Here's the situation. Using a 56 chrysler dash in my hot rod, including the original gauges. The fuel sender I want to use (fuelsafe SU-024) requires me to specify the Ohm rating of my gauge. So, I thought simple enough, hook a multimeter to the gauge, move the needle, find the rating. Wrong! Called fuelsafe, they say sorry, no help. Sooo, I'm guessing that I can rig something up to find the gauge's rating. My crude schematic is attached. Am I on the right track, or am I about to smoke my coveted NOS gauge? Or better yet, does somebody know the ohm rating?
couple of web sites says , The fuel gauge sender should vary from around 10 Ohms for full to around 70 Ohms at empty.
the ohms(the amount of neg. resistance there is at full and empty. in referance to the voltage 6 or 12. but yea if it is a 12 volt dash then it should be 10-70 if its 6 it should be half that?
You could go to an electronics store and buy some fixed resistors, maybe some 10 ohm ones and some 50 ohm ones. Usually resistors come in packs of 5 and they're pretty cheap, so you could add some in series to get a variety of different resistances. When you put resistors in series, you just add the values together to come up with the total resistance. Put a 50 and two 10 ohm resistors in series and you've got your 70 ohms to test the gauge with pretty accurately. Resistors are rated in watts. I don't think the gauge draws much current, but to play it safe you could get 1 watt resistors. 1/2 watt or 1/4 watt might run pretty warm depending on how much current the gauge coil draws.
Thanks everyone. I've seen in charts that "most" chrysler and ford are 10-70, but know with my luck that I'd have the one that wasn't. Rustybolts, that's an interesting angle on the test. Might have to try.
I am confused with your diagram. As far as I know, you do not want to send positive juice to the sender. Sender feed goes to gauge (multimeter +/- to ground) with a ground wire from sender to frame ground.
Yeah, I wasn't following your schematic either. I'm not sure what you were trying to measure there with that multimeter. If you want to use a rheostat, hook up your ohm-meter alone to the rheostat and dial it until you see 10 or 70 ohms. Then don't turn the dial while you hook the rheostat to the sender terminal of your gauge with 12 volts going to the gauge power. The other side of the rheostat goes to ground. Don't have the multimeter hooked up to anything while you're doing the test. Whenever you measure resistance (ohms) in something, it needs to be taken out of the rest of the circuit. A battery in your ohm-meter provides the voltage to do the test. You would probably burn out your ohm-meter if you had your multi-meter turned to ohms like you show in that diagram. I think a pack of resistors is only 69 cents or so. That's probably the easiest way to check it. Some rheostats are 0-10,000 ohms or so depending on what they're used on (like a volume control on a stereo), so you're not going to get much accuracy down in the 10 to 70 ohm range. Another name for a rheostat is a potentiometer. A wire wound rheostat might have definite steps in resistance as the rotating arm sweeps across one coil of resistance wire after another.
The stock sender is just a variable resistance ground. My attempt with the rheostat is to vary the resistance in the ground wire, mimicing the sender. You still have to run 12V to the gauge, in order to have something to ground. It would be less confusing if the multimeter went just to the rheostat posts, but still should work fine the way I've drawn it. The multimeter is drawn in to measure resistance to correlate to gauge movement. Rusty's point about dialing in the rheostat and unhooking the meter is a good one. Testing would involve setting the rheostat to a couple different places and seeing if the gauge trend followed. Either way, the rheostat's sensitivity is a moot point. Just visited my big box of household electrical stuff and didn't find the rheostat switch I expected to find. Suddenly a bag of resistors is the cheap solution. Thanks again for the idea rusty
Just don't have the battery and the multimeter hooked up at the same time. If you do, the juice from the battery will go through your multimeter and burn it out.
OK, update 70-10 is not this gauge's range. Bought a pack of 10 ohm and 33 ohm resistors today and did testing. Everywhere from 10 to 165 ohm pegged the gauge full, although higher resistance values didn't seem to peg it as hard. 10 ohms that puppy almost shot off the gauge. Going to try again with a series of higher range resistors to narrow in on the range.
What they are asking is for the resistance of the sender that works with gauge. Most instrument clusters I've worked with have a regulator supplying regulated voltage to the gauges. So stock gauges might not work with 12 volts to begin with - probably six. You need the resistance reading that gives the gauge an empty reading using the correct voltage to give a full reading. Tracy
Looks like maybe the drawing is wrong, you need to have the gage housing connected to the battery too....here's a typical gage connection diagram. You would replace the sending unit with the resistors for your testing. I would be really surprised to see a gage that did not show some reading with 20 to 50 ohms worth of sender resistance on it.
Hmmmm, me thinks the Squirrel may be onto something. Thus far I've not grounded the gauge body, only the terminal. Looking at the gauge movement I can see copper bits riveted to the metal back panel that's insulated from the terminals, a sure indication it should be grounded too. Will try again tomorrow and thanks for the input everyone
Leave the multimeter off for the time being. Adjust the rheostat until your meter reads full. Yank all the wires and measure the rheostat. At least my meter reads empty on 0 ohms and increases as resistance increases.
Oh this F-ing thing is killing me. So today, I follow Squirrels advice to ground the gauge body, and voila! It works! Using different series of assembled resistors (no rheostat on hand), determine the gauge needs 10-20 ohms for full, around 170 empty. This matches up pretty closely to double the standard mopar sender ohm rating of 70-10. What might do that??? Maybe something like a 6v gauge running on 12v is what I think. So now it's back to a question I thought I answered a long time ago----Is a 56 chrysler 12V or 6V???? Last time I checked around the consensus was 55=6V and 56=12V. Even double checked the ohm ratings on an old junk fuel gauge that I took out of a 56 personally. Same range as the NOS gauge I plan to use. To didgeytrucker's point, I've never seen a 6V dash converter unless it was a 12V converted car. But then again I've mostly dinked with GM stuff. Do you suppose in 56 chrysler went 12V but the dash had a 6V regulator so they could continue using same-as-55 gauges? 55 & 56 were the same body, only dashboard changes were shifter placement Arg. At least I think the sender question has been answered. 70-10. Now it's just a matter of deciding how to deliver 6V to the dash. On a side note, do ammeters need converted to switch 6V to 12V? Part of me says yes, part says no. On a second side note, the gauge does work on "ground comparison". For shits and giggles I fiddled around with adding resistance to the gauge body ground too, and it directly subtracts that amount of resistor reading from the sender wire resistor sum. Thus proving why good dash grounds are so important! Thanks again for any advice
the 60s mopars used a voltage regulator for the gages, so it would not surprise me if it was designed to run on 6v. A 6v ammeter will work fine on 12v.
Well there's the final piece of the puzzle right there. If they were doing that in the 60s it's a probably a moot point what year 12V started. Here I was all happy to have found 56 "12 volt" gauges and wouldn't have to dink around with having any 6V in the car. What a dumbass. Thanks again everyone, even if the answer was in the first reply, the journey was interesting and I learned something.
http://www.jalopyjournal.com/forum/attachments/gasgagewiring-jpg.361079/ Check out my attachment. I would just install the ohm reader between the terminal on the sender and the negative connection on the gage. Then as you move the sender float, your ohms will change to show the minimum and maximum ohm reading.
Per my poorly written notes, 56' has a 12 volt Autolite gauge, 200-10 ohm. Chrysler used Autolite gauges for many years, from 40-48 they had the 2 wire style dual bimetallic thermal gauge (a real pain) and the 6 volt magnetic gauges were 120-0 ohms. I'll double check, but I think they went to the King Seeley thermals with external voltage regulator and 78-10 ohm sender around 1960. www.morrisgauge.com
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