Dash guage issues

[d&dmach1 1]

Oil pressure & amp guage are not working although water temp & gas are (alternator is charging).  Had the dash out during resto and just replaced it recently.  Any suggestion as to where to examine for these two not working?  Everything else (lights, radio etc.) work fine.

Thanks for any input

[Mach1 Driver 560]

Gauge Operation

EDIT: When saved the drawing that went here was deleted. Dang this doesn't make a whole lot of sense without it! I'll try attaching a Word file.

To fully understand this you have to understand that everything electrical needs power to come in and go out. The two “Chassis Grounds” (one at each end) are connected together through the metal of the chassis. They did this to save money. Instead of running two wires they only run the power wire and connect to the chassis for the other. So current leaves the battery, goes through the sender, gauge, dash regulator, and back to the battery. Power in and out. Actual current flow is from negative to positive, not positive to negative as Benjamin Franklin arbitrarily assigned it. Heck he had a 50-50 chance, but maybe the whole thing with the lightning, kite and key used up some of this luck.

1. The Battery: (or alternator- it doesn’t matter which) supplies the power.

2. The Dash Voltage Regulator: knocks the voltage down to 5v. Why? Because that's the voltage they designed the gauges to run on. They didn't want sparks in the gas tank. Those old regulators are quirky little devices. They heat up a bi-metal that opens a switch, and that cools it off, and it does it again and again continually until the key is turned off. This makes the output: on-off-on-off-etc. They have it set up to be “on” for 42% of the time so over time you get an on-off cycle that averages 5v. The gauges are “dampened” to react slowly to this on-off so it seems like a steady 5v to them. Today we use a solid state circuit to get us an instantaneous solid 5v. You can replace your old style dash regulator for one if you like.

3. The Gauge: this is really a simple motor that turns less than 180 degrees and has a pointer stuck on the end of the shaft. As I said, it is dampened so it responds slowly to input.

4. The Sender: is a variable resistor. Its a resistor that is variable. A resistor impedes (limits) current flow. You can think of it as a valve if you like, that can be open a little or a lot. All the senders on our old Mustangs vary from 10 to 78 ohms. It doesn’t matter if its an oil gauge or a gas gauge. So if the sender was at 10 ohms you would get the maximum amount of current through the motor (gauge) and get HI pressure or a FULL tank, but if it was at 78 ohms you would get the minimum amount of current through the gauge and get LOW pressure or an EMPTY tank. 

So your oil gauge doesn’t work; connect the wire at the sender to the block for a short period of time and see if the gauge responds. Don’t leave it on too long because the minimum resistance is supposed to be 10 ohms, not zero. The gauges are dampened so a short time won’t hurt, just don’t peg the gauge all the way over. If the gauge responds then replace the sender. If it doesn’t then it could be an open circuit (wire off) from the dash regulator to the gauge, or from the gauge to the sender, or a bad gauge. Instead of running wires on the back of the dash they saved money again and used a “Circuit Board”, which is copper printed on plastic. These can delaminate and get open spots over time but are available from the popular parts houses, or you can use real wires from one of our members- Midlife who makes a nice harness. Next, connect an ohmmeter to the wire at the gauge and the other at the sender end of the wire with it disconnected from the sender. It should read very close to zero. If it doesn’t then the wire is bad. The wire is #31 (white with red stripe) and goes from the sender through a plug (somewhere near the sender) then up to the plug for the circuit board. Its right in the middle of the plug, look for the white/red wire and connect the ohmmeter there and at the sender end. You can also test the gauge here. Connect a wire to chassis momentarily to see if the gauge responds. The key has to be on so make sure nothing is shorted with the gauge cluster pulled out, then connect to the white/red wire and chassis. If the gauge doesn’t move it is either the gauge or the circuit board.

Ammeters in these beasts are a real weak link and usually only work for a short period of time. I can’t remember if mine ever worked. You can send them out and have them converted to a volt meter, which makes more sense anyway. They can keep the stock dial so it still looks like an ammeter if you are in to originality.

 

 

 

Gauge operation, oil pressure.doc

[LoneWolf2U 136]

1 hour ago, d&dmach1 said:

Oil pressure & amp guage are not working although water temp & gas are (alternator is charging).  Had the dash out during resto and just replaced it recently.  Any suggestion as to where to examine for these two not working?  Everything else (lights, radio etc.) work fine.

Thanks for any input

While the amp gauge to work is much more involved, Test the oil pressure first by grounding the wire at the sensor and flipping the key to the on position.

If the wiring is good the needle will peg to the left. You sending unit my be bad. But also check for the eng to body ground straps are in place and any other grounding points. thsi is a main problem with miss reading guages.

[Bob & Sue 110]

Mach1 Driver thanks for posting this info. Good write up on gauges I also saved the word doc.

I had an article from Mustang 360 bookmarked it is almost identical to your write up with pics on how to.

http://www.mustangandfords.com/how-to/interior-electrical/mump-1005-ford-mustang-instrument-panel-troubleshooting/

[Mach1 Driver 560]

55 minutes ago, Bob & Sue said:

Mach1 Driver thanks for posting this info. Good write up on gauges I also saved the word doc.

I had an article from Mustang 360 bookmarked it is almost identical to your write up with pics on how to.

http://www.mustangandfords.com/how-to/interior-electrical/mump-1005-ford-mustang-instrument-panel-troubleshooting/

Yeah, but they have pretty pictures and we are limited in what we can upload.

[Midlife 814]

Mach1Driver:

Excellent description, and you took more time and effort than I can do it justice.  But, but, BUT: you had to let out the secret that electrons flow from negative to positive?  For that, you should be banned from the Exalted and Holy Electrical Union and forced to turn in your union card!  *G*

[Mach1 Driver 560]

12 hours ago, Midlife said:

Mach1Driver:

Excellent description, and you took more time and effort than I can do it justice.  But, but, BUT: you had to let out the secret that electrons flow from negative to positive?  For that, you should be banned from the Exalted and Holy Electrical Union and forced to turn in your union card!  *G*

I know, my bad- its more than most people can bear knowing, but I saw it on Wikileaks and just had to report on it !!! Full disclosure.

[RPM 1,190]

I'll keep an eye on him Randy.

[Midlife 814]

Dang carpetbaggers...

[Mach1 Driver 560]

18 hours ago, Midlife said:

Dang carpetbaggers...

 

car·pet·bag·ger

ˈkärpətˌbaɡər/

noun

derogatory

 

1. a political candidate who seeks election in an area where they have no local connections.

   • historical

     (in the US) a person from the northern states who went to the South after the Civil War to profit from the Reconstruction.
   • a person perceived as an unscrupulous opportunist.

     "the organization is rife with carpetbaggers"

      

      

OK, I was just checkin', that's me- the last one

[RPM 1,190]

Too funny! I can't even remember the topic of this thread.