Okay, well it's not the same as the other temperature gauge threads. I've noticed some silly readings between my OEM oil and temp pressure gauges (they tend to go up and down together, varying a decent amount but always together) and am looking for recommendations for an analog gauge. I guess small changes in either voltage or current are causing them to move around strangely. I may try swapping out the sender, but I can't imagine two sensors would fail together. I figure if I'm installing one for the temperature I may as well install one for oil pressure too. Any recommendations?

Something I just learned while trouble shooting the temperature issue - IR thermometers can vary wildly based on the emissivity of the material being measured. Higher-end thermometers allow the user to adjust for this, matching the setting to the emissivity of the surface, however cheaper ones are generally pegged to 0.95. That setting is good for wood and sheet rock, but not at all for polished metal, like some thermostat housings. For instance the IR reading of my grill was off by more than 20 degrees. I've using a cheap one the whole time, so who knows what the actual temperature has been. Just ordered a Thermoworks last night

 

The EZ one first the IR gun. You just want to know close what the temp is and if it is off at the stat cover, they are never what the sat is just a little lower from what I seen.
So if off at stat if you go to the outlet of the radiator you are only looking for the spread or delta T between the 2.

Now the gauges lets get to the root of your gauges, is the fuel gauge doing the same thing?
I ask because the 3 gauges, fuel / temp / oil, all get power from the IVR (instrument voltage regulator) and if it is going bad could be a cause.
But first lest start at the battery as we need to check and clean the grounds.
Battery to motor, motor to frame, and a big one motor to firewall. If the cab is not grounded good, strange things can happen like the gauges freaking out.

If you want to test the IVR I believe with 12 volts in you get 5 to 6 volts out and that goes to the gauges and then out to the senders. See if the motor ground or the sender ground is bad it can also throw the gauges off.

If you want mechanical gauges note for the oil DO NOT use the plastic tubing that comes with the oil gauge. Over time it will dry out and break spraying oil all over and could start a fire if it hits the hot EXH manifold. They do make a copper line kit think Auto Meter is who I used.
On the water temp the sender is pretty big so fitting it thru the firewall can be fun and then you have where to install it? IIRC the factory sender is a small dia unit so the gauge sender may not fit the intake manifold. After market manifolds have a large port for the aftermarket sender.
Go to Summit or Jeggs, heck maybe a local parts stores will have gauges.
Dave ----

 

You are correct, the cheap IR guns are best used for relative measurements, not actual. A black plastic surface on the radiator will give you a different reading than a silver aluminum surface on the radiator.

On the gauges, you probably get what you pay for, but I usually use the cheap units from the parts store. I try to find the units that sweep almost 360 degrees on the dial face, that spreads the numbers out further. But sometimes you can't find them locally. You can find what you want online most likely. I do like the mechanical ones that use the small tube with fluid in it. The only problem with these, you can't take the large probe off the end, so you need a decent sized hole in the firewall. An easy place to get it through is around the steering column, cutting a slit in that large rubber piece. And you have to be careful and not kink the tube, there is fluid that runs back and forth and makes the gauge work.

It seems easy to use a plugged threaded port on the thermostat housing that was used for a missing emissions device. But that puts the probe beyond the thermostat,, so the gauge will not read until the thermostat opens. That is no good. Since you are not worried about the original gauge, you can take the original sender unit out of the intake manifold.

 

Just saying, make sure all your electrical connections, including grounds, are good, clean and tight. I swear after replacing the two wire terminals going to my coil that finally fixed my temp gauge reading funny. That was some 8-10 years ago, I have changed a few other connections too.

 

Thanks for the info, I'll check all the connections first. My fuel gauge has never worked properly. The rear tank only ever shows half full, front seems to be a bit more accurate. The stock oil and temp have worked pretty well for the 4 years or so I've owned the truck, up until replacing the timing cover, water pump, and thermostat. If that doesn't fix it, on to a more accurate gauge.

 

Rear tank level. Does the gauge move lower as the tank gets near empty ?
You make it sound like it only reads 1/2 full all the time.

FWIW, my single tank truck, the fuel gauge will drop quickly to a 1/4 tank left, then I can drive for many miles, just above empty. When empty, it's below the E. I don't drive it much, so when it gets to just below E, I carry a gas can and funnel.

I also believe there is a under dash ground wire that may need cleaning.

I think you know how to view the EVTM from Gary's site. https://www.garysgaragemahal.com/1986-evtm.html

 

Sorry, my post was unclear. On the rear tank full is halfway, then it does the same as yours and drops to just below E. once i get about a millimeter past E, i need to be looking for a place to fill up. Fortunately the front tank still works (knocks on wood) so i use that one first and then the rear only ever gets about halfway before I fill it again.

 

I'm a balance guy. I would use the rear tank first, then switch to the front tank. Just switch when the rear tank gets down to the E. You know you'll have a little left, just in case.

 

That's a good though. The gauge is more accurate, but I believe that if selector fails, it fails into the rear configuration. Also for some reason the pump in my front tank has failed twice, while the rear pump is just fine. In practice, none of thsi probably matters because I only drive it about 25 miles a week and am always in town. But it's a good thought exercise.

 

Only 25 miles a week, you drive your truck more than I drive mine. I keep only a few gallons in it. No need to carry around all that weight.

 

I see you have a 460 with in tank pumps. Your selector valve has a little motor in it that does the fluid line switching and the sending unit electrical switching. Unlike the simpler 3 -way selector valves which have a spring, I can see your valve failing in either direction depending on where the motor position was last.

These valves you have are prone to failure, I have the same thing on my 89 diesel and it failed years ago. I have been running on only one tank and it runs fine, but I find myself in a situation once in awhile where I am at a place with very cheap diesel. I want to get as much as I can since it stores well. I just bought the 38 gallon tank conversion and will be installing it in the rear position on my truck soon. So I can take advantage of cheap fuel when I run across it. No more failure probe valve for me.

 

Haha! I thought about all that extra fuel when I was at the landfill this week. I'm used to seeing the truck weight around 5280lbs and when I was crossing the scales empty it was in the 5400s.

That's good to know. I knew I had the problematic selector valve, but didn't know it could fail in either direction. Initially I wanted to keep the 2 tanks so I could mount the spare tire under the rear tank. Then I realized there is a 5th wheel hitch mount there so the tire mount doesn't fit anyway. Tire is now mounted in the bed, so switching to a 38 gallon rear tank sounds like a good idea.

 

You are right at least on mine if the switch or power to it fails it defaults to the rear tank.
That is 1 reason why I run the front tank first. I was on the front tank and had the fuse blow. At the time when I would use the truck I was putting 80 miles on a day, 400 a week.
I did not see the front tank level drop, I know both my tanks read right as I have tested when I installed them. When I switched to the rear tank I seen the level had dropped!
So I was showing front tank level but pulling from the rear.
The other reason the run the front down first is to keep the weight over the rear wheels for a better ride not that the ride is bad, it is not.

Shortly after my truck was back on the road the rear tank level would not move off E? Both tanks had new senders installed as part of the rebuild.
I ran the rear tank out of gas as I knew I had to drop it. When it spit & sputtered I then switched to the front tank and it took right off again, I have a carb not EFI.
When I dropped the tank I found the float full of gas. I took the float from the old sender and has been fine since.

That is what is nice with 2 tanks you can run the tank that has to be dropped dry.
Dave ----

 

I think someone mentioned firewall to engine ground. We've seen missing ground cause guage readings as you describe. A good ground path from firewall to engine, frame and ultimately to the negative battery terminal. Clean, tight and bonded.