restoring dash

[Denny]

Can someone tell me how I can bench test fuel gauge and clock, I assume the clock would be just 12 volt hot and ground?

[1970mcss]

The clock you would ground the body of the clock and 12 volts to the terminal. I believe the fuel gauge is the same

[Scott S.]

The fuel gauge has two contacts, one in (positive) and one out (negative) that runs to the sender unit.

There are several posts that discuss troubleshooting any issues you may have.

But to just test the gauge, grounding out the negative contact will spike your gauge to the 3 O'clock position...... It won't tell you if it works accurately.

[Rob Peters]

2 hours ago, Scott S. said:

But to just test the gauge, grounding out the negative contact will spike your gauge to the 3 O'clock position...... It won't tell you if it works accurately.

If I am not mistaken this is incorrect information.  If you ground the that wire it would be like putting 0 (zero) ohm to the gauge which would make the gauge read empty or near empty.  more troubleshooting in a minute.  I just went through all of this a couple weeks ago so it is still fresh in my memory.

rob

[Rob Peters]

A good test to check the wiring from inside the trunk to the gauge and the gauge is with a "Decade Box".  I got this information from Joe so I ordered a decade box from Amazon, they are not very expensive. 

A decade box allows you to dial in a known resistance going to the gas gauge.  At different known resistance readings dialed in from the decade box to see how the gas gauge reacts will tell you if the wiring is OK and if the gauge may be reading correctly.

The first test would be to disconnect the tan wire inside the trunk near the trunk latch catch.  This creates an open in the circuit to the gas gauge which should cause the gauge to read around the 3:00 o'clock position.

^^^^^   Needle points to the 3:00 o'clock position with the tan wire in the trunk completely disconnected (an open circuit)

 

The next test using the decade box is to induce a known resistance to the gauge.  You would run one wire from a good ground to one leg of the decade box and another wire from the other contact of the decade box and connect that other wire from the decade box to the tan wire going to the instrument panel and gas gauge.

After you have the decade box wired into the circuit you set the decade box at 0-1 ohm and then turn the key to the "ON" position without starting the car.  The gauge should move to at or near the Empty position on the gas gauge.

^^^^^     Gas Gauge reading Empty with decade box wired in at 0-1 ohm.  Just a note, 0 ohms is NOT the same as an open circuit which is what it would be with the wire completely disconnected.

 

Next, turn the key off again and then dial in 40 to 45 ohms into the decade box.  With 40 to 45 ohms dialed in on the decade box you would then again turn the ignition to the "ON" position and give the gas gauge a few minutes to settle down and the gauge should settle in to the needle pointing in the area around 1/2 tank position.

^^^^    This is the gauge reading 1/2 tank with 43 ohms dialed in on the decade box.

 

Next turn the ignition switch back to the "OFF" position and then dial in 88 - 90 ohms on the decade box, and then turn the ignition switch to the "ON" position and wait awhile to give the gas gauge to settle in and at that point the needle on the gas gauge should read around the F (Full) position, as seen below.

^^^^   This is the needle on the gas gauge at or near the F (Full) position with 88 to 90 ohms dialed in on the decade box.

I hope this helps you.  The decade box is a great tool to check your gas gauge and wiring from inside the trunk to the gauge.  If all of these checks work properly it verifies the wiring from inside the trunk and the gauge.  So, if this checks out good AND your gauge still does not operate properly then your issue is either your sending unit is bad, the ground wire going to the sending unit or the tan wire from the sending unit to and running into the trunk is bad or has a bad connection.

Let me know if you have any questions.  The decade box I bought from Amazon cost me around $12.00 plus shipping (I don't have amazon prime)

rob

[cbolt]

Which decade box did you purchase Rob?

Those things are pretty cool but I have yet to find a solid need for one in order to justify the purchase. Like a signal generator or an oscilloscope. Would love to have all that stuff handy because when you need it, you need it. 

[Rob Peters]

Jim,

With the cost of the decade box being under $20 I felt it was well worth it.  It told me the wiring to the gauge and the gauge were good.  It saved me from shot-gunning buying parts and swapping them out until I found the actual problem.  This isolated it to the fuel sending unit.  I suspected it was the fuel sending unit but would have really been ticked off if I had gone through the expense of draining the tank, dropping the tank buying and installing the new sending unit and tank straps only to find out it was not the problem.... definitely well worth the cost of the decade box.  Not only that, it seems a lot of people are having the same issues with gas gauges reading improperly.

I am going to start carrying my decade box in the car because it if I come across someone with similar issues I will be able to assist others with trouble shooting/isolating their issue.  Joe brought one to last years Eastern Meet in Carlisle to troubleshoot my gas gauge issue but I was too busy to work with him to troubleshoot it. 

rob 

[cbolt]

10 hours ago, Rob Peters said:

Jim,

With the cost of the decade box being under $20 I felt it was well worth it.  It told me the wiring to the gauge and the gauge were good.  It saved me from shot-gunning buying parts and swapping them out until I found the actual problem.  This isolated it to the fuel sending unit.  I suspected it was the fuel sending unit but would have really been ticked off if I had gone through the expense of draining the tank, dropping the tank buying and installing the new sending unit and tank straps only to find out it was not the problem.... definitely well worth the cost of the decade box.  Not only that, it seems a lot of people are having the same issues with gas gauges reading improperly.

I am going to start carrying my decade box in the car because it if I come across someone with similar issues I will be able to assist others with trouble shooting/isolating their issue.  Joe brought one to last years Eastern Meet in Carlisle to troubleshoot my gas gauge issue but I was too busy to work with him to troubleshoot it. 

rob 

The question remains though, which decade box did you purchase?

[Rob Peters]

1 minute ago, cbolt said:

The question remains though, which decade box did you purchase?

Jim,

In looking at them on Amazon it looked like the majority of them may be made by the same company and then sold by numerous different vendors so it was more a pick and choose from a bunch of different vendors.  In a case like that I TRY to find one that is "Shipped By Amazon".  I will try to find the one I selected and post that.

rob

[Rob Peters]

3 hours ago, cbolt said:

The question remains though, which decade box did you purchase?

OK Jim, here you go.  Since we all love pictures I will include a few.  Just a note, the one ordered did not come with test leads so I made up my own.  So I could do my decade box troubleshooting from the drivers seat I made my test leads extra long so I they could reach from the trunk to the drivers seat.

Here are the pictures:

Decade Box outer box.  Yes it is from China.

 

Label on the Decade Box 

 

The Decade Box

 

 

My decade box with leads 

rob

[Denny]

Once again thanks to all for the valuable info.

[Denny]

Finally getting back to the dash restore, I do have a new sending unit now not installed.Correct me if Iam wrong but can I hook up a 12v lead to one side of the sending unit and ground to the other and bench test the gauge by moving the sending unit arm? I should note dash and tank, sending unit is out of the car . TIA

[cny first gen 71]

1 hour ago, Denny said:

Finally getting back to the dash restore, I do have a new sending unit now not installed.Correct me if Iam wrong but can I hook up a 12v lead to one side of the sending unit and ground to the other and bench test the gauge by moving the sending unit arm? I should note dash and tank, sending unit is out of the car . TIA

It's not 12 volts to the sending unit, I fried one doing it that way. Don't remember exact voltage maybe 7.

[72 Monte Carlo]

Rob,

That sure looks like a super handy tool for faster and easier diagnosis and troubleshooting the sending units of fuel tanks, especially when we have to work on rigs by ourselves.

I'm going to have to order one of those units to make myself one.

Thanks for the info on that!