jagevileye21

iTrader: (3)

From my understanding the oil pressure switch keeps the fuel pump energized in case of fuel pump relay failure. As long as the oil pressure stays above a certain psi. Question: Would a bad oil pressure switch keep the fuel pump relay from energizing the pump?

Thanks,
Jay

BigBadLou

iTrader: (3)

Answer: Nope.
Explanation: The two circuits (relay and oil pressure switch) are wired in parallel so either of them can provide power to the fuel pump. They cannot "cut" power to each other.

Check the connector C313 that's above the rear differential. It might be a little oxidized inside, not allowing good contact. It's one of the most common causes for the fuel pump circuit failure.

You can also check out this short article on the FP circuit troubleshooting: http://www.austinthirdgen.org/index.php?pid=35

Hope this helps.
Lou

loneroad

iTrader: (3)

Ive had 2 oil press switches burn out. it doesnt keep the relay from coming on. but it will not let the fuel pump run after the initial start up.

turn the ign on. yo0u should hear the relay tick and the pump prime for abot a sec or 2. if you start it from there but it doesnt run long, then more than likely the o/p switch is bad. You can jump the pigtail with a wire or a paperclip to make sure, but not sure of which 2 wires at the moment

jagevileye21

iTrader: (3)

Check this thread for the whole story and initial problem.


https://www.thirdgen.org/forums/elec...-no-power.html

loneroad

iTrader: (3)

wow been there except for the whole headlight thing.. lou can prob help you with that.


I spent a lot of time looking for prob. I did the whole relay switch and the fuel pump change....... if you pull the pigtail off the op switch, it should be 3 wires. try jumping the 2 on the bottom and see if the pump comes on. (This is from memory, i wont be able to look at mine till the AM.)but if i remember correctly there is a diagram in the back of the haynes manual that shows that circuit.Like I said from memory.... it is either a tan or purple wire that you need to jump, and the other is power.

BigBadLou

iTrader: (3)

I replied in your other thread, you should have it solved in no time with your skills.

Lou

BigBadLou

iTrader: (3)

Just to clear this up a little:
if the ECM is controlling the fuel pump relay correctly, it primes it for 2 seconds after the ignition circuit is activated and then it turns the pump on again once it senses pulses from the distributor.

Also, the oil pressure switch powers the pump when there is enough oil pressure built up in the engine. It is only a BACKUP system.

If the ECM fails to operate the fuel pump, the pump will still run (off of the oil pressure switch).
If the oil pressure switch fails, the pump will still run (controlled by the ECM).
These two systems work in parallel, they are redundant, so that we don't get stranded in our cars 50 miles from the nearest town just because a $5 relay went bad. GM did something right for once.

Hope this helps.
Lou

loneroad

iTrader: (3)

Lou, your absolutly right. the switches didnt burn up until after the ecm was disc'd. Thanks for pointing that out. hey jag sorry for the confusion.

ScottyB724

This is my Oil Pressure Switch / Oil Pressure Sending Unit, correct ?

BigBadLou

iTrader: (3)

Yup, that's the one!

Lou

camaronewbie

A bad OPSU can do all kinds of weird things. Mine went bad, causing the fuel pump to run continuously even with the key in my pocket! I found out the OPSU gets direct current from the battery, regardless of the key position - but it took me a few days of dead batteries to track it down.

I've since switched to carb, so no ECM. I don't get priming of pump anymore, and my starter has to run an extra second to crank the car (waiting for oil pressure). Also, if I don't start the car every week or two, it's a bear to get enough oil pressure built back up there to start it (I usually jump the wires at the relay to make the pump run to get it cranked, and then disable a minute later). LOL - with the $4 gas prices, I'm going to rig a switch inside to jump this for me, 'cause I may not be able to afford to crank her up every week anymore!

1BADKIDD

anyone know what psi the opsu will shut down the fuel pump? I heard it was like 10psi.

ScottyB724

The same thing is happening to me. I'll be putting in a new one tomorrow.
Any special tips on how to remove it / installing a new one ?

Thanks.

ScottyB724

Wow I really can't get this sucker out. Nothing fits! How are you supposed to take this thing out?

BigBadLou

iTrader: (3)

I used a short crescent wrench, it helped a little.

Lou

ScottyB724

What size did you use? its like somewhere between a 25mm and 30mm right?

BigBadLou

iTrader: (3)

I used an adjustable crescent wrench. No idea on the nut size. I even think it's an octagon, not hexagon but I'm not 100% sure.

Lou

The_Phoenix

iTrader: (3)

Did replacing the OPS fix the problem with the fuel pump staying on??

BigBadLou

iTrader: (3)

It better!
It's usually either the FPOP switch or the FP relay that stick closed and keep feeding power to the fuel pump.
Not an uncommon problem.

Lou

The_Phoenix

iTrader: (3)

Today I checked for any corrosion on the c313 connector by the fuel tank and nothing, unplugged the relay but it still stayed on, took the fuse out of the f/p weatherpack connector by the battery and that shut if off finally (with the key out). I'm missing the orange wire and connector that comes from the black plastic cylinder (by the battery) and plugs into the female end of the connecor on the f/p weather pack connector; can this be the cause of it staying on? I'll try to wire it somehow so both ends connect and see what that does. The fpop was replaced when my engine was rebuilt a while back, can it still be a bad unit? My new harness was installed in my '89 GTA today and I finally got it running after 5 months!. Sorry for posting all over your thread jagvileye you can get me back on one of my threads lol.

phoenix

ScottyB724

For me, it actually didn't. I replaced the OPS, cleaned and greased the c313 connector with di-electric grease, but it still did not fix the problem. I ended up just wiring up a switch for it. Works for me.

The_Phoenix

iTrader: (3)

Thats wierd . About the OPS switch, I've heard of members of this forum who have purchased for example a brand new ignition control module and it turned out to be bad, they exchanged it for another one and that one turns out bad as well. This might be the case with my OPS but that'll be the last thing on my list to replace if anything. I'll eventually get to the bottom of this with the help of a friend of mine and when I do, I'll post the problem.

Later guys,

Phoenix

BigBadLou

iTrader: (3)

The simplest check is: if you disconnect the FPOP switch, the fuel pump should not run (with key off). If it does, there is a problem in the wiring, not the switch.

Hope this helps.
Lou

The_Phoenix

iTrader: (3)

I'll definately try that today. Thanks for the valuable info Lou. I'll post my results with the test later today. Who knows, it just might be the FPOP thats bad .

Thanks again,
Phoenix