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Old 05-01-2012, 07:09 AM   #1
Critter
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When the A/C is on, Oil Pressure and Fuel Guage decrease

Hello,

I have a automatic 1985 Corvette Coupe; when I turn the A/C on, my oil pressure drops to '7' or '8', and my fuel display goes down about a quarter of a tank. When I turn the A/C off, everything returns to normal. (This is all on the dash)

I don't have much experience working with cars, so I'm sorry if I'm using the wrong words. (Gotta start somewhere!)

Does anyone have any ideas on where to start, or what I should start checking, or know why this happens?

Thanks!!!
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Old 05-01-2012, 09:56 AM   #2
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that could easily be a symptom of a loose electrical ground , bad battery connection or low battery, defective alternator, loose serpentine belt, under drive crank pulleys can all effect this,if the engine rpms drop a few hundered rpm due to the load on the engine running the compressor both the oil pump and alternator also are spinning slower so that can reduce voltage and oil pressure. Id suggest cleaning the battery terminals and checking grounds, gauge readings depend on current flow, if the current draw rate increases past the alternators ability to supply current your lights dim, gauges read funny etc.
these links should help

http://forum.grumpysperformance.com/...i+meter#p21360

http://forum.grumpysperformance.com/...ternator#p8575

http://forum.grumpysperformance.com/...ternator#p5672

http://forum.grumpysperformance.com/...hp?f=70&t=6882
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Old 05-01-2012, 10:00 AM   #3
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Thank you very much! I'll check them tonight!
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Old 05-15-2012, 06:45 AM   #4
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I just wanted to give an update: I checked the battery and the alternator, everything looks fine voltage wise. This weekend's project is to check the ground wires, which are *hopefully* on the left side of the steering column.
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Old 05-15-2012, 03:18 PM   #5
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Quote:
Originally Posted by Critter View Post
I just wanted to give an update: I checked the battery and the alternator, everything looks fine voltage wise. This weekend's project is to check the ground wires, which are *hopefully* on the left side of the steering column.
Most grounds are on the drivers side of the block above the oil filter. There are also a few on the frame under the battery tray as well as one on the drivers side behind the kick panel and one passenger side behind kick panel. There are a few more but I dont remember offhand exactly where. Those will cover most of them though. Ones under the battery tray are the worst ones as far as corrosion from my experience. Sand the frame to bare metal and use dilectric grease to prevent future problems.
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Old 05-16-2012, 05:53 AM   #6
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Awesome! Thanks for the info!

Dumb question: am I able to get to the ones under the battery tray by removing the battery, or should they be accessed from underneath the car?
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Old 05-16-2012, 03:35 PM   #7
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Awesome! Thanks for the info!

Dumb question: am I able to get to the ones under the battery tray by removing the battery, or should they be accessed from underneath the car?
Easiest way is to remove the driver side fender gill, remove battery, remove battery tray ( four bolts, 2 in the tray and two down low into the frame ) then you will have all the access you could need. Got the grounds on the block this way on mine this way while it was apart. A bit tricky to acess them this way but at the time I was too lazy to put it up on jack stands. Plus I didnt have to lay on my back to do it. Let us know how it goes and if it fixed your issues
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Old 05-17-2012, 07:26 AM   #8
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Thank you sir! I'm planning on giving it a whirl on Saturday, so I'll post back on how it went. I also found what looks like a good tutorial on how to remove the battery at:

http://www.corvettemagazine.com/2001/october/battery/bat1.asp

(Wanted to share it in case any one else was looking)
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Old 05-17-2012, 03:29 PM   #9
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No need to call me sir, I answer just as well to many other names - dumbass, a-hole, you get the idea. Let us know how it goes. BTW you are very welcome.
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Old 06-05-2012, 01:46 PM   #10
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Update: Both good and bad news!

Good News: Was able to take off the side panel, battery, battery pan, and clean the grounds! They were quite rusty and I was able to clean 'em, cover them with dialectic grease, and re-assemble. (Getting the car side panel aligned properly took a little adjustment, but nothing too difficult). So, that all looks good, but.....

The Bad: Some guy hit the front of my car a few weeks ago; a little scuffing, nothing to bad, but it was in the body shop for the past 2 weeks. so riding home from the body shop, it was running rough. (Shaking as I applied gas, and was surging at idle). I had changed the spark plugs a few weeks before, and had a cap/rotor and new wires to add during my next project. So, I got home and changed the wires, cap and rotor, and the car didn't get any better. I did some research, and next two steps are to clean the MAF Sensor and change the Fuel Filter. (Which should happen this weekend)

So, all in all, I didn't want to take it for too long of a cruise to test the cleaned grounds on the A/C, so am waiting untill I get it running properly.
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Old 06-05-2012, 03:19 PM   #11
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Was it running ok prior to taking it to the body shop? Odd that it would do that all of the sudden. Keep us updated

BTW if you missed getting the main groound strap ( braided bare wire ) back on it will do strange things. Thant is the main ground between the frame and engine block where there are 7 other grounds. Should go from frame to just above oil filter on the block. Some are also on the upper bellhousing bolt. Might also check that you got the small grond from batt neg to frame. Main batt nec goes to the engine block IIRC.
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Old 06-05-2012, 03:21 PM   #12
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I have the same issue, but in my '89 chevy truck. Mine didn't start until I put in my dakota digital dash. Has me wondering if there is a bad ground within the dash cluster. Not that it'll totally relate, but sounds way too similar. I won't be checking my dash out for a few weeks, but I'll let you know what i find if you don't find your issue before i get to mine. Good luck.
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Old 07-03-2012, 02:35 PM   #13
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Update: Finally fixed the previous problem, the studder-ing as I applied gas. I changed the fuel filter, and it fixed it. About a week later, it had the same symptoms, so I ended up taking out the fuel pump and the strainer was pretty shredded. I replaced that, and the filter again, and she's running like a champ!

But, the A/C problem is still there, but I've also noticed something else that I want to throw out there in case it sheds some extra light on the topic. I looks like the fuel guage and oil readings on the dash also drop when it's extremely hot out, and I've been driving for awhile.

I've picked up the FSM, so this weekend's project is to find EVERY ground I can. (and I figured I might as well do an oil change and replace the oil filter as I get to the ones above it)

I'll keep you posted!
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Old 12-10-2012, 12:35 PM   #14
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Wanted to post an update! I put the A/C issue on hold, as some other expenses came up. (And, the weather in NC this time a year is quite nice, so no A/C required). But, my horns stopped working, and someone mentioned a first step was to replace the fuse.

I pulled the fuse, it didn't look bad, but I figured for $10 might as well replace all the fuses since the car had been sitting for a couple of years before I bought it.

And, lo and behold, it fixed the issue with the A/C causing my fuel and oil pressure drop! I checked the fuses when the issue first came up, but I figured as long as the little metal piece in the fuse wasn't broken, they were fine.

Thanks everyone for all the help and advice!!!
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