The Stovebolt Site Forums The Shop Area The Electrical Bay Continuing the Wiring Project

Hello everyone. I haven't been on here in a while. I've been knee-deep in rewiring the 49 Chev. So far, everything is going well. I am to the point where I don't know if my gas gauge is bad, or if the sending unit is bad, or if my wiring is bad somewhere.

The gas gauge did not work prior to starting this project. The truck has the original cloth covered wire from the tank to the gauge. I have replaced it with what came with the new harness from the gauge to the connector for the sending that connects underneath the cab. I have not touched the original wiring from the tank to the ground and what connects to the new wire.

How can I check to see which is bad, fuel gauge or sending unit? Thanks for all the help.

Darrell

Please read through this thread and see if it helps at all.

https://www.stovebolt.com/ubbthread...21/gonew/1/gas-gauge-misread.html#UNREAD

Thanks Rusty.

This truck was converted from 6 volt to 12 volt in the past. Reading the thread above, it looks like it should have an inline reducer on the fuel gauge, assuming a new 12 volt gauge was not put in. If the gauge is the original 6 volt gauge, is it possible the gauge might be burned up since there is no inline reducer? Is there a way to tell if this gauge is the old 6 volt gauge or a new 12 volt gauge? Would it be better to just by a new sending unit and gauge and install?

Yes, it is possible the 6v gauge could be damaged if a Runtz-style 12v-6v reducer is not used.

However, a few members have posted that they have not used a reducer when switching to 12v, and they have had no problems. I did-not/do-not gamble - I have (for many years) used the reducer from Classic Parts.

I do not know a way to determine 6v or 12 gauge.

I'd buy the gauge first; and, if that does not solve the problem, then buy the sending unit?

Check sender with ohm meter. If good, probably gauge or ground issue. Gauge and sender are both 30 Ohms, use a reducer into gauge. I read that you may have old cloth wire and you may have connected new wire to old sender wire. When you rewire a truck you only use new wire, everywhere. NO old wire or old terminals. Eliminate all under cab, under body inline connectors. They are not needed for any reason. Direct wire. The only connectors you keep are the ones that connect directly to a component. Like headlights bulbs and some switches. Where practical, remove those also and use blade or ring tongue terminals. (or get new connectors). 70 year old connector internal contacts can be a real pain. I'm sure you have also seen the plastic connector shells disintegrate in your hands.

Most gauge/sender, switch, motor issues are better tested/troubleshoot on the bench. Isolates the components and identifies whether component or truck wiring.

When turning the key on, the gas gauge flickers and goes slightly below empty. Turn key off, and it slightly rise back to empty.

Disconnected sender wire from back of gas gauge. Most forums say the gas gauge should go to full. Gauge still flickers and slightly goes below empty.

Checked sender with ohm meter. Measures around 2.6 to 3.3 at bottom. As soon as I start moving the arm upwards, meter goes to 1.

Made temporary wire with connectors and connected sending unit directly to sending post on fuel gauge and when the key is turned on, fuel gauge still flickers and goes slightly below empty.

No matter what I do, I get the same result. Gas gauge flickers when powered on and moves slightly below empty.

I don't really know what to rule out because I don't get the results the forums say I should when I do the different tests. I'm leaning towards bad sending unit, but I don't understand why the fuel gauge won't go to full when I disconnect the sending unit wire from the gauge. Worst case option would be to buy new sending unit and new wire from sending unit to gas gauge.

Looking at a picture of the back of the fuel gauge on another thread, I'm not sure which wire goes where. The wiring diagram I have makes it look like the hot wire from the ignition goes to the full side of the gauge, the side closest to the ignition and the sender wire goes to the empty side. That is the way mine is hooked up. I saw another thread where the hot wire was hooked to the empty side and the sender wire was hooked to the full side. I tried this on mine and I get no flicker, nothing at all. I would like to know which way is correct, though.

