International Full Size Jeep Association
Home Forums Reader's Rigs Tech Library Trail Stories FSJ-List
International Full Size Jeep Association  

Go Back   International Full Size Jeep Association > Tire Kickin' > General FSJ Tech

Reply
 
Thread Tools Search this Thread Display Modes
  #1  
Old 06-14-2003, 05:39 PM
PhilSine PhilSine is offline
Bleedin' Gasoline
 
Join Date: May 25, 2002
Location: Superior, WI
Posts: 2,084
Post

What's the difference between an oil pressure switch and an oil pressure sending unit? I think I know but need to be sure.

Here's the story....

I finally rebuilt my oil pump and the gears were bad, I mean really worn and some good scoring. While I was at it, I replaced the oil pressure sending unit so I could be sure I was getting an accurate reading. The old one would drop to 0 psi at idle in drive at stop lights or while driving real slow. I knew I had pressure based on how the engine ran. If I put it in neutral, my pressure would go up to 10 psi and at highway speeds it would display 40 psi. My theory on that was that the sending unit would fail when it got too hot ie. at idle in drive when the engine wasn't cooling its best.

So, I take it all apart, put it all back together, and take it for a spin. The pressure jumps up to 42 psi at cold idle (too low for a new pump) and drops to 20 psi at warm idle (about where it needs to be). The cold idle wasn't too cold so I dismissed the lower reading. I take it on the highway (the true test) to see what I get. According to all I have been taught, I should get 60 psi or better at 65 mph. well, the needle never goes above 38 psi. For the most part it stayed right around 30 psi and would drop to 20 psi at idle.

I got the part at NAPA and later looked it up and the description says "Oil Pressure Guage Switch". The connector was different from my original but I was able to adapt it easily. I go to CSK/Parts America's web site and do a search and fid both an "Oil Pressure Sending Switch w/Guage" and an "Oil Pressure Sending Unit". Now, the "Oil Pressure Sending Unit" looks exactly like the one I removed including the connector. The "Oil Pressure Sending Switch w/Guage" from CSK and the "Oil Pressure Guage Switch" from NAPA are identical. I can only presume that there is a difference and that I should return the "Oil Pressure Guage Switch" to NAPA and go buy the "Oil Pressure Sending Unit" from Checker's. Does this sound right? Here are pics of the 2 different units.

"Oil Pressure Guage Switch" from NAPA

Note the connector on the end ^^^

"Oil Pressure Sending Unit" from Checker's vvv

And notice the different connector on this one that matches my original.
__________________
I can neither define beauty nor obscenity but rather to state that I know them when I see them.
Reply With Quote
  #2  
Old 06-14-2003, 05:56 PM
Crazy_Jeepman's Avatar
Crazy_Jeepman Crazy_Jeepman is offline
Sloppy Shifter
 
Join Date: Jan 16, 2001
Location: Norway Michigan
Posts: 15,996
Post

Well I think the illustration is a bit off on the oil Pressure switch. The only oil pressure switch I know of on the AMC is the one for the electric choke circut. It juices up the electric choke when oil pressure comes up. Try a manual gauge, only true way to know what you are getting for oil pressure. Electric gauges are merely a decorative feature and serve no real purpose or accuracy.

[ June 14, 2003, 11:57 PM: Message edited by: Crazy_Jeepman ]
__________________
Daily Driver 2008 Wrangler Sahara
Going, Gone, or Thinking Fishing
Reply With Quote
  #3  
Old 06-14-2003, 06:35 PM
PhilSine PhilSine is offline
Bleedin' Gasoline
 
Join Date: May 25, 2002
Location: Superior, WI
Posts: 2,084
Post

Quote:
Originally posted by Crazy_Jeepman:
Well I think the illustration is a bit off on the oil Pressure switch. The only oil pressure switch I know of on the AMC is the one for the electric choke circut. It juices up the electric choke when oil pressure comes up. Try a manual gauge, only true way to know what you are getting for oil pressure. Electric gauges are merely a decorative feature and serve no real purpose or accuracy.
Thanks Crazy, you just cranked my gears over and they're moving now. You answered my question and may not even realize it. The final answer is YES, there is a big difference between an oil pressure sending unit and an oil pressure switch.

