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Help needed re: Tank sender Unit
#521509 December 17th 2011 11:27 am
Joined: Aug 2010
Posts: 323
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Hi all,
Currently have the tank out of my van (79 Dodge B200) as the positive post of the sender unit broke off. Have replaced it with the post from my old sender unit and thought I'd test the sender unit out of the tank before putting it all back. However when I connected it up the gauge now moves but will only show half full when the sender is set at full.

Any ideas on some tests I can do to isolate the problem?
Thanks in advance
Mark

Re: Help needed re: Tank sender Unit
3pinplug #521516 December 17th 2011 12:00 pm
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I had the same issue with my 85 GMC pickup, I ended up replacing the gauge for a JY part and it went all the way to full like it's supposed to, not sure if it'd be the same deal but if you have a multimeter pull out the gauge and run the test leads on the contactors for the gauge set for resistance and as the resistance goes up the needle/ float goes down.


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Re: Help needed re: Tank sender Unit
aftoyboy #521546 December 17th 2011 2:48 pm
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Hi aftoyboy
Thanks for the suggestion will give this a go in the morning, I'm over in the UK and its nearly 7pm and pitch dark now,

Re: Help needed re: Tank sender Unit
3pinplug #521547 December 17th 2011 2:52 pm
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You got it man.


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Re: Help needed re: Tank sender Unit
aftoyboy #521719 December 18th 2011 2:13 pm
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Hi Aftoyboy,
Pulled the gauge cluster forward but its an all in one unit and I have still have it connected at present, but the back is one large PCB but all accessible. Can you let me know how to proceed from here, can I test the resistance at this point or is some more work involved?
Many thanks
Mark

Re: Help needed re: Tank sender Unit
3pinplug #521900 December 19th 2011 3:54 am
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3PinPlug, how long did you leave the sending unit in the full position? The instrument cluster gauges are actually thermal devices and move extremely slowly.

Also, the gauge pointer may not settle in exactly the same location... Chrysler actually recommends tapping the instrument panel (-yes they do!) to position the pointer, and to allow at least two minutes (!!) for the gauge to reach it's correct value.

Empty is when the sending unit float is all the way down against it's stop. In this position the gauge should read "Empty" plus one or minus two pointer widths. With the sending unit float all the way up against it's stop the gauge should read "Full" plus two or minus one pointer width. When installed in the tank, the float should just develop interference with the bottom of the tank.

A lot of people have reported problems with the ground wire from the sending unit to the chassis. Have you inspected it? The strap on the sending unit which connects the sending unit's resistance element ground to the fuel line may also develop high resistance.

Chrysler fuel sending units are usually around 73 Ohms empty, and 10 Ohms full, though they may read anywhere from 10-30 Ohms full... due to manufacturing tolerances I gather.


If the instrument cluster loses it's ground, the fuel, oil and temperature gauges will usually read higher than normal. The ground for the instrument cluster connector should be a white wire with a black tracer, which goes to the round connector.

It's been a while, but I think that one of the white wires with a black tracer attaches to the dash panel metal for it's ground... perhaps down by the cigarette lighter? As a test you could attach any of the white wires that have a black tracer to the panel; they actually all tie together inside the wiring harness. (BTW, the headlight switch and delay wiper also tie to the dash panel for their grounds...)

The voltage limiter on the instrument cluster circuit board is the power source for the various instrument sending units. You could try re-seating it. It looks like a small rectangular metal can, a lot like a self-resetting fuse. It has a couple of blade terminals, which simply seat into contacts on the circuit board.

If you want to check the function of the voltage limiter, put a voltmeter between the temperature sending unit and ground, turn the ignition switch to the "ON" position, and you should see a slowly fluctuating voltage... somewhere around 5 volts, sort of. If it's a steady voltage or no voltage at all, there's a problem with the voltage limiter. Since the gauges are actually thermal, heat-driven devices, rather than voltage-reading devices, the exact voltage isn't critical, it just needs to rise and fall over time... the voltage limiter is exactly like a turn signal flasher relay - it heats up, then opens.

So in a nutshell, you're most likely looking at a faulty connection, or a faulty sending unit, which would also be a faulty internal connection issue. I included the info on the voltage limiter just so you can be certain of it's condition.


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Re: Help needed re: Tank sender Unit
Ram4ever #521938 December 19th 2011 8:48 am
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Hi Ram4ever,
Thank you for the extremely comprehensive list of pointers, i'm hoping you may have solved it first off!
I turned the ignition on and the gauge shot up to halfway then stopped so assumed it had finished and turned it straight off, would have onl been on for about only 10 seconds or so. Will try it again tonight but leave it for two minutes, didn't realise they were that slow so fingers crossed its this.
Checked the ohms on the sender unit (its brand new) and get 19 (definitely) to 97 (i think from memory)at the two extremes, but again will double check this evening. Wll also check the voltage regulator as suggested as well.Will work in that order then check the earth wire that you mentioned as well, I saw the white wire bolted to the back of the printed circuit board but have not carruied out any tests on it as yet.Thankyou for taking the time to type this up, much appreciated and will let you know what I find. Regards Mark

Re: Help needed re: Tank sender Unit
3pinplug #522086 December 19th 2011 5:02 pm
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Hi Ram4ever,
Have carried out the checks you suggested and its definitely the sender unit for sure, gutted about this as its brand new I brought it a while back and now fitting ti find it is faulty. The shop is refusing to take it back and wanted me to buy another from them - I don't think so!
My mission now is to find another unit in the next couple of days, so fingers crossed. Thanks again, your a credit to this site.
Regards
Mark


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