Here are a couple of pictures from a recent thread.

First one shows the location of the oil pressure sending unit with the blue wire attached and also the oil pressure switch with the red wire attached

Second one shows the location of the high water temperature switch with the red wires.

The oil sending unit has nothing to do with your check engine light, it feeds the gauge on the dash, so no need to mess with that as long as you have good oil pressure indication.

If you unhook the wire from the oil switch and the light goes out, then you have a bad switch. If the light stays on, hook it back up and unhook the wire from the water temperature switch and the light should go out.

It'll be one switch or the other causing the problem.

Your wires may be a different color like green or some other color. Those in the pictures look like they've been redone at some point in time with the red wires and crimp connectors 

A little bit of thread sealer isn't a problem to prevent leaks. It's not some kind of super electrical insulator that will keep the pipe threads from making metal to metal contact with the female threads in the engine/manifold.

Or in other words, it won't affect the operation of the switches 

Lots of replacement sending units/switches come with a little sealer already applied.

You also have a water temperature sending unit over on the right side of the engine, directly behind the thermostat housing near the coil. It'll have a tan wire attached to it. No need to mess with that either, it feeds the gauge on the dash and has nothing to do with the red alarm light.

PS....you don't need the globs of RTV or whatever it is on those connections either

Not needed but... 40 bucks extra to do it up now and not do it again for a while.  I was surprised at the amount of oil sludge in the T fitting and how a change of a "working" sender altered the observed oil pressure. Go figure. 

Keno's write up and pics are PERFECT - BTW!

Maybe, maybe not  

On a gt40 which I'm more familiar with in those model years, the hi temperature switch is behind the thermostat housing 

That's what Air206 said in his post too.

So what you should do is, look for the green wires, those go to the dash light 

Not to be picky  but with Kens steps it is not mentioned that the engine must be running when the wires are removed. 

Ask me how I know!

Yeah........what Marty said 

And if somebody happens to have a gt40 and is looking for sensors, switches, sending units for oil pressure and water temperature, here's a link to a thread by Marty showing locations

http://www.correctcraftfan.com/forum/forum_posts.asp?TID=49744&title=98-gt40-sensors-and-swithces" rel="nofollow - link

Totally agree - I find most of my engine warning light troubles go away when the engine is off!     Steve

There's no need to disconnect the battery

Quit calling it a sensor,     It's an on-off switch 

For hi temperature it closes at about 200 to 210 degrees and opens when temperature drops to 190 to 200 or so and for oil pressure it closes when pressure drops about 5 psi and opens when pressure gets to 7 or so.

They're normally open when the engine is running and either one closing completes the circuit for the check engine light to come on.

You can spray some oil if you want but shouldn't really have to. You can use an open end wrench or the box end of the wrench or if you have some deep sockets they'll work good too. No special tools needed.

http://https://nautiqueparts.com/oil-pressure-switch-r020015a-pcm-pro-tec-1989-1994-all-gt40s-and-gm-to-2006/" rel="nofollow - http://https://nautiqueparts.com/oil-pressure-switch-r020015a-pcm-pro-tec-1989-1994-all-gt40s-and-gm-to-2006/

For sure the oil switch above - I just replaced on 97 PCM carbed non-GT 351.  Have you figured out which switch is in error yet?

BTW - You use a wrench .  I didn't want to say that above for fear of sounding like a jerk, but the oil pressure switch area is very tight to wrench, that's why I took off, cleaned and replaced the gauge sender and the T-fitting.  I made it all a little easier and got it all done at once. You can do it, if I can...

If it's in the hole by #5 cylinder (green arrow) it has 3/8 NPT threads and takes part number R020014

If it's in the hole by #1 cylinder right behind the thermostat housing (tan arrow) it has 1/2 inch NPT threads and takes part number R020014A

Switch in one hole, sending unit in the other

And like Air206 said the oil switch is R020015A. It's the only one they sell these days, no other choice.

I think the rest is up to you and Nautiqueparts 

*EDIT* I just read this: "PS....you don't need the globs of RTV or whatever it is on those connections either" so that answers that question. 

Do these switches kill the engine or just light up a warning/red light? 

I have a 1980 and am interested in installing a temp/oil pressure warning light for those who might be driving my boat on the rare occasion that I get to ski/board behind it lol. 

You're catching on here Otto, that's been mentioned more than once in this thread.

One is the sending unit for your temperature gauge

The other is the switch for the dash light.

Now go back and read about the colors of the wires attached to each of those.

The sending unit which has nothing to do with your problem will have a Tan wire

Most anybody can post pictures these days even from a phone if they follow the directions below Steps 1-9 and watch the Youtube video in the link.

