Fuel gauge-BT7

[Perry]

When I turn the key on, the fuel gauge goes immediately to full and stays there. How can I determine the cause? Any help will be appreciated.

 

[Dave Russell]

Perry,
There is possibly an open circuit between the gage & the sending unit on the fuel tank. A quick check is to disconnect the wire on the sending unit & ground it. the gage should go to empty.

If not, disconnect the wire to the sending unit at the back of the gage & connect a temporary ground wire to the gage terminal. If the gage now reads empty the gage is probably ok & the trouble is a broken or disconnection between the gage & the sending unit. Or possibly the sending unit itself.

The sending unit itself should measure around 75 ohms from its terminal to ground with a full tank & around 7 ohms with an empty tank.
D

 

[Stretch]

The fuel gauge is controlled by a variable resister in the sending unit. It's located on top of the fuel tank. Take a voltmeter and measure the voltage from the terminal on the sending unit to a good ground point on the car chassis. If the voltage is 0 to 2 volts the sending unit is the problem (stuck or shorted). The sending unit is a generally a poor performer.

 

[Stretch]

Wups! According to Dave Russell I got it backwards. Sorry for the bad memory.

 

[piman]

Hello Perry,
In my experience, Smiths type gauges work opposite to Dave's advice. Shorting the sensing lead gives full scale deflection, (this applies to fuel and temperature gauges). So a short circuit to ground anywhere from the gauge itself to the sending unit will have the instrument reading 'full' The easiest check is at the sender unit and disconnecting will tell you if it is the sender unit or a fault in the wiring.
A possibility if the wiring has been disturbed, some sending units have a second terminal for a fuel warning light. If the gauge wire is connected to this terminal, and the tank is nearly empty then the gauge will read full.

Alec

 

[bighly]

Perry,

I had a float come off once. Replaced it with a wine cork until a replacement arrived. The best part about this work around is emptying the wine so as to provide a donor cork.

This is only temporary, I believe the cork would eventually breakdown.

More likely you have a bad electrical connection tho.

Tracy

 

[Dave Russell]

Alec,
I have to disagree with you. I just did the experiment that I listed above & it worked exactly as stated. Here is a very good fuel gage "how to" that also bears out my statement.
https://mgaguru.com/mgtech/electric/fg_05.htm
D

 

[piman]

Hello Dave,

Well, if you do as you say on my Triumph, it will read empty, short the sender unit to earth and I get a full reading.
Looking at your link, I wonder if there were different types of instruments. Incidentally, do the Healey's use a stabilised power supply?

Alec.

 

[piman]

Hello Dave,
further to my last post:-
I looked at an old electrical manual of mine and there are two types of instrument. The moving iron type as described in the MG guru and the bi-metallic type to which I am familiar with. The latter have a voltage stabilisor and they run on about 10 volts. This type are slow to react when switched on, the earlier type do not have a stabilised voltage supply and give an instantaneous reading when switched on.
Interestingly, my manual asys that a high or erratic reading can be caused by a poor case earth of the instrument.
So it seems we are both right as long as we identify the type of instrument. I don't know how long the bi metallic types have been fitted, but I have always (I'm talking 30 + years) checked instruments by shorting the sensing wire to earth and looking for full scale on the instrument.

Alec

 

[Dave Russell]

Hi Alec,

It appears that there are several types of fuel gage used on the various LBC's. The Healey gage is a very crude "thing" which is completely non-damped & uses no voltage stabilizer. To it's "credit" the bridge electrical design is not supply voltage sensitive & needs no regulator. The gages are notorious for being little better than a wooden dip stick.

