The Stovebolt Site Forums The Shop Area The Electrical Bay What happened to my gas gauge?

Hi guys,

Everything was working just fine on my 37 1/2 ton, all my gauges, etc. Then for some unknown reason, after driving about 10 miles one day, I blew a fuse in the ignition switch. I changed the fuse and everything worked again, except my gas gauge. When I check with my light tester, I have power on the ignition side of the gauge when the ignition key is turned on; but there is nothing on the other side of the gauge or at the sending unit. I figured that the gauge was blown, so I just changed it with another (supposedly NOS) gauge that I had. Same thing: power to one side when the key is on, but not to the other side or the sending unit. Any thoughts? Is this NOS gauge shot also or am I missing something? Thanks for your thoughts.

Jim

Hey Jim- Long time no see! I'm not an electrical expert, but I'll try and confuse you some. Oops, I meant help you some. First, I'm not sure your ignition switch should be fused. When this fuse blows, you engine dies? Or is this just fusing accessories powered when the ignition switch is on? Where is this fuse located? Second, I think the sending unit works like a variable resistor to ground. As the float rises and sinks it changes the amount of resistance at the sender. The gauge reads this amount of resistance and displays it on the gauge. If I'm correct in this thinking, one side of the gauge is hot (like you have) and the other is ground. (sender side) I think if you ground the sender side of the gauge it will read full. This will verify that the gauge works. (try this on your old gauge, if I'm wrong I don't want to owe you a NOS gauge!) The problem is most likely the sender. Are you still 6-volts? I would like to know more about the fuse that blew and what you think caused it to blow.

Hi Barry,

Thanks for your reply. I've been lurking on the site, but pretty busy so I haven't been saying much.

I misspoke about the fused ignition--I should have said that the fuse on my ammeter blew. And I am really puzzled why it did. I have converted the truck to 12 volt, following Randy's booklet, and I've put a Runtz voltage reducer on the gas gauge as per his instructions. The sending unit is a fairly new after-market item, which up to that point was working well. Do you or others know what voltage the fuse should be in the ammeter--20 or 30 V? Is it possible that somehow there was a power surge that went throught the Runtz in the gas gauge and cooked the sending unit? What might have caused the fuse to blow in the first place? My thought was that maybe two wires/connectors got jostled around and inadvertently made contact, or some such thing. I also have a fused light switch and a fuse on my windshield wiper motor (which is an aftermarket electric one). I haven't tried your grounding suggestion yet on the sending unit, but I will if that's the way to determine what's wrong. Anybody else have a thought on this?

Again, any further advice or wisdom is greatly appreciated.

Yours,
Jim

Have you checked the sender unit?
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Hook up an ohmmeter and you should get a steady increase/decrease in your reading depending on what way you move the float-arm

The Runtz has a built In circuit protector (think time delay fuse) so in the event of a short in the wiring harness caused by a bare wire, or a pinched wire, or some other defect, the Runtz will protect your gauge. You gas gauge runs on about a 1/4 of an amp, the Runtz will act like a fuse when anything over 1 amp makes it way to you gauge.

In this case you could have destroyed the Runtz as a result of the short in the wiring harness. That is of course better than destroying your gas gauge. I would try and find out what caused the short in the first place. Usually it is a place where the insulation has fallen off or a wire has gotten pinched. Once you have fixed that, check the Runtz it should have about 7.85 volts on the output side. If it has zero output you can simply replace the Runtz and you will be back in business.

Also...it works best to power your ignition switch with power directly from the battery. Simply run a 10 gauge wire from the positive post of the battery to the ignition switch, Many modern battery cables have this wire already there.

The reason for this is to prevent voltage spikes. When the contact points inside of the starter solenoid close, or in the case of the foot starter button, when that contact is made a voltage spike is created. It is about 300 volts!!! It does not last long, just a mega second but it is enough to destroy solid state accessories like a modern radio (you will see the led numbers go away first) or things like cell phones plugged into cigerette lighters (you will send your startac back to the homeland) and a Runtz on a rare occasion, or the fuse to the ignition switch.

If the power to the ignition switch comes from the battery the battery will absorb all of the voltage spikes. You should have what the engineer types refer to as clean electricity, which will prevent future social embarrasment and disgrace.

Thanks, Randy

Thia is a little off the gas gauge subject. Most of school bus fleet has digital radios and in about 1/3 of them, the lights and LED are burnt out. Could our Start All be killing them whem we have to jump start one with 300 + amps? Your power spike makes me think so. Joe

Joe....The odds are very good that is what's happening. You might want to be sure the radios are turned off when jump starting or better yet, like in the previous post, move the power source of the ignition and radios to the battery, which will asorb the voltage spikes. It took me a while to figure this out also, only about one in 300 of the Runtz voltage drops would be affected. Because they are solid state I finally narrowed it down to the Flathead F*** cars and trucks having the most failures. Their solenoids create a nasty voltage spike when they get some age on them.

Stray voltage spikes have also been known to zap an electronic ignition module now and then....just for future reference. Randy