'll double check to see if there is a fray close to the connector that I moved when unplugging, but, from what I saw last night (in the dark with a flashlight!) the wire itself looked to be in good condition.

As you can tell, I'm a bit prejudice towards that damn switch. Switch don't like me, I don't like switch. I'll double check for a bad wire.

Thanks,

Eric M.
 
I'd leave it. No need to carry a spare at that point.

Obviously you are having a problem, but yours is so rare, I have to concur that something else HAS to be up. (sure, I'm talking in absolutes which is bound to be proven wrong lol)

Really look over the wiring again. If you disconnect it and the problem disappears, is the short perhaps AFTER the switch? Obviously if the relay works, it would start, fuse blows and it dies if that was the case.

Check the connector wiring, and I might suggest replacing the oil pressure sender/switch with a Delco if you determine the replacement IS the problem. They are not a failure prone item, yours is the first I've ever heard of having this problem.

You could always go with the older style that is solely a switch if you don't intend to use the oil pressure sender portion. Should save you quite a bit of money. My '87/88 Camaro TPI setup used a separate oil pressure sender and a switch, I simplified by going with the later switch/sender, just had to change the connector. So going the other way would be fine. I *may* even have the old switch/connector pigtail laying around if you want it.
 
Dorian,

I did a little more investigating. The switch is the problem, the hot lead is shorted to the housing (ground).

Using a Delco item sounds like a good plan, I've noticed a lot of nonfactory replacement parts failing much quicker on my fleet.

I'm still wondering if I even need this item. I can't imagine GM spending money on a part that really wasn't necessary. If you have a wiring diagram for a TBI CK Chevy, can you take a look at this switch and it's relation to the fuel pump relay and fuel pump? It looks like it's only function is to back up the relay.

Thanks,

EricM.
 
I think the post is in the 73-91 forum where we hashed this out. The wiring diagram for ALL GM's this vintage is very clear, the oil pressure switch is a failsafe to the relay. To prove it, one of the members disconnected the relay with the truck running, and guess what? It kept running. :)

Not only is it a good failsafe, it also tells you when the relay has failed. With no oil pressure switch, you'd have to diagnose a no-start condition. With the oil pressure switch, you know if your start time takes longer, the relay has likely failed.

But no, it's not necessary unless the relay fails.
 
How does the 3-wire sensor work? It has to be able to bypass the fuel pump relay and also turn the relay off. This implies that the switching operations are sequenced. If it disabled your relay anytime oil pressure was low, you would never be able to start the truck (bar using the bypass mode).

Or am I missing something?

Also, when you stall does this switch make re-starting harder? Do you have to switch the key "off" and then back "on" first?
 
I'm not trying to hijack but can someone explain how the darn switch works. Why is there a need for a back up to the fuel pump relay. What purpose does it serve? From what i've got its normally open and closes at 4 psi oil pressure? If we figure out its purpose I think you will find your answer on whether or not you need it!
 
Blue85 said:
How does the 3-wire sensor work? It has to be able to bypass the fuel pump relay and also turn the relay off. This implies that the switching operations are sequenced. If it disabled your relay anytime oil pressure was low, you would never be able to start the truck (bar using the bypass mode).

Or am I missing something?

Also, when you stall does this switch make re-starting harder? Do you have to switch the key "off" and then back "on" first?
Click to expand...


The switch and the relay are in parallel, not series. If the relay has failed and you crank long enough to build oil pressure the switch closes and powers the fuel pump.
 
dyeager535 said:
With the oil pressure switch, you know if your start time takes longer, the relay has likely failed.

But no, it's not necessary unless the relay fails.
Click to expand...

I can see how this would make diagnosing a failed fuel pump relay easier, but if my truck starts in 6 seconds instead of 2, I'm probably not going to assume anything is wrong.
Well, actually now that you've pointed it out, maybe I will!
Still, with all the issues my trucks have had and will have as they get older, this just seems like an odd place for a back up system.

I'm on the fence with this. I'd hate to go install another one and have it be the cause of yet another issue down the road. Or just leave it off and risk having the fuel pump relay go out and leave me stuck. Realisticly, there are about 50 other parts that could fail and leave me in the same situation, especially with 160K on the truck.

