Fuel gauge power problem? potentially the alternator? grounding wire?

[jcarney1995]

Hi guys, ive got a 98 300tdi that seems to be playing up ever since it got a new battery ( could be that? ) ive noticed after a full charge over time my battery loses charge, or so it seems. It starts with my fuel gauge not showing a full tank when it has a full tank, the more electronics i turn on in the car, i.e radio, headlights, indicators etc. the lower the fuel gauge goes, i cant figure out if this is an issue with the alternator not charging, the battery being a dud, or my uncle said something about potentially a faulty earthing wire? was hoping someone could help nail down what it is before i go buy a new version of it haha, also if its the alternator does anyone know how much hassle it is to install an aftermarket uprated one? and if it is worth it.

 

cheers

john

Edited November 9, 2020 by jcarney1995

[Peaklander]

Check the earth point on the side of the transfer box. It’s on the right side underneath the driver’s side on a rhd vehicle. You can see it from underneath. 
 

Also you can’t assess the state of charge without a voltmeter. 

[cackshifter]

2 things to do before replacing anything: Clean the the connections carrying high current, esp between engine and chassis, and get yourself a multimeter. Measure the voltage at the battery when the engine is running reasonably fast , ie battery charging, and then turn stuff on and see what happens. It sounds like at a complete guess maybe a diode or two has failed in the alternator, you can get some charge but not the full current. But otherwise you are faced with randomly changing things at considerable expense. To fit a 100amp alternator is fairly easy (Discoveries had them) but you need to run a thicker wire  from alternator to the starter to handle the current.  There are some 120amp Chinese versions about too, or there is a 90amp Mondeo one that fits. But if you have poor connections it will be no use.

[jcarney1995]

18 minutes ago, Peaklander said:

Check the earth point on the side of the transfer box. It’s on the right side underneath the driver’s side on a rhd vehicle. You can see it from underneath. 
 

Also you can’t assess the state of charge without a voltmeter. 

when you say check it, do you mean just check it has contact with the chassis?

[jcarney1995]

19 minutes ago, cackshifter said:

2 things to do before replacing anything: Clean the the connections carrying high current, esp between engine and chassis, and get yourself a multimeter. Measure the voltage at the battery when the engine is running reasonably fast , ie battery charging, and then turn stuff on and see what happens. It sounds like at a complete guess maybe a diode or two has failed in the alternator, you can get some charge but not the full current. But otherwise you are faced with randomly changing things at considerable expense. To fit a 100amp alternator is fairly easy (Discoveries had them) but you need to run a thicker wire  from alternator to the starter to handle the current.  There are some 120amp Chinese versions about too, or there is a 90amp Mondeo one that fits. But if you have poor connections it will be no use.

is there any advantages to the bigger alternator? i just assumed it would be a good investment as i begin to add more electronics to the car, i.e a diesel heater, led headlights etc.

[Peaklander]

@cackshifter gave a very comprehensive reply which is all valid.

The reason that I was specific about that particular transfer box earth point, is that my fuel gauge (1996 300TDi) started to go haywire, especially when I pressed the brake pedal. After cleaning the other earths that I was aware of (chassis strap, bulkhead earth ring stud near the pedal boxes), I found the other one on the transfer box and that was definitely the poor one which fixed the problem.

[jcarney1995]

15 minutes ago, Peaklander said:

@cackshifter gave a very comprehensive reply which is all valid.

The reason that I was specific about that particular transfer box earth point, is that my fuel gauge (1996 300TDi) started to go haywire, especially when I pressed the brake pedal. After cleaning the other earths that I was aware of (chassis strap, bulkhead earth ring stud near the pedal boxes), I found the other one on the transfer box and that was definitely the poor one which fixed the problem.

multimeter is now in my amazon basket, will have ago at testing whilst running and clean up the earth points

[Peaklander]

5 hours ago, jcarney1995 said:

when you say check it, do you mean just check it has contact with the chassis?

