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Old 08-01-2007, 08:28 PM
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sending unit safety

While on this fuel pump/tank project I replaced my sending unit and gas gauge also and when wiring it up I found a tag on it warning that improper wiring would damage the sending unit and could be a fire hazard. That makes sense because if one should apply full voltage to the sender wire I would think the sender would get hot in a hurry. If I understand correctly a fuel gauge is an ohm meter that measures a variable resistance from the sender and should not pose a danger but is there any situation where a properly wired unit could be dangerous such as a damaged sender or maybe the wrong sender? I don't have a problem with mine (at least I hope not) but I could not help but wonder about this.

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Old 08-01-2007, 09:40 PM
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Doc here,

YES..

Most modern Senders contain the rheostat OUTSIDE of the tank (above the sender unit in the Little round cap) And are connected by means of mechanical linkage from the float which is in the media..

I have found (and surprised) that very old units (50's and under) were not that safety conscience..and had it all in the media, float, rheostat, wires the full monte..not a swift move...

A spark can occur at either end of the system..ground or power..it just doesn't care..if an immersed rheostat, even referenced to ground, were to wear out at the slide points, It could spark..sitting in 20 gallons of gas.."Boom"...

The Gauges are, (most) A Coil of bimetallic strip, with a mechanical linkage connected to the index pointer..As power is fed to the GAUGE and the strip is referenced to ground via the sender, the coil heats and cools according to the float (resistance value) level..moving the index pointer accordingly for us "Humans" to be able to read..

The power is ALWAYS at the gauge, dissipated through the strip coil to internal ground at "0" (gauge body) and varies from the change in resistance from the sender value..

POWER is NEVER run to a sender..EVER!

Some with Electric Pumps MAY appear to be that way, but it simply is not so..that is pump power only.

Another approach is a "Whetstone Bridge" gauge..which is a magnetic motor, connected to 4 IDENTICAL resistors in a "Diamond pattern" across the gauge..Power is on one side of the bridge, and a resistive value is added to the other (the sender)..All 4 values being identical (with no driven resistance) the motor does not creep or move..the system being ballanced..add the changing resistance value of the sender, and the motor drives in an attempt to Balance the system resistance out..you won't run across too many of these systems ...


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Old 08-01-2007, 09:59 PM
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Wow, That was not what I expected to hear but it is what I was afraid I would hear. As you said in this Ford set-up the whole shooten match is submerged in the tank and when I looked at that curved strip with bare wire wrapped around it and that little metal sweeper with a wire attached it I did get to thinking and that is why I asked. Ford still uses this system but in this case I have a 65 Mustang tank with an Autometer gas gauge but a Sun-pro sender,the Sun-pro (made in China ) was what the auto parts had in stock and the 33-240 ohms range was what was called for but now I am not so sure about this set-up. Using an Autometer gas gauge and the stock tank could you suggest an alternative to using that open sender? I am not a bit afraid to fabricate things and if there is a better set-up out there that I could adapt to this thing I would probably do it.
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Old 08-01-2007, 10:19 PM
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hi doc, ive read a lot of your previous posts and respect your vast electrical knowledge. i did a fuel pump in a c4 last week and it did indeed have the reostat IN the tank, all wiring for pump and sender went into the tank. the reostat was halfway down in the right where the float pivot arm attaches. the pump comes with a new wiring harness and they will not guarantee it unless the wiring harness is replaced at the same time, maybe thats why??
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Old 08-02-2007, 04:21 AM
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Quote:
Originally Posted by oldred
Wow, That was not what I expected to hear but it is what I was afraid I would hear. As you said in this Ford set-up the whole shooten match is submerged in the tank and when I looked at that curved strip with bare wire wrapped around it and that little metal sweeper with a wire attached it I did get to thinking and that is why I asked. Ford still uses this system but in this case I have a 65 Mustang tank with an Autometer gas gauge but a Sun-pro sender,the Sun-pro (made in China ) was what the auto parts had in stock and the 33-240 ohms range was what was called for but now I am not so sure about this set-up. Using an Autometer gas gauge and the stock tank could you suggest an alternative to using that open sender? I am not a bit afraid to fabricate things and if there is a better set-up out there that I could adapt to this thing I would probably do it.

Doc here,

Best I can offer on that is try and find an enclosed rheostat type, that has a larger resistance value than stock, will mechanically mount, (if stock value can not be had..)

then add an EXTRA external adjustable potentiometer within range (like 0 to 500 ohms at 5 watts) to the sender line to calibrate the sender within range of the gauge.

OR leave it as is..just if it starts to malfunction, be sure to replace or disconnect it right off..after all, it HAS been in there for years boom free..


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Old 08-02-2007, 04:34 AM
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Quote:
Originally Posted by techron
hi doc, ive read a lot of your previous posts and respect your vast electrical knowledge. i did a fuel pump in a c4 last week and it did indeed have the reostat IN the tank, all wiring for pump and sender went into the tank. the reostat was halfway down in the right where the float pivot arm attaches. the pump comes with a new wiring harness and they will not guarantee it unless the wiring harness is replaced at the same time, maybe thats why??
Doc here,

By C4, I assume you mean Jeep? ..

