MG-Cars.net

Welcome to our resource for MG Car Information.

Recommendations

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG TD TF 1500 - TD Fuel Tank Sending Unit

My Fuel warning light has not worked since our restoration. A recent leak around the sending unit revealed that the float arm was frozen. i pulled it, exercised it, wd40'd it. it now moves easily. (Yeah.) Before i found the leak, i thought it may have not been grounded correctly. The ? is, Where does the ground come from as the only wire to the sending unit is the hot side. Are we grounded from the tank itself, the fuel lines to the pumps? I just want to handle this when i replace the unit not after i fill up again. Help.
Earl Ehrie

The tank sits on rubber isolators. The fuel line is the ground. If you have an extra filter in line, it may not be grounded especially if rubber fuel hose is used to connect the filter into the line... so then you need to ground the tank with a separate wire to the chassis.

warmly,
dave
Dave Braun

Dave,
I guess sometimes, “ya can’t see the forest through the trees!” We have been driving our TD since I rebuilt it in 1996, and the fuel light never worked. It was just one of those things that I figured I didn’t need at the time, and would get around to fixing it later. We could drive almost all day on one tank, and as long as we filled up towards the end of the day, all was well.
Well, one of the first things I did when putting the car together was install a fuel filter in the line near the tank connected with rubber fuel line! And to make matters worse, it was a plastic filter !!!!! Now I have a good idea why the light never worked! DUHHH!
Thanks Dave, you’ve done it again!

SPW
Steve Wincze

OOPS,,, that should have been "since I rebuilt it in 1986" not 1996
Steve Wincze

That's OK Steve, you've helped me too. I'm building the AMCO rack support you demonstrated here. I'm currently in the varnish stage...

Oh, and as an aside, a fuel filter on the inlet side of the pump, if it ever clogs, will fail the pump. The pump is quite happy with a clogged fuel filter on the outlet side, as that is no worse than closed fuel inlet needles (full float bowls). But since you have 20 plus years of driving with your filter in the present location, you must be safe. You may want to service the filter though.

Have fun restoring your tank ground. I bet your light will start working!


warmly,
dave


Dave Braun

Dave,
Thank you! I have exactly the same issue. I had left this task for a winter day thinking it would take a fair amount of time.
Raj
R Thadani

Earl, Test the cicuit/socket/bulb by grounding the wire to the sender (it is not hot, it just completes ground)-make sure the bulb lights (key must be on too). If the sender is still off of the car, use a meter, and make sure the contact is actually working and closing electrically. I think the Abingdon Spares sender gaskets (some kind of a greenish-fuelproof stuff) are better than anything else- I think they have a set with the round one, the rounded/rectangular one, and for the top terminal. A lot of the black rubber ones turn to mush after a year or so. I also ran a ground wire from somewhere (don't remember - maybe a frame bolt) to one of the sender mounting screws. George
George Butz

Steve - "Well, one of the first things I did when putting the car together was install a fuel filter in the line near the tank connected with rubber fuel line! And to make matters worse, it was a plastic filter !!!!!"

Not only that, but you are in danger of causing damage to your fuel pump with a filter between the tank and the pump. See the article, SU Fuel Pumps Facts and Myths in the SU Fuel Pump Articles section of my web site at: http://homepages.donobi.net/sufuelpumps/ Cheers - Dave
David DuBois

Dave,
It looks GREAT! And you need the extra benifit that the support provides,,, It acts as a stabilizing fin for those high speed runs!!!

SPW
Steve Wincze

Hey George. Help me talk thru this...
1) I Checked continuity from the pivot bar inside the sending unit to the electrical connection stud at the top of the same unit. I can swivel the float and see continuity come and go. It seem like the sending unit is working to me. What are your thoughts on that so far?
2) On the back of the fuel warning light, with the ignition on... i do not have continuity between the hot and ground posts like i do on the ignition light. I also have continuity between the hot on the ignition light and the ground on the fuel warning. I have also check both bulbs and they are fine. I feel like Im now narrowed down to a defective fuel warning socket.
3) The wire from the fuel tank to the fuel warning light seems fine too. (no cuts, shorts or anything.)
Anything you can think of is always very helpful. I appreciate all the info. Earl...
Earl Ehrie

So how do you take that sending unit apart? Mine is not working cause the float arm is extremely stiff. I finally got it loose somewhat but still won't move under it's own wieght. So I think I need to take it apart but can find no hints anywhere on how it comes apart. Anyone have a link or any guidance they can provide?
l rutt

Not George but...

Item 1) is fine.
Item 2) continuity across the posts on a lamp socket just says the lamp has a good filament, and the contacts in the socket are making good contact with a good filament. The lamps for the ignition light and the lamp for the fuel warning are wired completely differently and comparing them is apples to oranges. One side of the ignition light sees power from the A1 terminal of the control box, via the white switched wire from the ignition switch, the other side sees power from the Dynamo. When it is not generating power, the dynamo wire acts as a ground and the light goes on. When it generates power, the power from the dynamo balances out the power from the control box wire and both light contacts are at the same potential, so the light is out. The fuel warning light gets its power from the A4 side of the fuse block (green, switched power and fused). The ground comes from the sender in the tank. The tank is grounded, the sender is grounded to the tank, and when the sender closes its contact, the ground side of the fuel warning light is complete. If you don't get continuity in the ignition light when it is on, it is because the circuit path is one of least resistance, and with the light on, you are simply parallel to the bulb.