Ok, I got back on the fuel gauge problem today and here is what I've found. I had the hot lead and the sending unit wire reversed on the gauge. I've now corrected that. I've checked the lead from the switch to the gauge and it is hot on the post of the gauge. With the key on and the sending unit in the tank and grounded, I am able to pull almost 30 ohms, so it appears the sending unit is in fact good. When it was out of the tank, I could not get a good ohm reading, but there was not power to it. I'm assuming the key needs to be on and power coming to the gauge for the sending unit to read ohms. Is this correct?

With the wires attached correctly, the needle stays on empty. When the key is turned on, there is no flicker and the needle does not move at all as it did before when the wires were reversed. I have checked all of my connections. Is there a way to check and make sure I'm getting a good ground? I've taken off both ends of the ground wire from the sending unit and the frame and cleaned and reattached. Still, no movement on the gauge needle from empty. Does it sound like the gauge is bad? Could I have messed it up when I reversed the wires?

Test in order, exactly as described.

1. Sender ohm test is done with NOTHING but the tester hooked up to it. One lead of tester on the center tap and one lead on the flange. Out of the tank and operating float by hand. 0-30 reading, sender is good. If your meter is set correctly (200 ohms) with leads connected in prober places on the meter and you do this test and it does not read approx. 0-30 ohms, the sender is bad.

Sender tests good? then:

(sender re-installed)

2. Power ON:
Disconnect sender wire from gauge and hold in air, gauge goes to "F", good gauge so far.

3. Power ON:
Take off sender wire on back of gauge, hold off to the side. Attach a jumper from the sender terminal on the gauge to GOOD KNOWN ground. Should read "E". That is the same as taking sender wire off at sender and touching it to a GOOD KNOWN ground. That second form of the test also verifies that the wire leading back to sender is OK. Goes to "E", gauge is good.

The gauge itself MUST BE GROUNDED. Depending on design, that usually is a lug on the back touching the gauge cluster metal and the gauge cluster metal touching the dash and back thru /body/chassis/motor... to battery neg post. This is a common place to lose ground as well as at sender. losing ground is Number one cause. But we shall see.

A gauge reading "E" may have the sending wire touching ground somewhere along the line. Especially if sender tests good. You must have a brand new, continuous wire from sender center tap to gauge "S" terminal. Period. No matter what's wrong right now.

There are other procedures and sequences to test fuel reading system. You may choose any system you like, but I don't know about those for sure.
You are at a point, in my opinion, that you have not proven anything.

Originally there was a red warning sticker on the sender post on back of the gauge. Warning ...click.... ...click...

You may have damaged the sender or gauge when you hooked it up backwards

The best way to test a sender is independently, no power, out of the tank, as bart stated

...click...

The full needle sweep as a test happens by disconnecting the ground at the sender flange. Not the sender wire,

"I had the hot lead and the sending unit wire reversed on the gauge. "

Unfortunately, I think DADS50 is most likely correct for your situation.

Thanks Bartamos.

1. When sender was out of tank on bench with nothing attached, ohm meter was set to 200 ohms with lead of tester on center tap and one on the flange where the ground wire was connected. I get a reading at empty of between 2.3 and 3.3. As I slowly move the float up towards full, the ohm meter starts to rise and immediately goes to 1. I'm assuming this means a bad sender. I was confused when I put the sender back in the truck and had everything hooked back up and with the key on, tested the sender again and got an ohm reading of 29.7, which would have been correct because the fuel tank is full. Sounds like this is an erroneous reading, though, based on what you said with wires being attached.

2. With power on and sender wire disconnected from gauge, gauge still reads empty.

3. Following this step, gauge stays on "E."

I'm going on the assumption that the sending unit is faulty and have ordered another one. When it gets in, I'll see if that fixes the problem. If not, then I guess it's on to the fuel gauge. I really hope it's the sending unit. I'm tired of contorting my body under the dash.

