Just did some gauge/sending unit tests on inop fuel/oil gauges.

[jogirob]

Just thinking out loud here, please feel free to correct me:

Did the gauge sweep test by removing the sending unit wire from behind the oil and fuel gauges and attached em to a good ground. Turned ignition key to on:
Fuel Gauge = Complete sweep
Oil Gauge = Completely dead
***Oil gauge needs replacement***

With ignition key to the on position voltage limiter reads ~12v on the input side and a pulsating ~0-5v on the other.
***Voltage limiter checks good***

Did a continuity test from the wire leading to the fuel tank sending unit:
Reads ~85ohms from the sending unit wire behind the gauge clusters
Reads ~85ohms from the sending unit plug right on the fuel tank
I have a little over half a tank so it should read around 23ohms.
I did a voltage check from the tank side of the sending unit wire and I get the pulsating 0-5v from the voltage limiter.
***Fuel tank sending unit needs replacement***

I measured resistance from the oil pressure sending unit over behind the gauge cluster and got a "1" or no continuity. I turned the engine on and let it warm up and still resistance read "1". I took a look at the oil pressure sending unit and there doesn't appear to have any wires coming out of it to test? There is a set of unplugged spades that looks like it reaches exactly to the little bell shaped contraption but then again this car is full of unplugged connectors that go nowhere. How do you test the sending unit itself?
***???***

 

[danf_fl]

I measured resistance from the oil pressure sending unit over behind the gauge cluster and got a "1" or no continuity. I turned the engine on and let it warm up and still resistance read "1". I took a look at the oil pressure sending unit and there doesn't appear to have any wires coming out of it to test? There is a set of unplugged spades that looks like it reaches exactly to the little bell shaped contraption but then again this car is full of unplugged connectors that go nowhere. How do you test the sending unit itself?
***???***

On a typical digital multimeter, I thought that the display would show O.L if no continuity, 0.0 if no resistance.

 

[FrnkNsteen]

***Oil gauge needs replacement***

Not so quick on this. You are probably right, but some things to check before you make that assumption. Are you getting the 5 volts you need to the oil gauge? I know you checked your voltage limiter and it checks out, but make sure that 5 volts is making it through the board to your gauge. You could have a bad spot in the board, or corrosion between the board and the gauge causing the gauge not to see the proper voltage.

To check your oil sender itself, set your meter to its Ohms setting and connect one lead of your meter to ground and the other end to the post of the oil sender. Check resistance with the motor off, then check resistance with the motor running. If resistance changes, your sender is working. I'm not saying it is working correctly,.... just that it is working and should give you a signal to your gauge if the wire is ok. Resistance range is typically 10 - 75 ohms for our gauges (10 ohms for high pressure, 75 ohms for low)

***Fuel tank sending unit needs replacement***

Again,... not so fast. The resistance range of fuel senders is typically 10ohms - 80 ohms. If you are getting 85 ohms, your sender may be working, but is just at the bottom of its stroke on the sweeper. I understand you have a half tank of fuel, but your float may just be bad. I have a '71 Chevy C10 that had a non functioning fuel gauge. I removed the sender to check it out and found that the sender worked fine and moved the needle when I moved it up and down. I looked closer and found that the float had a crack in it and just filled with gas and sank to the bottom of the stroke. Before you assume it is bad, pull it out (watch to see if it looks like gas is running out of the float), ground the sender body to chassis and move the arm up and down (with gauge wire connected) and watch your gauge. If your gauge reacts,... the sender is working and you need to check out the float to look for cracks.

On a typical digital multimeter, I thought that the display would show O.L if no continuity, 0.0 if no resistance.

I agree,.... on most Volt/Ohm meters, OL is an open circuit, and 0.0 is straight connection with no resistance (like if you touch the probes together).

 

[jogirob]

Not so quick on this. You are probably right, but some things to check before you make that assumption. Are you getting the 5 volts you need to the oil gauge? I know you checked your voltage limiter and it checks out, but make sure that 5 volts is making it through the board to your gauge. You could have a bad spot in the board, or corrosion between the board and the gauge causing the gauge not to see the proper voltage.

