Good evening,
When I purchased this truck a month ago (79 F100, 351M), the fuel and temp gauge were not working. After grounding the temp gauge wire, the gauge did peg out so today, I changed the sending unit. Now the gauge is reading in the normal range when the truck in still cold. Fuel gauge barely moves when the key is turned on.
I have tried searching how to test to see if the stabilizer is working or not. When i put a meter on it, it seems to be fluctuating from 5 volts down to 0, I dont like just throwing parts on hoping to fix the problem but...is it safe to say this 42 yr old part is probably not working correctly? I do have dash lights, oil pressure gauge works as well as there is movement on the ammeter.
If changing it out is the best option, how does it get disconnected from the printed circuit?

 

The fluctuating is right. That's the way its supposed to work and the voltage is also right. So there is no reason to change it out. If its not reading at the gauge start testing at the tank. Or your out of gas.

 

Based on your description, the cluster voltage regulator is fine and should be left alone. Your issue with the fuel gauge sounds like a sending unit issue. Ground the wire like you did for the temp sending unit to test the gauge.

 

So maybe I got a bad new sending unit for the temp? I mean it is possible. I appreciate the feedback guys, better to ask those who have more knowledge.

 

Dave said it best. I'm just not that articulate.

 

I would question the VR, they should pulse rather quickly. The bi metal strip does go bad, all the ones I have had to fix I have changed them to a solid state regulator.

Here is the soid state mod https://squarebirds.org/Electrical/C...StateRetrofit/

 

In doing some more reading on threads, one of the posts on the stabilizer should have 12 volts and the other should fluctuate. I am not sure which post should have which but I took 2 videos of my multimeter screen. Both posts fluctuate but the post furthest away from the speedometer goes up to 12 but fluctuates and the closest post has the lesser. Hopefully I can attach these.
Well, that's not gonna work but on the outer post the highest reading is 13.6 and drops to 9.6. On the inner post the highest reading was 6.5 and the lowest was 1.2. I took the videos while the truck was running. Does this still jive that it is working correctly?

 

You've checked everything at the dash. Have you checked anything at the sender?

 

Checked everything? All I have checked is at the VS. What else should I check? And when you say at the sender...you mean what voltage I have at the plug that hooks to the temp sensor?

 

The Ford Truck Shop Manual says the instrument voltage regulator (IVR) controls and maintains an average pulsating value of 5.0 volts at the gauge terminals.

 

I was referring to the fuel sending unit. That said the quality of the replacement parts is very questionable. Its easy to have new parts that are defective and happens all the time.

 

The temperature gauge sender can be checked with an ohm meter. (As can the fuel level sender.) It should have a range of resistance from cold to hot. I think you can probably find that info online, or perhaps someone here has it.

Also, clean every connection you can find, from those senders all the way to the individual gauges. That might gain you some more gauge response.

 

You state your oil pressure gauge works, that means that your instrument cluster voltage regulator is working correctly. That is the part you call a stabilizer. No need to test it further, or replace it.

You grounded the wire to the temp sensor, & the gauge moved full scale. That means the gauge, as well as the wire to the cluster, are good. No need to test those further. Replace the sender, even if it is new. The parts store may have given you the wrong sender for the gauge, or they may have given you a switch for a warning light, rather than a gauge.Or, new parts these days are often junk, right out of the box. Do not wrap the sender threads with multiple layers of teflon tape.

Go under the truck, remove the wire that plugs on to the sender in the fuel tank. It will be hard to get to, & hard to remove. Hook a jumper wire to the sender wire, connect the other end to a good clean ground. Turn the key on, watch the fuel gauge. If the fuel gauge rises to peg the gauge in a minute, your fuel sender in the tank is most likely bad, replace it. Also check for a ground wire coming off the body of the sender, make sure it goes to a good clean ground. If the fuel gauge doesn't work when the sender wire is grounded, start checking that wire for continuity back to the gauge cluster plug. our fuel gauge could be bad, but that is unusual. If you have to replace your fuel sender, plug it in to the wiring, & check it for correct operation BEFORE you install it in the tank & reinstall the tank. Many new senders do not work, or read incorrectly.

Again, your oil pressure gauge works, that means your instrument voltage regulator is working, no need to fool with it further.

 

The sensors work by changing resistance to the ground, and the less resistance, the closer to a 'good' ground you get and the higher the needle goes on the gauge. So if you ground the wire to the sensor, you should see the needle peg all the way over. On the gas gauge, the sensor runs between about 20 and 70 ohms resistance as I recall, so when the tank is close to empty you should see about 70 ohms resistance between the posts on the sensor. If you don't, you may have a bad sending unit. If you see high resistance on an empty tank and the gauge pegs at full when you ground the wire, you may also have a misadjusted float and will need to bend the float so that it is almost on the bottom of an empty tank at rest. Should be able to test the water temp sensor similarly, try measuring the resistance between its post and outer grounding shell - I am thinking when its cold you should see close to 70 ohms, and when you put it in hot water the resistance should head toward 20 ohms. Be sure the screw threads on the sending unit are clean and nice - no plumber's tape or paint overspray. They will change the resistance in the circuit and screw up the readings.
Another thing to check is the wiring between the sending units and the dash - make sure there are no worn spots in the insulation, partially severed spots in the wire, etc. These will alter the resistance in the circuit and hence your gauge readings.

 

Thank you for the responses and insight. I did go to NAPA and get another sending unit for the temp and made sure it is for a gauge. Everyone has said do not use thread tape but is there anything needed so it doesnt leak or because its brass it should seal?