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  #1  
Old 05-11-2008, 11:53 AM
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Stock fuel gauge/modern sending unit

I replaced the tank in my '48 panel truck with one I bought on uPay quite some time ago. The guy I bought it from said it was a take off from a new IH diesel truck being turned into a firetruck. They used the tanks to transport the trucks to the factory and then installed larger tanks.

The questions I have are:

Will the sending unit work with my stock guage. I searched the site trying to find information before posting this question but came with one post that had a little information related to my question, the other were about swapping gas tanks from Mustangs, not specifically about gauges/sending unit combinations:

http://www.ford-trucks.com/forums/63...auge-56-a.html

Quote:
Originally Posted by Randy Jack View Post
The stock gage is looking for 73 ohms full and 10 ohms empty.
I checked the sending unit and I get 74 ohms at full and 10 ohms at empty. Randy I think was talking about a '56, since the subject was a '56 and Randy's truck is a '56. Are the gauges basically the same as the '48-52 gauges? I would assume so but I could be wrong.

Also, since this sending unit is out of a tank that is about ten years old it had to be a used in a 12 volt system. Does this matter? I wouldn't think so, but again, I could be wrong. It does have a pretty thin wire on it but I could change it if it needs to be larger to handle the 6 volt system.

One thing, as I mentioned above, the tank was out of a diesel truck. Would they have used a different type of sending unit in diesel than in gas? Diesel is less flamable, and explosive, than gas. Is using a diesel sending OK to use in a gas tank. Again, I would think so but I don't want to find out the hard way.

I have always found it interesting to have what is basically an electrical switch, which I think could produce a slight spark, or at very least some small amount of heat, sitting inside a gas tank. My thoughts are that the completely submerged unit is denied air, one of the three needs for ignition, but what about when the tank is close to empty? Also, this might play a factor in it, in the industrial control systems I build the relays that control the motors are usually wired with the neutrals being switched off by the thermal overload protection device. I asked once why it's done this way and was told it prevented arcing in the device. Maybe someone more knowledgeable of the science of eletricity will be able to answer this question.
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Old 05-11-2008, 03:01 PM
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Bob -

The only info I have is this chart, which talks about "pre-1987" Ford bi-metallic gages. That may or may not mean as far back as '48.

I would think 12v vs. 6v does make a difference.

Click the image to open in full size.
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Old 05-11-2008, 07:35 PM
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Randy why would voltage matter? I know I asked about this in my original post but I would like to know what the effect is. I would think the rheostat is allowing the correct percentage of voltage for whatever voltage system it's working with.

I think the sending unit would be safe to use in a gas tank as it is in a diesel tank but I just wanted to hear if anyone would have an opinion on using it.
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Old 05-12-2008, 07:40 AM
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Fomr your readings you need a pre-87 Ford gauge...
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  #5  
Old 05-12-2008, 08:02 AM
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I'm using the stock gauge in my '48 panel.

I'll play around with it tonight to see what I get.

I haven't heard any negatives about using the diesel unit in the gas tank so I'll try it. I hate screwing around with gas, it's so unpredictable. One thing for sure it will go boom! if you don't respect the dangers Click the image to open in full size.
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Old 05-12-2008, 02:43 PM
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Hi Bob!...Is this a test?..LoL I've always been taught voltage is consumed over the load..If the component works,there will be 12v
(6 v in your case)before the component...and less than .01 volts after...voltage drop. I am not sure what your gauge is looking for, but I suspect it is resistance in the sender , causing a voltage drop.
No voltage, no spark. Its always good to be safe though, and I like to find answers to questions that make things go BOOM.
Since your going to poke around and have some fun with it tonite anyway, let's measure the voltage 'To" the level sensor ,AND lets measure the ground side of the circuit and then we can maybe apply some Ohm's Law...Just for even MORE fun , let's measure the resistance Thru the level swx..we're bound to fiqure out this riddle!
This is FUN isn't it!!?..LoL..DVOM #'s only please..they work better in the calculator...Sorry if I sound like a smart-aleck, it's slow around here today..and I think BobbyT is wearing off on me..LoL
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Old 05-12-2008, 02:57 PM
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Sending units have different ohms and the ohms need to be the same as the gauge or it won't work. I recently went through this costly episode with changing out my gas tank. I assumed my gauge was stock 56 Ford truck gauge. So I ordered a sending unit from CPP for that. The gauge didn't work and the needle slamed to full as if there was a short. So I purchased a different ohms gauge for aftermarket gauges. (there are only two supposedly). Installed that, and the gauge worked better but dropped from half full to empty when the sending unit should of showed full. Finally, I just decided to take the mechanism off of my original tank and transfer it to the new sending unit which I purchase. As soon as I hooked it up, the gauge worked. I purchased a voltmeter/ ohms meter (the digital) type that tells you what ohms your sending unit is sending at. You can also take your guage and sending unit to a gauge place and they will adjust you gauge or set you up with something that will work.
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Old 05-12-2008, 03:10 PM
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For what it's worth, the explosive range of gasoline is 1.4% to 7.6%. Anything richer that 7.6 percent won't ignite. I don't know what the concentration would be in the vapor space of your gas tank when near empty, it would vary with the ambient temperature.
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Old 05-12-2008, 03:32 PM
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I measured the potential of the newer unit with a digital meter. I got the 74 ohms at full and 10 at empty. I just dug out a stock sending unit out of my parts stash (it's at my shop, I was at home when I first posted) and took a reading of the potential. It gave me some wierd reading so I suspect it might be defective. The readings I got off the stock unit was practically 0 at empty and any where from 64 to 130 at full. The meter jumped around at full and most times settled on about 65.