Good tip. OK I get the pulsating 0-5v coming out of the board wire that goes into the oil pressure gauge. That 5v gets lost once it goes into the oil gauge and I get 0v out of the other post (sender side). So should be the gauge for sure right?

To check your oil sender itself, set your meter to its Ohms setting and connect one lead of your meter to ground and the other end to the post of the oil sender. Check resistance with the motor off, then check resistance with the motor running. If resistance changes, your sender is working. I'm not saying it is working correctly,.... just that it is working and should give you a signal to your gauge if the wire is ok. Resistance range is typically 10 - 75 ohms for our gauges (10 ohms for high pressure, 75 ohms for low)

I get a reading of .02 ohms with the engine off and 3.1 ohms with the engine running. Which means I should be getting those same readings on the lead right behind the gauges.hmmmm
Side note: How in the world does the sending unit transmit the reading without wires? Wi-fi?

@danf_fl I've got one of those cheap-o-meters from harbor freight. As part of my initial tests I touch the leads to each other and get 0. Non touching = 1.

 

[FrnkNsteen]

Good tip. OK I get the pulsating 0-5v coming out of the board wire that goes into the oil pressure gauge. That 5v gets lost once it goes into the oil gauge and I get 0v out of the other post (sender side). So should be the gauge for sure right?


I get a reading of .02 ohms with the engine off and 3.1 ohms with the engine running. Which means I should be getting those same readings on the lead right behind the gauges.hmmmm
Side note: How in the world does the sending unit transmit the reading without wires? Wi-fi?

@danf_fl I've got one of those cheap-o-meters from harbor freight. As part of my initial tests I touch the leads to each other and get 0. Non touching = 1.

Yep,... looks like the gauge is shot! Sounds like the oil sender is sensing pressure changes, but not putting out the resistance you need to be in correct 10-75 ohm range. I would imagine if you had that sender hooked up, your gauge would be pegged all the time.

Well yeah,... of course it uses WIFI! I would assume it uses on of those dangling wires you mentioned. Unfortunately I have nothing to take reference pictures of. My Charger is up in Minnesota and has all the wiring pulled out of it, and my Barracuda uses a mechanical gauge mounted under the dash. If you can find a wiring diagram, either in one of the factory service manuals, or online, you should be able to determine which wire should be connected to it.

 

[danf_fl]

Take a look at my post in the fuel section.
I was reading 80 in resistance at the sender unit before pulling tank and sender unit.
I posted a picture of the old sender unit in the thread.

 

[jogirob]

If you can find a wiring diagram, either in one of the factory service manuals, or online, you should be able to determine which wire should be connected to it.

Oh yeah I have the 73 manual. The schematics have a line drawn from the gauge cluster to the oil pressure sending unit. The line could signify an actual wire or simply that it's attached somehow? I can't find a good clear visual on Google of the sending unit for a B-Body Charger other than a couple images glancing at the top nipple area, which shows nothing attached to the top usually.

I know there are some similar looking sending units that have the top as a threaded post where you plug in your lead and tighten it down with a nut. These units don't seem to have that.

 

[kiwigtx]

I made a discovery about 9 years ago....if you connect up a similar make temperature gauge to the oil pressure switch, you get a reasonably accurate reading. must try that the other way around.

It was found that the internals of my test OIL and TEMP gauges, that they have the same electrical properties....meaning a swap out is easy - except if you have any lettering on the gauge face.

I have successfully run a blanked out temp gauge for the oil pressure for the last 8 years - in my A100. Only thing missing is the numbers....BTW the truck only had an idiot light previously. Swapped out the sender unit also of course.

Note that the numbers are missing on the OIL gauge (right hand end)

 

[jogirob]

I made a discovery about 9 years ago....if you connect up a similar make temperature gauge to the oil pressure switch, you get a reasonably accurate reading. must try that the other way around.

It was found that the internals of my test OIL and TEMP gauges, that they have the same electrical properties....meaning a swap out is easy - except if you have any lettering on the gauge face.

I have successfully run a blanked out temp gauge for the oil pressure for the last 8 years - in my A100. Only thing missing is the numbers....BTW the truck only had an idiot light previously. Swapped out the sender unit also of course.

Good to know. After all, all we really need to be in the know is if it's starving or about to blow.