KENO wrote: Mpost wrote: 75 Tique wrote: Per Ken's request/suggestion, and per Ken's efforts in finding a relatively easy way to post photos, here is a step by step process. 1.  Have photos you want to post in your phone gallery or on your computer. 2.  Open website imgbb.com (no need to sign in, sign up, or register unless you want to) 3. click or tap on "select images to upload" 4. Go to your gallery or computer and select photo(s) (Leave autodelete option on "dont delete") 5. click or tap "Upload" 6.  In the "Embed codes" box, scroll to and select  "BBCode full linked" 7. in the box at the bottom of the page where the photo URL shows up,  tap or click "copy" 8. Open your CCF reply and tap and paste or click and paste. 9. Voila, its there. (Note: it wont show up until you view preview or post it) I suppose 8 steps sounds like a lot, but its mostly quick easy point and clicks (or taps)  Goes quickly and easily.  Give it a try. Added this here so it is sticky or pinned  And added this link to the You Tube video that Larry made so there are written instructions and a video showing how to do it. https://youtu.be/0UYbzewyjXw" rel="nofollow - https://youtu.be/0UYbzewyjXw Written and video instructions all in one spot  Good idea Marty

On a gt40 they turn on the dash light and at the same time, tell the computer to put the engine in SLOW mode often called limp mode, but they don't kill the engine.

On the carbureted engines with the same switches, they'll turn on the dash light and nothing else. Then you look at your temperature gauge/oil pressure gauge to see if you have normal readings with a switch malfunction or an actual problem.

For your 1980, with a little wire and a couple of switches, a tee for the oil switch, a light and some connectors and a horn if you were so inclined, you could make your own setup for a whole lot less than the cost of the PCM labeled switches at Nautiqueparts. 

Alright the slow guy figured it out!   Here are pictures I took today....I am a preventative guy so would it be best just to replace them all?

LOOKS like a quick job...I have had many of those...experienced comments?

Also one last diagnostic to narrow down what switch...the light doesn't light up until the engine is warm than comes on intermittently at an idle, then on.  As soon as I get above an idle it goes out...Amateur guess says oil pressure because water temp would increase at higher rpms????

So the green wires go to the Engine Light, and the other two are different colors because they go to different destinations...AKA different gages!  Ok....baby steps!  You ar very good with the colored arrows corresponding to the wire color!   I am NOT worthy...but I am THANKFUL!

It looks like #1 has a tan wire so it's the temperature sending unit and #5 has a green wire so it's the high temperature switch.

It sounds like the oil switch would pretty definitely be the problem based on your description.

The oil switch is part # R020015A like mentioned earlier.

The temperature switch is part # R020014............there's no A on the end. The one with the A on the end would be for the gt40 switch location which is different than your location.

I'd pull the wire from the oil switch, tape the end so it can't ground itself on the block and take the boat for a ride  Your light won't come on when you turn the key to the Run position and it shouldn't come on at any time if the oil switch is the issue. If it does come on during the drive, that would point to the temperature switch.

Then you can decide whether you want to replace both switches or just the bad one.

I wouldn't mess with either sender if you have good, normal oil pressure and temperature readings on your gauges.

PS.........if you hit the space bar between pictures, they won't all run together 

And don't forget to use the not too well publicized PND discount code for a 10% discount on the order 

Thanks for the info KENO!

I was on Nautique Parts website looking at the switches mentioned in this thread and I saw they sell a temp/oil pressure switch and buzzer kit for about the price of one of the other switches. I need to know the thread size on the temperature switch before I can order it but I'm thinking about going that way along with an alarm indicator light they sell. They also sell the oil pressure tee fitting, so conveniant! Any reason I shouldn't get the alarm kit and just stick with the parts you listed instead?

Kit SKU# 6257 https://nautiqueparts.com/alarm-buzer-low-oil-overheat-system/

In the picture below you'll have to figure out where your temperature sender is and use the other hole that would have a pipe plug in it.

Your 80 probably has the sender in the hole with the Tan arrow, so you'll want a 3/8 NPT alarm switch to fit in the other hole with the Green arrow.

The kit should work, you'll have to verify with Nautiqueparts the size of the temperature switch threads in the kit.

Here's an example in the link   and the T is easy to find also

http://marinepartssource.com/audible-warning-system-mp41400" rel="nofollow - link

And a link to the T, you can find them locally too at an auto parts store, hardware store

http://www.amazon.com/s?k=Legines+Brass+Street+Tee+1%2F8+NPT+Female+x+1%2F8+NPT+Male+x+1%2F8+NPT+Female+Pipe+Fitting%2C+Barstock+Male+Run+Tee%2C+Oil+Pressure+T+Fitting+Splitter+%28Pack+of+2%29&ref=nav_bb_sb" rel="nofollow - link

This version of the TRB diagram is probably the closest you'll get for the engine wiring with your PMGR starter.

The wiring for the Red check engine light isn't shown, since it wasn't on his boat probably 
 

Here's a link to a YouTube video complete with a cutaway of an oil switch.

The title says "Oil sending units explained, but he's really explaining an oil pressure switch 

http://www.youtube.com/watch?v=CZGg4x5VmkA" rel="nofollow - link

Be nice to your boat, just replace the switch

I will let you know...I will try to clean and reinstall the switch prior to pickling for the winter!

You already have the new switch, just put it in.

That poor old "original" switch cycled itself every time you turn the engine on and off.

if your oil is cruddy enough to plug the old switch, you probably have other things to worry about

I'm trying to make this like 

Joe Dirt or Who shot

JR!!!!!

Ok - So I'll give you an update on my fix:  Ski 176 drives great and Red Light off after the oil switch replaced; ready for another 25 years and hundreds of hours!   Woot!

Here we are 2 plus weeks out Otto............so how'd the "cleaning" job go?