What you want is to see if either wire has 12volts at the fuel light when you switch on the ignition, then with the ignition on, jump the OTHER contact to any convenient ground behind the panel (any black wire) the light should light. If not, replace the bulb and try again, or it could be a defective socket (to veryify that the socket is bad, try the test with a bare bulb, hooking wires to the bulb contacts directly). If the light goes on, try grounding the wire back at the sender, the light should also go on.

warmly,
dave
Dave Braun

Earl, Dave is totally correct as above. In addition, check continuity between the outer case of the sender and the terminal post (when moving the bar) to make sure the rest of the internal circuit (ie the fine wire that goes to the casing) is not broken. As far as the rest, Dave is also right that the wiring between the two lights is different. Just make sure doing Dave's test you ground the wire going to the tank, not the one from A4- I don't have a wiring diagram handy so can't tell you which color is which. Did you ever try grounding the wire at the tank to a good known ground and see if the light works (w/key on?). George
George Butz

Since these sending unit seals are prone to leakage, you might want to get a neoprene seal from Peter Edney in the uk (search for his website). It has dual seal ridges (inner and outer) and a dimple for each screw. I put one in and it works great.

I changed my gasket without removing the tank from the car.

Larry Ayres
L Ayres

Here's an image of what the tank side of the sending unit looked like when I removed it. It had been sitting in the tank for 30+ years. There was still some semblence of petrol in the tank. After chipping and scraping the deposits off of the unit I began soaking the pivot point in a variety of penetrating fluids. I would periodically attempt to rotate the pivot arm. It took about two months of effort, but eventually I got it to free up.


Bud Krueger

Dave B..
Nice looking rack holder. At first glance I thought you had taken an old wooden stocking stretcher/dryer.
Sandy
SANDY SANDERS

Sandy,

You're right... that's exactly what it looks like. I didn't notice, as I borrowed the design from Steve Wincze, and always thought of it as a rack holder. Now I know it is both a vertical stabilizer and a sock stretcher!

warmly,
dave
Dave Braun

Alright Guys... The Saga Continues. I thought i could use the ignition light as a replacement for the fuel light. i disconnected it and reconnected the fuel wires to the ignition light. with key on, no light.

Aside from that, I had another ? about the sending unit. There is a little tiny wire connected to the pivot bar inside the unit. It comes out of the unit along side but isolated from the electrical connection stud atop the unit. Where does it go from there. Should it be twisted around the stud, and tightened with the nut and washer. Or does it get sandwiched between the body of the unit and the fiber washer that goes on the stud first?

Earl Ehrie

The wire goes out and is sandwiched between the fiber sealing washer and the body of the sender. When the float drops and the contacts touch, current flows from the terminal stud and fixed contact to the movable contact, then through the attached tiny wire to ground, and the circuit completes. The photo attempts to show the inside of the unit with the wire running up under the insulator (this is an original sending unit- the one you have could look a little different). I'm a little confused about switching the lights? I'll try to post a little about electrical basics and a hopefully simple test for you to sort this out once and for all in a bit- have to do some work now. George


George Butz

Earl, here is how to hopefully once and for all troubleshoot your circuit. A "stick tester" light- has a sharp tip on one end and a small gator clip on the other is easiest to use. All numbers refer to the number in the factory manual diagram. Current flows (OK guys- I'm not going into the pos/neg. ground, electron flow thing, 'cause it doesn't matter!) from the hot (non-grounded) side of the battery to the starter switch, then via brown (33)wire to the ammeter, out of ammeter brown/white (37) to terminal A on the regulator, out of the regulator (36)brown w/blue to the ign. switch, then out by (9)white to A3 on the fuse block (or A3 if early 9 post reg), through the fuse, and out terminal A4 green (17) to the fuel warning light. At this point, with the key on and the gator clip grounded (always test the tester light to make sure your ground is good- trust me on that!), use the pointed end to verify the bulb lights at each and every one of these connections. If you have juice to one side of the fuel light at this point, ground the other terminal of the socket and the bulb should light. If not, either bad bulb or socket. (if this is the case, take the bulb out, touch the base to the known hot terminal and ground the bulb case- if good- bulb lights and there is a problem with the light/socket. If not, bad bulb, replace the bulb and try again. When this part is good, then ground the wire at the tank sender and the bulb should light. If not, there is a problem in the wiring harness between the sender and the dash. I hope this made sense and helped. Feel free to e mail me off the board if you want at the above address for any more questions. George
George Butz

Thanks! That's exactly what we did. It seems we have a brand new bad socket. I've reattached the sending unit and will deal with the socket later.
I appreciate all your help!
Earl Ehrie

This thread was discussed between 20/08/2009 and 28/08/2009

MG TD TF 1500 index

This thread is from the archive. The Live MG TD TF 1500 BBS is active now.