Jeep has it's own way of doing things electrically..not saying it's good, bad or indifferent..it's what they do.

Yes, you will STILL find those out there..even on some new cars..there is probably no set guideline for it..

BUT: here is some food for thought:

Remember when you were a kid, and had a Lionel Train set..and you'd run it around the track at different speeds ALL day long by the transformer controller handle..??

And by day 2, the handle would throw tiny sparks as you moved it and smell of ozone?

ALL that was...was a Rheostat connected to a power transformer ground side, then to one of the tracks..as you moved it along the rheostat to change the speed all you were doing is mocking a "Fuel Sender"...

Makes ya wanna Think , Huh?


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Old 08-02-2007, 08:34 AM
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After searching some I am not finding much in the way of sending units except for the in-tank type so I guess I will have to use that. The reason I was so concerned with my set-up is that it is not factory and consists of an after market gauge and a cheap imported Chinese sender of questionable quality (I have not installed it yet). I found that Autometer has a gauge that is set up for the factory resistance range and uses the factory sender which will obviously fit the tank and will not require making a hole in the top to install that junk I have now. Even though there are millions of those things on the road I still can not help but be a bit nervous at the thought of sticking that open coil into the tank and attaching a wire with power on it no matter how low the current, I feel a lot better using a factory sender and after this discussion I will no longer even consider that cheap one.

You are right it does make me wanna think, a LOT!
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Old 08-02-2007, 08:47 AM
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I have a hard time seeing a fuel tank "blow up". Because of the fact that no matter how much fuel is in the tank there is VERY little to no Oxygen in the tank because the Gas fumes displace all of it.

Now if you had an open system I might worry, but ALL gm trucks that I have changed sending units out on have had the contacts in the tank, right next to the pump. (and the pump is electric )

Until I hear of some cases of this happening I think your worrying for nothing.
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Old 08-02-2007, 09:02 AM
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You are right about the fact that the fire trucks are not exactly busy extinguishing fires from these things but that is factory parts. I have a non-factory gauge/sender and the sender that would be in the tank is a cheap Chinese made outfit that I will not use. As far as the oxygen being too low to cause an explosion I would have to disagree with that and I can just about guarantee a spark inside that tank would start a fire, a big one! I could get a better made autometer sender but I just feel better using the Ford part because it obviously has a decent track record, not so sure about the Sun-pro.
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Old 08-02-2007, 09:25 AM
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Quote:
Originally Posted by oldred
You are right about the fact that the fire trucks are not exactly busy extinguishing fires from these things but that is factory parts. I have a non-factory gauge/sender and the sender that would be in the tank is a cheap Chinese made outfit that I will not use. As far as the oxygen being too low to cause an explosion I would have to disagree with that and I can just about guarantee a spark inside that tank would start a fire, a big one! I could get a better made autometer sender but I just feel better using the Ford part because it obviously has a decent track record, not so sure about the Sun-pro.
its your own decision I mean lots of cars are out there using aftermarket senders with no problem.

and the Chinese unit works on the same principal.

Also, check your bearings the next time you put one in. I'll bet it says made in china or Taiwan. The reason is not realy the materials.(how ever SOME cheap stuff is made outta melted down bikes and garden gnomes) Its the fact that there is so much time consuming labor involved. Same machines are used, the difference is whether its a American pushing the button or a Chinaman. And the fact that there are no environmental standards to meet.

So were saving the ozone - they're cuttin holes in it!!!
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Old 08-02-2007, 10:52 AM
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All this talk of sending units in the gas tank makes one wonder exactly how safe it is to have an electric fuel pump in the tank...
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Old 08-02-2007, 11:05 AM
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docvette, shame on you, you have a C3 in your avatar. C4 is a vette also. and i used to have a C3 also, nice car. and i just reread your post, i ran my lionel train allday, how did you know that???

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Old 08-02-2007, 11:08 AM
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Quote:
Originally Posted by joe_padavano
All this talk of sending units in the gas tank makes one wonder exactly how safe it is to have an electric fuel pump in the tank...
perfectly safe, manufacturers have been doing it for years and i never heard of one blowing up!!!
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Old 08-02-2007, 12:57 PM
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Quote:
Originally Posted by techron
perfectly safe, manufacturers have been doing it for years and i never heard of one blowing up!!!
With factory parts I agree and that is what I am going to use, it is that Chinese sender I am concerned about.
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Old 08-02-2007, 01:07 PM
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[QUOTE=Holder350]and the Chinese unit works on the same principal.



The Chinese works on the same principle-Yes but is it the same quality? NO WAY! It is almost impossible to say quality and China in the same sentence.


[QUOTE]Also, check your bearings the next time you put one in. I'll bet it says made in china or Taiwan. .[QUOTE]


I will bet you anything you want to on that one! I REFUSE to buy cheap Chinese bearings and having been in the mine equipment repair business for many years I assure you I know the difference and where to get the right ones. Most auto parts have good bearings made in USA or Europe (or Japanese for that matter are excellent bearings) it is just that most of the time you have to ask for them. The funny part is that most of the time they are hardly more expensive at all.
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