The Stovebolt Site Forums The Shop Area The Electrical Bay Fuel Gauge Again

The fuel gauge on my 54 Chev 3100 is acting up again. The gauge needle tracks great when I raise the sending unit arm, but only to about 1/3 full. No matter how high I move the sender arm the needle stays at 1/3 full. When I bring the arm back down it tracks great again from 1/3 to empty. The gauge is the original 6 volt gauge with a Runtz voltage reducer.
I also have a new 30 Ohm sending unit that I bench tested with a multi-meter and it tracks 0-30 Ohms when moving the arm from the empty position to the full position.
I jumped the sending unit post to the sending unit cab ground wire and the gauge needle pegged to empty. I removed the sending unit wire to the gauge and the needle only pegs to 1/2 full. My understanding is that it should peg to full. I'm thinking the gauge is shot or in need of calibration but could I still be having a ground issue? Any suggestions are much appreciated.

Might as well ask to be sure: 12v electrical system (necessitating the Runtz reducer)?

Yes, it was converted to 12 volt by the previous owner. The gauge was not working and had no reducer. I pulled the gauge out and a small wire on the back was fried. It was repaired and I was told calibrated. I installed it with a reducer and it seems to now have a life of its own.

Now I have to ask....when repaired and calibrated , did the shop convert it to 12 volt internally?

Good question, I'll have to check.

Before I remove the gauge cluster and fuel gauge would it be to risky to remove the voltage reducer and try the gauge through the 12 volt system? I've read forum entries where a reducer was never used, once converted to 12 volt, and the 6 volt gauge worked fine.

Check with Fifth Avenue Garage, Randy Rundel has a whole page on fuel gauges and 6-12volt gauges great stuff.

First of all, some guys get away with the conversion without the reducer, some don't. Sounds like your trucks PO didn't. OK if you want to try it again, 'it's only money'.
Disconnect the battery if your working under the dash. Question, what kind of reducer do you have?

Just got back from the speedometer shop and had the fuel gauge checked out. Turns out the gauge is working great at 6 volts but to jumpy/sensitive when tested at 12 volts. I brought along the Runtz reducer I had on the gauge and it tested bad. I had a second Runtz I brought along, that I already thought was bad, and it also tested bad. Now I'm on the hunt for a good reliable reducer. One suggestion by the shop is to use a 1973 Dodge van instrument cluster reducer from Napa, about 40-50 bucks. They were not to keen on the Runtz reducers and neither am I at this point. Said Runtz have a bad track record.

Where did you buy your Runtz reducer?

The ones I have bought from Classic Parts have worked.

Bought them May 2015 from "Truck Shop" in Orange, CA. They were manufactured by Counterparts.

Never ever having the need for a Runtz, I may be taking outta my hat. But I do know they have current limits. I am under the impression that a Runtz is in the 10 to 15 buck range. What about running two Runtzs in parallel to share the load? Also, what if anything shares the output of the Runtz?
Be careful on the MOPAR reducer. I have worked on some that at first, confused me. They would power up certain gauges then power would go off. Found out the reducer was built like a turn signal can with a heated bi-metallic strip.

Voltage has no effect on the gauge sender, The sender is a variable resistor and the gauge reads the resistance. 6 or 12. If it reads only half way something is bad in the swing arm or the winding in the sender

I have seen where there is a short in the winding from powering the wrong side of the gauge

Beltfed- Runtz was on the fuel gauge only.
Dave- I tested the sender with my multi-meter and the swing arm tracked great 0-30 Ohms as I raised the arm. Wouldn't the multi-meter reading equate to the gauge needle moving from empty to full?

I just checked the power lead to the fuel gauge. It comes from a fuse block and not the ignition. The fuse is a BUSS AGC 20 AMP 32V. I noticed on Speedway Auto Parts the Runtz reducer has a 4 AMP max rating. Could this be causing my reducers to fail?

Well, the fuse is too large for sure. The Runtz failure would be downstream of the device if current related.
I don't know your year of truck at all but want to take a stab at something, please no unkind words.
You said your sender is 0-30 ohm's. Is your gauge calibrated for a 0-30 ohm sender? I have worked on many gauges and the sender resistance for different models can be all over the place, i.e. my Packard was like 87-8 ohm's (no that number is not backwards).
Have anyone ever lifted the battery cable off the battery while the engine was running and if it's got an alternator? Hotrod informed us that that is a big no-no on modern alternator cars. The battery acts as a load and without it the alternator voltage goes way up and might fry a Runtz type device which I believe is a semiconductor.

Truck is a 54 Chevy 235. Yes, the gauge was calibrated from 0-30 ohms last February and rechecked today at a local speedometer shop. It was right on the money. I watched them check it on what I would call a Resistance Checker box with a rheostat knob and a number counter, set at 6 volts without the Runtz. He checked with the Runtz on at 12 volts and the gauge hardly moved even as he turned the knob up. The truck has an AC Delco 1959 12 volt 30 AMP generator with a power steering pump on the back, Model 1102115. No, the battery cable has never been removed while the truck was running.

Thought I'd ask.

No problem. I appreciate all the input I can get.

If it were mine, I would go to the trouble to remove the gauge cluster and the sender and test them on he bench. After you are done and go to put the gauge cluster back in, do yourself a big favor and use knurled brass nuts which you can tighten without tools. I got mine from Ace hardware.

FYI, I bench tested one of my Runtz reducers through a 12 volt car battery with my multi-meter and it is working properly at 6 volts. The other one I tested jumped all around never settling down to 6 volts. I then bench tested the Runtz on the fuel gauge with the sender and again the needle would not move. I started to think the gauge is now the culprit but I wanted to check the Runtz again. My thought was that somehow the Runtz gauge mounting hole was not making contact with the hot post on the gauge. The little 1/16" sides of the post hole on the Runtz would be the only part that really makes contact with the gauge post. The bottom of the Runtz, with the most contact area, is sitting on the cardboard post divider. With the Runtz hooked up to 12 volts I used my multi-meter to probe the mounting hole sides on the Runtz where I felt it would come in contact with the gauge post. Well, it was sporadic at best. Some spots would get a 6 volt reading and some got no reading at all. I then tightened the Runtz on the gauge but gently pulled the Runtz to the side hoping the hole sides would contact the post better. I then bench tested everything as a unit and the gauge worked perfectly with my sender swing arm. I'm not going to use a Runtz,too iffy, but use a 6.8 Volt Zener Diode, Part CZ5342B CT-ND instead.

Thank You everyone for all your help. I'm terrible at working through electronic problems but with your help I was able to finally get a positive outcome.