Appreciate all of the help.

Darrell

Thanks. There was no red tab on mine. The wiring diagram I had from the wiring harness I bought looked like the hot wire went to the full side. I'm hoping it just killed the sending unit and not the gauge. The bench test makes it look like the sending unit is bad. I get a reading between 2.3 and 3.3 at empty, but as I start to move the float up, the ohm meter goes to 1. I've ordered a new sending unit. Hopefully, that's all it is.

Thanks for the info. This is definitely a learning process for me.

Darrell

I guess that is a good plan at this point. New sender. It is possible you somehow sent voltage to sender and burned it out. It is also possible you sent wrong voltage or voltage to wrong gauge post and burnt it out also. It is possible you did nothing wrong and have bad parts due to age and etc.
Don't throw anything away.

In continuity mode, multimeters have an error display of 1 or OL, which means open loop, broken wire, not connected, no continuity. Sorry that I did not pick up on that error reading of "1" the first few times you said it. Seems like sender is bad. At first you are reading good becase the wiper on the sender rheostat has not reached the broken section. When it does, meter reads 1, the display for open/broken wire. Hopefully that is the explanation. BE SURE to test the new one on the bench. (just multimeter hooked up). Always touch your ohm meter leads together before test to get a small reading, showing that meter is good.

If bench test of new sender shows approx. 0-30, then test sender laying on the seat of the truck, hooked up to gauge and to a good ground. (preferably direct to battery neg post). BE SURE YOU HAVE THE CORRECT VOLTAGE AND KNOW WHICH POST IS HOT ON GAUGE. You may need more study and advice on that. When setup is ready turn the key on and operate float arm.

If gauge still reads wrong, unhook sender from gauge and ground. Retest sender, with multimeter, right away on the seat. If it still is good, it's the gauge OR the gauge ground. Then you take out gauge and take sender and gauge to bench and do the DADS50 test as seen in his video. Again be sure what voltage the gauge wants and which post is the hot post. (Once the correct hookup is determined, you will proceed with the test). This test will eliminate any bad wires you have in truck and will eliminate any grounding issues you may have in the truck. If the parts work together on the bench, all you have to do is install all new wire and fix all the grounding spots. Please report test results mentioned before going "haywire" on us.

The hookup of both parts on the bench is: Power source positive wire (battery charger or a battery with correct voltage needed) to hot post on gauge. Wire from sender post on gauge to sender center tap. Power source negative wire to sender flange AND to proper gauge ground metal. Post a picture or two of back of gauge here. Should be a round ground "slug" between posts. NOT SURE. DADS50 can verify where he attached ground on gauge for his video. I believe the posts on the gauge are isolated/insulated.

You can us the same multimeter, set on DC voltage, to measure output of power source to see if it has correct voltage for gauge. Be careful, be safe, double check setup before power on. Have good connections of test leads and wires.
We will get there and you will fix this problem and start answering questions about this issue for others.

P.S. A reading of 1 in the left side of display can also mean "out of range". Which means wrong scale or resistance unmeasurable. When you turn on your meter and select ohms, it should display 1 if leads are not touching. Touch leads and it should read 0.2 to 0.5, test lead resistance. If when testing a component/wire and you get "1" and you suspect you are on the wrong scale, change scales. If, each time you change scale, the "1" stays at hard left but decimal moves to different spots, you do actually have an open loop. It's not a range/scale issue. Some meters, in ohms mode. will have OL meaning overload. Which can still mean open. It can also mean unmeasurable range.
It is also possible to damage meter if power is on during continuity tests. At the least, it will NOT give an accurate reading.

All the above verbiage above is the reason I and others suggest leaving the DVM in the toolbox for other jobs and using the old, but much better suited to this task, analog meter. I will let someone have my trusty Simpson 260 when I’m done with old rusty tin but not before.
I do agree with bartamos if you just gotta use a DVM.