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Oil Pressure Woes

Discussion in 'Chevy Trucks' started by Welby, Sep 25, 2002.

  1. Welby

    Welby Junior Member
    Messages: 2

    First the background:

    I have an '86 Chevy Scottsdale that was handed down to me from my grandfather and father. It has a 5.0 liter V8, automatic tranny and 96,000 miles.

    This truck has not been driven for the past two years with the exception of an occasional jaunt around the neighborhood.

    My questions concern the oil pressure gauge and the oil pressure sending unit.

    I noticed the oil pressure gauge reads very low. The needle actually hovers around the mark just below 30. I thought perhaps the oil pressure sending unit, or switch, was faulty, so I decided to replace it. This is where I ran into my first problems.

    Regardless of what parts retailer I went to, they all showed a different switch than what was on the truck (which, by the way, is supposed to be the original one). The switch they showed had the correct thread on one end, but only a single electrical terminal on the other end. The switch on the truck has two electrical terminals on the other end.

    I had to actually look through their parts catalogue to find a switch that looked like the one on the truck. When I found such a switch, I was relieved to read that this switch was available on 91 - 77 GM trucks. My first question is why wasn't this switch listed in the database as a replacement instead of the one they first gave me?

    Also, can anyone tell me what the normal oil pressure reading is for this truck? And, if this is not a good pressure, what other cause can be for the low reading?

    How can I tell if it is a main bearing or oil pump showing a low pressure?
  2. Alan

    Alan PlowSite.com Addict
    Messages: 1,393

    I would bet that the standard instrument panel only had an idiot light for oil pressure. That would use a single terminal switch like you kept finding. The guage cluster, being optional, would have used the two terminal switch and might not have made it to the aftermarket listings. There is a chance that you may have a second switch in there somewhere to run the idiot light, which you probably also have. I know that on the computerized engines there are two oil pressure related switches/sending units.

    As for oil pressure, I've heard that an engine will live on 10 lbs of oil pressure per 1,000 RPM. That translates to 10 at idle, 20 around town and 30ish on the highway.
  3. Kurt B.

    Kurt B. Junior Member
    Messages: 29

    I had several Scottsdale's up to a 1985. In the beginning I often thought the same thing about the oil pressure gauge setup as you but generally came to the understanding (from multiple trucks and talking to others) that the GM factory oil gauge or its setup was poor and not very accurate as they aged. You can have a mechanic check the oil pressure to see if its ok or buy an aftermarket gauge with an under the dash mount setup. This is the easiest and most accurate way to go if you are concerned about monitoring the oil pressure. I did this in one truck but let the others go and never had a problem.
  4. ShafferNY

    ShafferNY Junior Member
    Messages: 16

    First of all, my '78 came factory with a gauge and it only had one wire on it. The sending unit is about 2 1/2" in diameter and next to the fire wall, on the back of the block.

    30psi at idle doesn't sound bad. I just rebuilt my engine(less, new cam bearings) and my oil pressure is 20psi(warm and idling) and 40 psi (warm and highway speeds).

    I wouldn't worry about the oil pressure.

    It sounds like the two wires on the sending unit are for the gauge and one for the check engine light or idiot light.
  5. Welby

    Welby Junior Member
    Messages: 2

    Thank you all for your responses. I appreciate them and your information very much.

    I think, but am not sure, I have found at least a possible source of the confusion. I believe that this truck came from the factory wired for the wrong oil switch. I know that sounds far-fetched, but here’s why I am slowly arriving at this conclusion.

    After going to three different parts supply retailers and having them all give me the wrong switch, I immediately was curious. As I said before, I had to pick the “correct” one out of a catalogue by looking at pictures of different oil switches.

    Last night at home, I was looking at the oil pressure switches in my LMC catalogue (2002 Fall edition, pg. 112). Lo and behold, there at the top of the page was the switch the retailers tried to sell me (part no. 36-0771) as well as the one I eventually purchased (part no. 36-0772). As I read through the list of applications for each one, it became apparent to me that the switch that is on the truck is for a model without the oil pressure gauge.

    So, based on this observation, one would surmise that because my truck DOES have the gauges, it should be wired for the single terminal oil pressure switch and that part number 36-0771 could be the correct switch. However, as stated before, the wiring harness on the truck is undoubtedly designed for the two terminal switch, part number 36-0772. This switch, according to LMC, is applicable to models without gauges only.

    Could my conclusion be correct? And, if so, is this a rare occurrence or a common problem?

    Finally, I don’t believe there is a problem with the oil pressure. I honestly believe the gauge is not showing it correctly. Aside from the carburetor being out of tune, the engine is just fine.

    This truck, by the way, is for sale if anyone is interested.