Yesterday I started up my 75 Swinger and the oil light came on and never went off.
I want to obviously get to the bottom of this so I am assuming that I should first pick up a mechanical pressure gauge and screw it in to where the sending unit plugs in. Then start car and see if I have oil pressure.
Question #1: Anyone have a recommendation on what mechanical gauge they have good luck with?
Question #2: I was reading on here about the fact that there were multiple sizes on the sending unit. It seemed like that may have been for older models. For s 1975 are there multiple sizes?
Question #3: Any recommendations for a reliable switch?

Thanks

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Is there oil in the engine?

Is the wire that connects to the sender still attached? If it comes off and touches ground it will keep the light on forever.

Definitely oil in the engine. I checked the connection at the oil pump and it was fine.

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Mopar used 2 sizes. 1/8" pipe is the V-8 size, I think the Slant is 1/4" pipe.

Put in a new sender and see if the light goes off. The switch can sludge up and quit working.

For just checking oil pressure most any parts store will have a gauge and fittings for not a whole lot of money.

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Official Cookie and Mater Tormentor.

I agree...no use in spending a lot on a gauge. AutoZone, $18 or whatever. 10 seconds or so is all you need it for.

The big fear is your oil pump gear gave up....not the end of the world but also not pleasant to repair.

If the wire from the pressure sender to the gauge touches ground at any point along its length the light will come on/stay on.

It is not the case that the V8 senders are one thread size and the \6 senders are the other. See here. A large-thread (3/8"-18 NPTF/Dryseal) sender will fit any Slant-6 oil pump. Use a reliable brand such as Standard (not their "Tru-Tech" cheap line) or Echlin.

It will be a good idea (whether or not it fixes the problem) to service the oil pressure relief valve to make sure it's doing its job, as described in these two threads: thread 1, thread 2.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

Many thanks to everyone who has responded so far. Much appreciated.
Dan, in one of those threads you wrote:
Here's a photo cadged from the oil pump gear failure article on this site. I've drawn green arrows to the cap plug behind which is the pressure relief valve and spring.

But the photo no longer exists. Is it possible you could send that to me? Thanks!

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I've restored the pic to the linked post.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

Fantastic! Thank you

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I installed the new sending unit but that did not solve my problem. So i am going to go ahead and change my oil and filter (wix 51515) and clean the pressure relief valve.
I was also going to go ahead and get a mechanical pressure gauge but I am having trouble finding the right one. So the sending unit is 3/8-18 NPT but I have been to several auto parts stores and all of their gauges have a much smaller connection. Nobody at the counters could help. I have looked online fruitlessly. I assumed you could buy one that just screws right in where the sending unit was.
If anyone could point me to one that would do the trick I would be grateful. I may be searching wrong. Thanks.

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Also, I was surprised that the owners manual says the oil capacity is only 4 quarts. I have always seen 5 quarts on 225’s.

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That's 4 qts if you don't replace the filter, factory manuals and owners manuals that came in the glove box would say oil filter only needs to be changed every other oil change, I don't know anyone who doesn't change the filter with every oil change.
On the oil pressure gauge having too small of thread, usually they come with a pile of adapters in the package of every aftermarket oil pressure gauge that I have ever bought. If not any hardware store that sells "any" plumbing supplies should have an assortment of bushings/adapters, in galvanized, black iron and brass to choose from. You should be able to find an 1/8npt to 3/8 npt adapter there for a couple of bucks. Problem solved.
Regular car parts stores (getting harder to come by) should have a similar assortment, usually with the brake lines/associated hardware.

Mine’s a 1969. Went to Home Depot and found a tee that allowed me to connect gauge and sender. Here’s a pic:

(I just noticed... did my car came with a 3/8 to 1/8 already (prev owner?)? Things you find when looking twice...)

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/6 Dodge Charger 69
...there's nothing like using the "Search" link above to solve most of your problems...

If all of that weight is hanging off the oil pump sender hole, you will likely break it off from stress/fatigue/vibration after some time. Ask me how I know...

Lou

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Home of Slant6-powered fun machines since 1988

It should be noted that when I took the oil filter off, there was essentially no oil in it. That clearly is not right.

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