Homemade gauge test tool, C-3826

[buck351]

Hello. I was working on trying to get my fuel gauge working correctly since the new ones you get at the parts store I put in some time ago are linear. Found two original fuel sending units and bought them. One looked in good shape kept that one and sent the other one to a place that rebuilds sending units. both units didn't put the gauge at full. So I made a device to test the gauge With a variable resistor and marked positions for F, 1/2 and E with ohm meter. See attached photo. After testing it looks like the old style voltage regulator wasn't working correctly so ordered one of the solid state replacements ones that are listed at helping the gauges be more accurate. Ordered from Classic Industries.

Even tried connecting testing unit directly to the battery negative and gauge still wouldn't go to full unless I decreased the resistance below the spec for full.

 

[chinze57]

There are many threads on these issues. Some of them are

1....broken/ loose harness connector pins at the PC board. Clean them and solder them to the board traces

2...Ditto the contact fingers for the IVR (instrument voltage limiter) Solder jumpers from the contact fingers to the board traces

3...Poor connections at the gauges studs. Loosen/ tighten the nuts several times to "scrub" the connectors clean, and consider replacing the nuts

4...Flakey / bad IVR replace with a commercial solid state replacement or google search one of the many DIY circuits

5...Add a grounding pigtail to a common point on the cluster and bolt it to the dash frame/ column support

6...Don't discount poor connections in the power circuit feeding the cluster, or to bad connections leading to the senders. This includes the left kick panel connector (tail harness gas tank sender) and the bulkhead connector (temp sender and oil if used) as well as poor connections at the senders that is the wire end itself.

And then of course the gauge units themselves can be bad

I’d be willing to bet money the gauges themselves are fine, and the issue is one or all of the above. My gas tank leaks when filled above 3/4 full, and I believe that stems from a bad gasket where the filler neck meets the tank, and as such haven’t messed with the tank sending unit as It’s no use until that leak is resolved.

The wiring being bad somewhere along the way wouldn’t surprise me in the least. The previous owner took the car to a mechanic who butchered the wiring. Completely bypassed the fuseblock, wired things in strange places, and just spliced and taped wires left and right. I’m planning on replacing my underdash harness with a totally new one with a blade fuseblock, and hope to get into all sorted out, which will hopefully fix my gauges. It’s possible when he was messing around, he connected things wrong.

 

[convx4]

Thanks to all who contribute to FABO and allow me to learn some thing new and be able to keep my old junk running.

I have put together some information that some might find helpful. I have not completed the how to make a gauge tester like the C-3826.

Fuel gauge calibration

If you have a miller C-3826, ford / mopar gauge tester, you can test the gauge in the car. The gauges are powered by 5 volts thru the cars wiring system, ***Do Not test the gauge with 12 volts*** The wire that goes to the sending unit is the activation / ground wire. Meaning the 5 volts of electrify flows thru the gauge then to the sender. The sender limits the ground with variable resistance, the 73,32,10 ohms.

IF all your gauges don’t work your voltage limiter (to 5 volts), on the back of the dash, could be bad. To test the limiter you need some wires and a 12 volt power supply and Digital volt meter. The center post is 12 volt in, The post on the end is ground and the last of the three posts is 5 volt out. The original limiter is mechanical so the volt numbers in the digital volt meter fluctuate. There are solid state voltage limiters that also look like the originals, they will put out a steady 5 volts.

The miller C-3826 tool uses the vehicles 5 volts to supply the energy needed. The C-3826 takes place of the sending unit to test the gauge. The gauge can be tested at the back of the instrument cluster or thru the wiring. The do the thru the wiring just disconnect the wire from the sender. Hook up the C-3826 at the disconnected wire and the other C-3826 wire to ground. Use the following instructions.

Keep the vehicles key in the “Off” position. To hook the C-3826 up you remove the wire going to the sending unit. Then hook up one wire from the C-3826 to the sending unit wire, the second C-3826 wire needs to be hooked to ground. On the C-3826 tester set the three position knob to low. Look at the gauge, better to draw several picture showing the different configurations during testing. Turn the key to “on”and note where the gauge moves then stops, make note. Move the C-3826 switch to Med, watch the gauge move then stop, make note. Last move the C-3826 switch to High, watch the gauge move then stop, make note.This completes the gauge test, remove the C-3826 and reinstall the sending unit wire.

To adjust the gauge, it needs to be removed from the instrument cluster. To adjust the gauge, that has been removed from the car and the instrument cluster, a self powered tester that preforms like the C-3826 is needed. The tester needs to limit the supplied volts to 5 and have a three position switch that has one (10,32,73) Ohm’s in each of switches the negative line. The gauge has holes in the back that allow a small screw drive accuses to the adjusters. Each side (High and Low) should have a means to make adjustment. The easiest thing to do is to do is start with the needle of the gauge in the middle. Then switch between the high and low making note of the needle locations. Make adjustments so the needle is should/needs to be. Retest

To test the fuel tank sender unit’s Ohm’, it needs to removed from the car. Use a digital volt meter set on 220 ohms. Connect one wire to the lead from the sending unit and the other wire to the body /ground. Move the arm of the sender, slowly,and make sure the limiters are set at 73 Ohm (low) and 10 Ohm (high). Verify empty and full are correct. 1/2 should be around 23 Ohm Adjust the stop tabs

As needed. ***aftermarket sending units don’t always use the 73,23,10 Ohm’s.

Now the sender should be check with the gauge that will be used with it.

All of this information can be used on the temp gauge and the oil gauge, because they all have the same 73,32 10 ohms.

*****A late 60s A100 truck sending unit has a long arm like original. It is totally accurate in a 70’S A-body.

Parts list to make a self powered version of the Miller C-3826.

Item Digi-key part number

Black Box 2.5x2.3x1.5 4gxv1

4 position switch 700-bl-xg

Linear Voltage regulator 5volt L7805CV

Or 5 volt power supply wall mount l6r06h-050

2.1mm female power jack 721A

Low 73.2 OHM resistor MFR-25FBF52-73R2

Med 32.2 OHM resistor MFR-25FBF52-23R2

High 10.2 OHM resistor MFR-25FBF52-10R2

 

[bcschief]

Wow talk about raising threads from the dead this thread was 11 years old.

 

[threewood]

While we are here, I'll post some pics of my gauge tester that someone can comment on in a few more years lol. Got the parts from Mouser.