 

[FrnkNsteen]

Yes,... the gauges, at least in a rally gauge set, are basically the same. There are slight differences, but I think it is more to identify one from another when they don't have numbers on them, but they basically function the same on Temp, Oil, and Fuel gauges. The difference is that there is an extra hole in the gauge face in different locations on each gauge so you know what it was originally used for (hope that makes sense).

If you zoom in on the attached picture, you can see a small white circle on the Temperature and Oil Pressure Gauges. On the Temp gauge, it is on the left side, northeast of the left side brass rivet. On the Oil Pressure gauge, it is straight east of the left brass rivet. No extra hole on the Fuel gauge. There is a corresponding hole on the gauge faces that tells what the gauge face was originally.

Don't know exactly what kind of car it is we are talking about, but Performance Car Graphics makes decals for some of the Mopar gauges. I bought a full set from them for the rally gauges in my Charger. The picture below is from using those decals to redo my instrument cluster.

 

[miller]

On your 73, can only assume those gauges work the same, as earlier years. Power supply to the gauge, should be 5 volts, from the voltage reducer, going through the gauge, to ground. In the ground circuit, toss in the sending unit, limiting ground to read the gauge. Probably already know this, but 12 volts will fry the gauge.

Anyway, how I tested my 64 gauges, was using two 'C' batteries, that gave just under 5 volts, that, and other side to ground showed if the gauge was good, or bad. Maybe yes, maybe no, on any help.

Ahh...just re-read, see you checked 'em.

 

[jogirob]

From what I've gleaned on another thread I shamelessly started, there is supposed to be a physical wire attached to the top part of the oil pressure sending unit (seems to be giving a very high oil pressure reading @ 3.1 ohms).
I found a disconnected, chopped off grey wire, which according to schematics should be the one that leads to the back of the oil gauge. Gonna run a continuity to make sure.

Gauge bad, wiring bad, sender bad!

If I get this sucker resolved I'm gonna have to buy the forum a round on me.

 

[miller]

Oil Gauge = Completely dead

Any chance you've pulled the oil gauge out of the cluster? Not always, but have looked at some, where it was something minor that kept it from working. Studs corroded, and the like.

the little bell shaped contraption

Is that one on the top of the oil pressure sending unit? Just in case you might not know, it takes one of the 'female' type spade connectors, that slip onto it. Normally would have one of the single plastic covers over it, that has a slot on the bottom, to slip past the pin connection. (Best way I can describe it.)

 

[jogirob]

Is that one on the top of the oil pressure sending unit? Just in case you might not know, it takes one of the 'female' type spade connectors, that slip onto it. Normally would have one of the single plastic covers over it, that has a slot on the bottom, to slip past the pin connection. (Best way I can describe it.)

That helps a ton! A member showed me a photo of what it's supposed to look like but I couldn't get a good look at the connection type. Good to know it isn't some fancy boot and just one of those dime a dozen connectors that'll glide right on.

I'll give the gauge a good once over once I remove the cluster. The entire assembly needs to come off so I can put some graphite on the spedo cable. (Bouncing readings)

 

[jogirob]

Ouch, those oem gauges are pricey. Thinking of just going with one of those Autometer kits that come with their own oil pressure sending unit since mine is bad anyways. Do you guys know if one of their 1/4" or 1/8" NPT will fit the existing sensing unit location?

 

[miller]

Yep, I added an Autometer oil gauge on my 64. Slips right in.

You saying the sending unit is bad? Or gauge, or both? btw, that grey wire you mentioned IS the right one for the oil sender.
Gauges...the oem ones are pretty stout, but like anything else, things can happen, and it quit. Mine just took a little tlc, and down the road.

 

[jogirob]

Yep, I added an Autometer oil gauge on my 64. Slips right in.

You saying the sending unit is bad? Or gauge, or both? btw, that grey wire you mentioned IS the right one for the oil sender.
Gauges...the oem ones are pretty stout, but like anything else, things can happen, and it quit. Mine just took a little tlc, and down the road.

Both. Resistance on the sending unit only read 0.2 ohms when the engine was off and 3.1 ohms with the engine running. If the gauge was working it'd be just about pegged to the high side. I'm going with a mechanical gauge and sender. If oil pressure is still that extremely high then I have bigger stuff to worry about!