Cascade Crew

Al Hasse · Posted: Sun Apr 15, 2012 4:43 pm Post subject: Auto Zone OPSU blues

Aaron and I tried to remove my oil pressure sending unit (for interference) last time we worked on the car together but all we were able to do with what we had was spin the housing. Needless to say, it finally started leaking, not bad but a leak none the less. I went to NAPA to get new oil, filter and since I was there, a new sender. Sender wasn't locally available from NAPA, so I bought one from Auto Zone down the street.

I went on up to Bangor Sub Base to use the hobby shop and change the oil and install the new sender. Everything went well until I turned the key to check the gauge for pressure, it pegged high as soon as it had power. I figure bad sender, or they sold me a switch instead. I left it in as I already discarded the leaky one and it plugged a hole. I stopped back at Auto Zone on the way home to voice my complaint and ordered another under warranty (to be in tomorrow).

After I get home, I hear my fuel pump still running after I shut the motor off. Unplugged the relay, pump still runs. Crawl under the car and play with the connector on the OPSU and the fuel pump shuts off but the gauge remains pegged high. I think I have a short in this sender....

Xophertony · Posted: Sun Apr 15, 2012 4:59 pm Post subject:

You may be right, and here is why...
82-89 (Pontiac, not sure on the years for the Chevrolet) have a sensor for the gauge up on the china wall, and a switch near the filter on the block. on your 89+ the china wall sensor is gone, and the switch on the block became a three wire switch and sensor.

Many folks do not know how this system works, thinking that the switch SHUTS OFF fuel if there is a low pressure situation. not the case. The switch portion TURNS ON the pump if it senses oil pressure. This will even occur if the car is not on. The point of this was to act as a backup for a faulty fuel pump relay. the relay fails, you crank the engine and build pressure; thinking "why won't the car start" and then it finally does thanks to the oil pressure switch.

If the mechanism that senses the pressure is sensing full pressure all the time it would send the "OH MY GAWD, TURN THE PUMP ON!!!" signal all the time. I think that is what is happening to you, it would explain the pegged pressure and the fuel pump. could be a short, could be a bad sensor mechanism. either way, replacing it is the way to go.

You may have known all this already, but i like the sound my keyboard makes, so i typed it out anyway. Smile

Al Hasse · Posted: Sun Apr 15, 2012 5:11 pm Post subject:

aaron_sK · Posted: Sun Apr 15, 2012 6:34 pm Post subject:

Yeah, the oil sender wiring bypasses the relay.

This is why you never buy anything at Autozone. That place exists for no other reason than to rent me tools that I break and then return. Laughing

iansane · Posted: Sun Apr 15, 2012 9:40 pm Post subject:

I enjoy hearing stories about vatozone. It makes me realize O'Reilly doesn't hire all the morons...

flea · Posted: Mon Apr 16, 2012 5:12 am Post subject:

Oh no Ian, there are enough morons for everyone. Smile

91RSVert · Posted: Mon Apr 16, 2012 5:05 pm Post subject:

I will buy felpro gaskets, and cleaning supplys at autozone, thats about it. Laughing

Al Hasse · Posted: Mon Apr 16, 2012 6:35 pm Post subject:

I picked up the second one today. As soon as I connected it (not installed), the fuel pump kicked on. Since the sender I removed looked different than the new ones, I'm inclined to believe that a previous owner did something with my harness. I did a little probing on the connector - The middle wire (tan) is the gauge wire. The two outside wires (orange? and tan/white) are 12V and 0V through the switch in the sender. 12V is battery voltage which seems odd to me, shouldn't it be switched on with ignition?

I think it's time for a one-wire sender and deleting the bypass. For this weekend, I'm pulling the 12V wire out and taping it off so I have a gauge and a sender in the hole. Then I'll figure it out when I have more time off...

EDIT: I found a schematic that shows the fuel pump relay bypass. My harness is as it should be. Also looked on the NAPA site and looked up the sender and pulled up its info, the contacts in its switch aren't supposed to close until pressure reaches 1.5 - 7.5 psi. Both of the Auto Zone parts I tried turned the pump on without any pressure, they also list the same part for V6 and V8. I have another on order through NAPA, who list separate parts for V6 and V8.

Xophertony · Posted: Tue Apr 17, 2012 11:51 am Post subject:

Vatozone.. Rolling Eyes

RSFreak · Posted: Tue Apr 17, 2012 7:47 pm Post subject:

Al Hasse · Posted: Thu Apr 19, 2012 6:17 pm Post subject:

Success!!! A new sender came in today at NAPA, first thing I did was check the two outside pins for continuity (or lack thereof), checked good. Installed, fired up the car and the gauge reads higher than it ever has, though not pegged. And the fuel pump stays off when the engine is off, unlike the previous two senders. Made the drive to Auto Zone and returned the second sender and got my warranty refund.

Red92Bird · Posted: Thu Apr 26, 2012 12:40 am Post subject:

glad to see you got everything figured out! it sucks when you think you've done something wrong and it turns out that the new part you just bought is faulty.
I've had quite a few problems with autozone myself. they didnt have a listing for the correct serpinetine belt for my jeep so i had to measure it out, and the clutch master and brake master cylinder i bought from them both started leaking within a year. the origional equipment lasted for 17 yrs! thank gawd for the lifetime warrantee, but im tired of re-fixing stuff i shouldnt have to be worrying about.