1979 fuel gauge issue

[sparks4540]

Hi guys. Quick question. I have a '79 Sea Ray 240cc. Fuel gauge went to full the other day. I pulled the sender, cleaned it and checked it with a meter. The resistance range on it is 200-40, so it appears to be ok. The gauge has 3 posts. I have 12V switched to 'I', good ground wire and black wire that's screwed to the side of the sender and into the tank to 'G', and the pink sender wire to 'S'. If I remove the sender wire alone the gauge does not respond at all. If I take my 20ohm resistor and jump S-G, gauge goes to Full. If I remove the good earth ground from the gauge I get no response at all. Does it appear the gauge has gone bad?? The only way I get a movement is if it's grounded at the 'G' post.

Also something interesting to note. If using the MM i measure from S to G with the black wire from the tank hooked up to 'G' and the pink sender to 'S', I show what the resistance from the sender is,,ie. 40-200 ohm's or so. As soon as I hook up my good battery ground the resistance drops to 45-65 if I am moving the sender float back and forth?? The gauge then moves slightly off full and back. I have run all new wiring from the sending unit to the gauge and I still get the same values. Any thoughts?? Thanks, Chris.

 

[glenn property of pam]

Re: 1979 fuel gauge issue

the gauge has 12v+, 12v - and sender terminals. the sender has 2 terminals which are sender and ground. the 12v- must be connected to the battery ground to power the 12v's the gauge requires. the sender ground (dc-) must be connected to the battery ground or connected to the dc- at the gauge which in turn is connected to the battery dc- post. the fuel gauge has its own resistance so if you measure the sender with the gauge connected you will not read the same ohms on a mutimeter. the only test is the one you have already done which is with the sender disconnected. i hope this ans your question.

 

[Silvertip]

Re: 1979 fuel gauge issue

You don't need a ground wire running from the metal shell of the sender all the way to the "G" terminal of the gauge. Run that wire to the NEG terminal of the battery since they are right next door. It is the entire "GAUGE SYSTEM" that needs the ground connection not just between the sender and gauge. If the gauge indeed has a solid ground back to the battery then grounding the sender to the gauge should work fine. If that ground connection is not good the system cannot work. As a test, run a jumper from the sender shell to the negative battery terminal. Leave the ground connection in place that you ran from the sender to "G" on the gauge. Now move the sender arm through its arc. Siince the gauge responds properly with a 20 ohm resistor then the gauge is OK. When you remove the ground the gauge, obviously the gauge can't work for the reason specified above. Current needs a complete path from the battery, through the ignition switch, to the gauge, biased by the sender, and back to ground. The gauge is essentially a voltmeter that is variable because of the sender.

 

[sparks4540]

Re: 1979 fuel gauge issue

Thanks for the input. I have tried every combination I can think of. I thought I had read that with 12V to I and a good ground at G with no sender wire, the gauge should go to E..Not correct? I actually have the sending unit out of the tank right now. I have a ground from the sender to the battery ground under the dash. I have 12V on the gauge, ground to G and also a sender wire from the unit to the gauge..all this setup right next to one another. Gauge simply stays near full and if I move the sending unit the gauge moves slightly less and the voltage measured between G and S varies from about 4.5V to 7V(as expected with the 45-65ohm measurement). Resistance with this setup is 45-65ohms depending on where I put the sending unit. Guess the question is why with the sending unit unhooked to the gauge, resistance is 40-200. Once connected the measurement is 45-65?

 

[glenn property of pam]

Re: 1979 fuel gauge issue

with out playing with it on the bench i think that its resistance in parallel.

if you put 2 off 100ohm resisters in series it makes 200ohms, if you put 2 off 100 ohm resistors in parallel then it equals 50 ohms. the gauge is a magnetic coil so it has a resistance calibrated to the 40-200ohms of the sensor if you try to measure the resistance while connected to the gauge then you are realy measuring a parallel resistance thus it is smaller than the original reading. the gauge may be bad as it should return to 0 when the sensor wire is unplugged.


to be honest ive never bothered reading a resistance with the gauge connected so i would ignore any result i got anyway. the only thing i can surgest is to remove the gauge completely if the dash is made of alluminum as the little plastic insulators on the back of the gauge may be broken giving a low resistance connection to the dash

 

[sparks4540]

Re: 1979 fuel gauge issue

Yep..that's what I was thinking about the resistance in parallel as well. The gauge is out of the dash right now, but the dash is wood and the housing of the gauge is plastic. I'm thinking that I will most likely just buy a new set of gauges and a new sender. These gauges have always worked flawlessly, but they are from '79 and it may be time for replacement. Thanks again..

 

[Silvertip]

Re: 1979 fuel gauge issue

Are you saying the other gauges are malfunctioning also? If so, then you need to revisit the ground issue. All of the gauges typically have +12 volts and ground daisy chained from one gauge to the next. If ground or +12 volts is broken at any point in that chain, the gauges after that point won't work. An yes -- if you are attempting to measure resistance with the sender hooked to the gauge, you are measuring parallel elements. Don't do that.

 

[sparks4540]

Re: 1979 fuel gauge issue

Other gauges work perfect and yes, the grounds are all daisy chained together. I'm just thinking the fuel gauge crapped out...I had an issue with it a few weeks back and disassembled it and found one tiny wire from the coil became unsoldered. I repaired it and it worked just fine for 2 trips. Last trip out, went to full and that was it..