When I first started my rig after the work, the oil guage stayed at 0 for about 2 seconds and then the needle jumped to about 20 psi. That would indicate that the switch does not turn on until the pressure hits a certain level. My old guage would move the second the engine was started and slowly rise as it was a sending unit, not a simple switch.

I realize that mechanical guages are the truest but at this point I am not in a position to convert. My dad has owned and worked on FSJ's since the early 70's and he has always maintained that the electric guages are not perfect but are usually accurate to within 3 to 4 lbs (at worst) and can sometimes be dead on. For now, that is good enough for me. An oil pressure sending unit works on resistance. That being as the oil pressure fluctuates so does the resistance of the unit and that is translated into psi at the guage end. It is the exact same thing as a fuel guage. I can imagine that the reason an electrical guage may be less accurate is because quite often the resistance of the wire may change based on factors such as corrosion or poor connectors.

Thanks.
__________________
I can neither define beauty nor obscenity but rather to state that I know them when I see them.
Reply With Quote
  #4  
Old 06-15-2003, 07:46 AM
Metal Man Metal Man is offline
Member
 
Join Date: Apr 30, 2003
Location: Sunbury PA
Posts: 49
Post

Quote:
The only oil pressure switch I know of on the AMC is the one for the electric choke circut. It juices up the electric choke when oil pressure comes up.
Sorry to jump in here,but I have a 1990 GW and just after I put my rebuilt motor in I had an oil pressure "switch" spring a leak.I couldn't see what it did (I was looking for a light on the dash)so I just put a plug in the hole.This is not the sender for the gauge,it has a moulded plug with two prongs.What is this for?Is this for the choke?

My sender for the gauge came from NAPA and is like the one in the lower right pic.

[ June 15, 2003, 01:48 PM: Message edited by: Metal Man ]
__________________
<a href=\"http://www.ironhorseband.com\" target=\"_blank\">www.ironhorseband.com</a>
Reply With Quote
  #5  
Old 06-15-2003, 07:49 AM
Crazy_Jeepman's Avatar
Crazy_Jeepman Crazy_Jeepman is offline
Sloppy Shifter
 
Join Date: Jan 16, 2001
Location: Norway Michigan
Posts: 15,996
Post

Yes the one with the molded plug is the Choke. Sunbury Pa HUH.............Wish I could remember how many loads I pulled out of Celotex!!!!! Nice town......Just hated Celotex loads!!!
__________________
Daily Driver 2008 Wrangler Sahara
Going, Gone, or Thinking Fishing
Reply With Quote
  #6  
Old 06-15-2003, 07:30 PM
jimzamx jimzamx is offline
Member
 
Join Date: Oct 26, 2000
Location: Kenosha, WI, USA
Posts: 34
Post

The switch is normally only used to send power to an idiot light, a sending unit works a gauge. The sending unit sends a voltage that needs to be in the correct range for your gauge. There should be specs on the voltage output for the sending unit. I would do as sugested and use a mech. gauge to verify your pressure readings.
These pumps many times require a new or remanufactured gear case / pump housing to get good oil pressure. The rebuild kit helps but sometimes won't bring the pressure up to spec.
__________________
\'90 GW all stock<BR>\'86 GW w/lift (laid over)\"If you lift it, It will roll\"<BR>\'69 AMX 390 4spd, Bittersweet Met.
Reply With Quote
Reply


Thread Tools Search this Thread
Search this Thread:

Advanced Search
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

vB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Forum Jump


All times are GMT -6. The time now is 04:45 AM.


Powered by vBulletin Version 3.5.4
Copyright ©2000 - 2020, Jelsoft Enterprises Ltd.
corner corner