The subject BT7 is at least 40 years old. I'm not sure, but possibly the late BJ8 used a "better" gage.
D

[ 08-25-2003: Message edited by: Dave Russell ]</p>

 

[CDK]

I have always enjoyed the challenge of figuring out about how much fuel I have and maybe being a liitle nevous about getting to the next gas station, as if they are all that far apart anymore.Todays tech stuff has us looking at input values that can be painted with a good size brush in six figures after the decimal.I do alot of calibration work and when we first got digital displays we would work like crazy to get perfect 0 to 100% readings on stuff that use to say open or closed.I just recently had the idea that part of why I like the Healey is because I have to drive it and somtimes grasp what its doing from real world inputs like sounds and smells.Some recent articals in racing magazines have talked about traction control and its place in racing,at first I thought you were not a real racer if you used tration control but then I relized that it only reflected todays driving, smash down gas, smash down brake,turn wheel, computor wiil figure out the rest, hit things and have the restraints save your life.Sorry I did not add much to the trouble shooting of gauges have not been posting much (hooked on the virtual rally game)and decided to have some deep thoughts and rant a little.Dave,I would have said wax philosophic but I did not think I could spell it.Enjoyed your description of the gauge system,very good.One last thought. My gauge is always around 1/4 to 1/2 since thats all I can afford to put in with present prices.I will try to give good input next time that deals with the subject at hand.
Thanks for the time
to let me ramble
CDK

[ 08-25-2003: Message edited by: CDK ]</p>

 

[Dave Russell]

Dave,
Right on. At least you didn't spell it "philogyny"
D

 

[CDK]

Dave,
Remember I spent years under the tutelage of the renowned philologist Proffessor Norm Crosby.
CDK

[ 08-26-2003: Message edited by: CDK ]</p>

 

[Perry]

Thanks everyone. I'm new to this and need all the help I can get. I'll try what you suggested, Dave. If it's the sending unit, do I just remove the six (I think) screws and replace it? (I tend to be nervous when I'm dealing with a large container of flammable liquid.) Any special precautions?

 

[Dave Russell]

Perry,

You can rough check the sending unit without removing it. Disconnect the wire & measure the sending unit terminal resistance to ground. If it between 75 ohms & zero ohms it is not the problem.It could vary 10 ohms from these readings & still not be a problem. See below for ohm meters.

If you decide to replace the sending unit, I would suggest that you start with a fairly empty tank & disconnect the battery ground,then you can remove the screws & the sending unit. Might have to pry it loose.

While it is out connect an ohm meter, (if you don't have one, Radio Shack has multimeters pretty cheap) from the gage terminal to the base. Move the float up & down & measure the resistance. It should read around 7 ohms empty & 75 ohms full. Check to see if the reading changes smoothly between full & empty. If it doesn't pass the tests, you need to replace it. There are ways to sometimes repair one but it is pretty touchy work.

When you replace it, be sure to get a new gasket & new sealing washers for under the screw heads. Don't lose any of the screws, they aren't very standard. I use Permatex "Hylomar HPF" on the gasket & on the screws.

PS - Check the new sending unit with the ohm meter before installing it.
D

 

[CDK]

Perry,
Every thing Dave has said will work well,Here are some options to consider.The day or two before removing the screws spray them up with your favorite penetrating oil,make sure your screw driver bit fits good and at the slightest hint of the slots getting chewed up switch to vise grips.These can be used to break the screws loose and if you are a little bit carefull you will not chew up the screwhead.Make sure you clean both gasket surfaces very well,tank and sending unit.This will require that you dont get any dirt in the tank in the process.At this point you could fool around a little bit with the float hight and see how its location compares to the reading on your gauge(and meter)and possably make slight adjustments by bending the float.You could also see if you can tighten up the float action any to reduce Healey tank swing.I use RTV to seal the gasket surfaces(this is a rtv not bothered by fuel)and never seize on the threads of the screws, you may take it apart again someday, remember in your zeal to get the screws tight dont snap them off as they are small gauge.Also this would be a good time to review the wiring the connections and the quality and location of the ground and make any needed repairs.Dont forget to check for leaks for a least two or three days after you finish.O yeah did the float face forward or to the back whish I had marked that when I took it out.
Good Luck
CDK
Ps
Trying to make up for previous rambling post

[ 08-27-2003: Message edited by: CDK ]</p>