I haven't had a chance to hook the newer sending unit up to the stock gauge yet but wanted to tap the minds on this board to see if anyone has done this before, using stock gauge with newer sending unit, before I hooked it up and wrecked something. I've read the posts about swapping different gas tanks but it seemed like everyone who did the swap upgraded their electrical systems and used an after market gauge that matched the modern sending unit.

I'll try to get out to the truck tonight and see how new unit works with the stock gauge. If the potential from the new unit is different that what the stock gauge is looking for I might be able to tweak it with a resistor or two.
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Old 05-12-2008, 04:27 PM
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Hey Bob, you wouldn't have a electrical schematic for that application do you? Just wondering if the sender info is grounded at the chassis ,or it returns to the gauge, I'll dig up some info in the meantime...Later....


Somehow I just "know" you have a schematic in your collection...LoL
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Old 05-12-2008, 04:37 PM
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Sure do. The sending unit is grounded:

Click the image to open in full size.
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Old 05-12-2008, 06:22 PM
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I thought for gas guage to sending unit matching it was merely a matter of the gauge and ohm range matching as has been discussed. To use a 12V sending unit with a 6V gauge I think you just needed a voltage reducer. I just bought a stock ohm gauge from F100 Parts and it came with a 6 volt reducer to use it with my original 53 gauge.
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Old 05-12-2008, 11:06 PM
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This isn't a test...it' a TRAP!!! You original guys are alway's trying to get us "darksiders" to blunder into some oldschool, WWII airplane technology "electrical" twighlitezone...
I ain't buying it!...LoL ..
I got a chance to look at Bobs schematic, and to tell you the truth, it looks exactly like the one in my 55 repair manual...But I dont see or understand the ground path at the level swx.
I also am wondering about the "bi-metal" swx/contacts , at the sender, ..on the 55, there are 'contact-points" on the bi-metal strip(inside the coil) which heat up as current passes, bending the stip, which breaks contact at the points ...untill it cools down enough to make contact again..then opens again...kinda like a WWII airplane circuit breaker...
If the tank is "full" the float increase's pressure on the contacts ..requireing more current to bend the Bi-metalic strip ..and moving the needle on the gauge to move likewise....Anyway , that's my story , and I'm sticking to it....LoL Good Luck!!
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Old 05-13-2008, 06:59 AM
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Prior to 57 the pulse senders were used for all fuel, temp, and oil press gauges. In 56 when Ford went to 12 volt systems the same pulse sender design was used but redesigned to work on 12 volts. In 57 for some reason Ford went to 6 volt gauges again and began using the constant voltage regulators (CVR) to drop the voltage for the instruments. Starting in 57 all sending units were changed to the resistance type.

So 56 gauges are very unique being 12 volt and not generally compatible with other years. Instruments are very sensitive to the supply voltage. If only half the voltage is supplied to the sending unit it will transmit only half the current. The panel gauge in all years works with a bimetal arm and heating coil. The amount of heat produced by the coil is proportional to the square of the current through the coil (I˛R). So with half the current you get one forth the reading.

My knee jerk reaction has been that the pre 56 gauges designed for the pulse senders are not compatible with modern resistance senders, but seeing that the design of the 57 and later panel gauges are practically the same as the earlier gauges I think they could work with a resistance sender. The resistance range required for the sender I don't know. Probably easiest to determine by experimenting with a known good panel gauge and a few resistors. You cannot determine it by measuring the resistance of the pulse sender. Once you know what resistance range you need for the sender then you may be able to make the sending unit compatible by lengthening or shortening the float arm or installing shunt resistors or something.

I recall an excellent thread about a year ago where a guy installed modern gauges in an old panel. I think he put something like 70's or 80's gauges in a 48-50 panel. The thing was the face plates were practically identical. That might be a good option for this situation. I've searched for that thread using all different key words I can think of but can't find it.

I've often wondered how having an electric fuel level sender or fuel pump in a gas tank can be safe. Seems to be more of a problem with modern airliners than our old trucks. I see where the electrical part of the old sending unit is separated from the fuel tank by a diaphragm. Is your diesel tank unit similar?
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Old 05-13-2008, 08:06 AM
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That's very good info there dmptrkr..Thank's!..clears things up,
I was thinking along the same line's last night,..find a known "good" sending unit, and measuring the resistance, to find a compatable unit. Searched my stuff till I realized, hey , this system does'nt work like like modern stuff , so why bother...
Pulse-senders...Hmmn..I did read in the 55 manual, to check the gauge , substitute a potentiometer for the sender, adjust it to 1.5 volts( across the poles on the fuel gauge) and the meter should read 1/2. Dont know if that helps or not...it also says if you nneed to check the sender, replace with known good unit,again not alot of help. Your explanation helps alot though...like dropping out of the clouds in a WWII airplane ,somewhere over Bermuda...
I think Bob is in a bit of a fix here...he ,needs a known good unit, and the question still remains..will the diesel unit work?
I think more research is needed on the compatability issue, in an effort to prevent a mushroom cloud where the old truck used to be parked...LoL
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Old 05-13-2008, 08:06 AM
 
 
 
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