One thing to consider. I've owned the truck 8 years, put 50K on it and have broken down twice ... both this switch. I believe you when you say mine is the only one you've heard of failing.

How does that saying go:
Fooled me once, shame on you,
Fooled me twice, shame on me.

I'll think about this a bit longer before deciding to repair or ditch. Maybe the Delco part might last longer.

Thanks,

Eric M.
 
If it's a money issue then i say leave it off, if money isn't an issue then leave it there. I also keep an extra fuel pump relay clipped onto the space next to the one that is being used in case it ever fails (and the oil switch fails at the same time :crazy:) then i just swap the connector over and keep going. The fuel pump relay is noly like $10.00, the oil pressure switch is like $35.00 (may be more since these are my cost on the parts).
 
Here's how the ENTIRE fuel handling works, except the oddball trucks with TBI that had hot fuel handling, and I'm not getting into that one.

Since the switch and relay are in parallel as mentioned, I'm going to explain how one side works, then the other.

Put key in ignition in morning.

Turn to "run".

ECM triggers the fuel pump relay for prime, roughly two seconds. (this is on a timer in the ECM...if the pump has been primed/run recently, you do not have a second prime cycle) The ECM is controlling the RELAY. Relay has 12V constant battery going in, when fuel is commanded, the 12V simply continues to the fuel pump.

Turn key to "start", engine rotates.

ECM sees spark signal from the ignition module as the distributor rotates.

ECM commands injectors to fire, AND triggers the fuel pump relay to run based on spark signal.

Now on the oil pressure switch side, one terminal of the oil pressure switch is simply spliced into the same battery 12V as the relay on the feed side, and the second terminal is spliced into the fuel pump 12V side that comes from the relay.

If the relay fails, the ECM still commands the injectors to fire (because that is based off of the spark signal from the ignition module) as the engine cranks. The ECM has no way of determining if the fuel pump relay is working or not. Since the switch closes at roughly 4PSI, the engine cranks and injectors fire, (with no fuel obviously) until 4PSI oil pressure is hit, the switch closes, and the fuel pump sees 12V.

If the relay works fine, the first set of conditions occurs, and of course, once 4PSI is hit, the oil pump switch becomes redundant, which is why you can disconnect the fuel pump relay connector while the truck is running and nothing changes. Disconnect the relay while the truck is off, and your start time will increase.
 
** Hi-jack on**

I have this problem!

My trucks engine will only fire around 200 Kpa (roughly 30 psi i think). I already replaced the fuel pump relay. So would my problem be the ECM then??

From what I gathered in the first post, the ECM controls the fuel pump relay. And since it is brand new, and my start time did not drop (sometimes up to a solid 15 seconds), the ECM is not telling the fuel pump relay to open up for the priming time of 1-2 seconds correct??

And I would prefer the answers to be sent in a PM. I don't want to clutter this thread with hijack responces. But I saw a bunch of electrical system pros talking fuel pump relays and oil pressure sending units I had to ask my question.

Thanks


** hi-jack off*
 
dyeager535 said:
Here's how the ENTIRE fuel handling works...
Click to expand...

OK, there are some good responses here, but nobody has explained how the oil pressure switch can turn the fuel pump off ( I am a skeptic).

If the engine stalls, the spark signal from the dizzy goes away and the ECM already monitors that.
 
Blue85 said:
OK, there are some good responses here, but nobody has explained how the oil pressure switch can turn the fuel pump off ( I am a skeptic).

If the engine stalls, the spark signal from the dizzy goes away and the ECM already monitors that.
Click to expand...

Oil pressure switch never turns the fuel pump off, only on. Turning the ignition key off is the only way to shut off the fuel pump (or the relay has to fail and the oil pressure needs to drop below 4 PSI). Switch and relay are wired in parallel. I think this would make much more sense if you looked a wiring diagram. Your original assumption that the switch was some sort of safety device which shuts off the fuel pump is incorrect.

Eric M.
 
You must log in or register to reply here.

Latest Posts

Top Bottom