Sorry the thread went weird and I didn’t see the reply. The earth point I’m referring to is a connection to the transfer box case. Undo it, clean all contact surfaces to shiny ( there’s two ring terminations I think, plus the bolt into the case), coat them with a smear of Vaseline and then reconnect.

The strap from engine to chassis and the cable from chassis to battery negative is the rest of the circuit. 

[jcarney1995]

10 hours ago, Peaklander said:

Sorry the thread went weird and I didn’t see the reply. The earth point I’m referring to is a connection to the transfer box case. Undo it, clean all contact surfaces to shiny ( there’s two ring terminations I think, plus the bolt into the case), coat them with a smear of Vaseline and then reconnect.

The strap from engine to chassis and the cable from chassis to battery negative is the rest of the circuit. 

cheers dude, its now on my to do list along with buy a multimeter! 

[cackshifter]

With the engine running and so alternator charging you should see about 14.5 volts at the battery. If not being charged it will probably be about 12.6 -13.  The fuel gauge is effectively a voltmeter so it suggests there may be something going on. If you put on headlights, heater fan HRW, and maybe wipers it should still maintain the 14.5 volt with the engine running. Ignoring any heated screens, bass amps, winches, beer fridges etc that may have been added that is about the most standard electrical load it has to handle. If that works Ok, the charging circuit is probably Ok, you may then be looking at a fuel gauge problem.

[jcarney1995]

2 hours ago, cackshifter said:

With the engine running and so alternator charging you should see about 14.5 volts at the battery. If not being charged it will probably be about 12.6 -13.  The fuel gauge is effectively a voltmeter so it suggests there may be something going on. If you put on headlights, heater fan HRW, and maybe wipers it should still maintain the 14.5 volt with the engine running. Ignoring any heated screens, bass amps, winches, beer fridges etc that may have been added that is about the most standard electrical load it has to handle. If that works Ok, the charging circuit is probably Ok, you may then be looking at a fuel gauge problem.

the problem is with everything on i sometimes have the issue of indicators not working or headlights turning off, thats when it gets really bad, i usually find to fix that i can just recharge the battery and put it back in but then it seems i lose charge over time. Ive also noticed the battery light coming on when i start the car but it goes away after a few revs of the engine. But yeah basically i tihnk that rules out it being an issue with just the gauge, it seems it can be any and all electronics in the car, fuel gauge just seems to be first to take the hit 

[jcarney1995]

2 hours ago, cackshifter said:

With the engine running and so alternator charging you should see about 14.5 volts at the battery. If not being charged it will probably be about 12.6 -13.  The fuel gauge is effectively a voltmeter so it suggests there may be something going on. If you put on headlights, heater fan HRW, and maybe wipers it should still maintain the 14.5 volt with the engine running. Ignoring any heated screens, bass amps, winches, beer fridges etc that may have been added that is about the most standard electrical load it has to handle. If that works Ok, the charging circuit is probably Ok, you may then be looking at a fuel gauge problem.

out of curiosity would an alternator charge a battery that is loosing charge, back up to full charge? perhaps its just an issue of losing charge and not necessarily the alternator ? 

[Peaklander]

The alternator will supply enough current to power the loads even if the battery is discharged and enough to charge the battery too. It takes time to recharge though and it could be a few running hours. With less load (no headlights, wipers, heated screens) then the alternator has more of its output available to push into the battery.

The multimeter will give you a much better indication of what's happening but there is no harm in checking the earths anyway. Just don't invest in a new alternator just yet.

[FridgeFreezer]

A bigger alternator is not going to hurt anything but may well be utterly pointless - most of the time the loads are below the alternator's output anyway, it only works hard for a few minutes after starting, maybe a bit longer in the winter.

TL;DR if your battery voltage is ~13.8V (or whatever your alternator's set-point is) when running, a bigger alternator is going to do nothing for you.

I posted measurements of alternator current in another thread but jiggered if I can remember where

The long version is add up your loads, add maybe 5-10A for battery charging and compare that to your alternator's rating - you've got to be trying pretty damn hard to continuously draw more than about 25A in a TDi Defender.