Subject: Re: [Mgs] Fuel smell in boot/trunk

From: Aaron Whiteman via Mgs <[email-masked]>
Date: Fri, 17 Apr 2015 20:07:32 -0700

> On Apr 17, 2015, at 4:32 PM, Mogrits via Mgs <[email-masked]> wrote:
> 
> Until you get that fuel smell sorted out, I would consider removing or disabling the boot light switch. Mine, in my '72, throws sparks when it makes and breaks connection. If you park the car indoors or undercover, leaving the boot lid up is a good idea, too. I have not had this problem with my B yet, but I had it in my '88 Jaguar XJ6. Turned out the tank was rusted out. They are totally different cars though, so no connection.

Fortunately(?), the boot light switch is not an issue. After two of those switches failed in the “on” state, even with the lid closed, I stopped replacing them and just leave the circuit disconnected these days. I’d rather have no boot light than a dead battery.

I didn’t give a lot of details about my ’75 last time, as it was a bit of an aside to natter about how nice the northwest weather has been this year, even on the side of Washington that gets snow with regularity.

I replaced the tank last year because I had a sediment issue, and given the smell in the boot and how the “top of the tank” never lasted (gauge would read 3/4 within 10 miles of filling). I figured I had holes in the top of the tank and was losing fuel that way.

Now, I have a new tank, new sending unit, new pump, and new primary fuel hose connections. I still lose the “top of the tank” rapidly (guessing gauge, I just haven’t followed the direction on Barney’s website to calibrate) and still get fumes in the boot. I’ll be buying new expansion/absorption tank hoses and a new filler neck soon; I’m 90% certain it’s the neck, but rubber is cheap enough I may as well tackle all three.

> Warren (making my first post here in years and years)

Welcome back!
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From: PaulHunt73 via Mgs <[email-masked]>
Date: Sat, 18 Apr 2015 09:17:32 +0100

Boot light switch - sounds more like an issue with the positioning of the 
switch holder or the actuator on the lid, given the simplicity of the 
switch.

Don't expect the gauge to be linear even when recalibrated.  If the top of 
the tank is rusted out it is very obvious from staining down the sides of 
the tank.  But that shouldn't really put fuel smells in the boot anyway as 
it is under the floor, it would be more obvious outside.

Incidentally replacement senders vary wildly from the originals and even 
each other.  because of that you can't reliably recalibrate the gauge using 
the resistance values given on Barney's site, you can only do it in the car 
with the appropriate tank and sender.  First fill the tank, then tweak the 
gauge to show F, then enjoy using your tank full of fuel.  When it gets near 
the bottom carry a spare gallon, and run the tank right out.  Now you can 
either adjust the gauge to E there and then before putting your spare gallon 
in, or you can do what I do and put your spare gallon in and then adjust to 
E, giving you a psychological 'reserve'.  Note that in my experience of five 
senders on two cars the gauge stops moving before you actually run out.  So 
if you notice the needle has stopped moving, even when going round curves, 
you are nearly out.

PaulH.

----- Original Message ----- 
> Fortunately(?), the boot light switch is not an issue. After two of those 
> switches failed in the “on” state, even with the lid closed, I stopped 
> replacing them and just leave the circuit disconnected these days. I’d 
> rather have no boot light than a dead battery.
>
> Now, I have a new tank, new sending unit, new pump, and new primary fuel 
> hose connections. I still lose the “top of the tank” rapidly ... 

[email-masked]

---

From: Charley & Peggy Robinson via Mgs <[email-masked]>
Date: Sat, 18 Apr 2015 09:21:16 -0500

From looking at the wiring diagram of the MGA I see that it doesn't 
have a voltage stabilizer to feed the gas gauge & sender;  it depends on 
the output voltage (V) of a terminal on the control box. I suppose 
that's a nominal 12V but I don't know the  delta V. Either MG A or B 
that supply V is going to vary a bit from car to car with the B being 
better regulated and you have to take into consideration the condition 
of the wiring harness, so I agree that each gauge should be calibrated 
on the car.   Also, I've never seen a gas gauge whose readings I have 
ever considered to be exact.  I see it more like a guide.

CR

On 4/18/2015 3:17 AM, PaulHunt73 via Mgs wrote:
> Don't expect the gauge to be linear even when recalibrated.  I
>
> Incidentally replacement senders vary wildly from the originals and 
> even each other.  because of that you can't reliably recalibrate the 
> gauge using the resistance values given on Barney's site, you can only 
> do it in the car with the appropriate tank and sender.
>
> PaulH.
>

[email-masked]

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From: PaulHunt73 via Mgs <[email-masked]>
Date: Sat, 18 Apr 2015 16:30:23 +0100

The MGA and MGB Jaeger gauges use a completely different method of voltage 
regulation to the MGB Smiths gauges, and the system voltage can vary 
significantly on both types - albeit a little less with alternators. 
Effectively the earlier system uses a Wheatstone Bridge, which is where with 
three known resistances you can arrange for a voltmeter to display the 
resistance of a fourth unknown resistance, and this is independent of supply 
voltage.  The three known resistances are in the fuel gauge (two being 
magnetic coils), and the unknown is the tank sender of course.

MGB Smiths gauges use a thermal voltage stabiliser which outputs an average 
of 10v over time regardless of system voltage, and thermal gauges, both 
using heating coils wrapped round bi-metal strips.  Despite being called a 
'stabiliser' the factory stabiliser is actually switching system voltage on 
and off about once per second, once the ignition has been on for a few 
seconds.  At high system voltages you get a relatively short 'on' time 
compared to the 'off' time, and at low system voltage you get a longer 'on' 
time, always averaging about 10v.  The tank sender simply varies the current 
through the gauge heating coils.  Because the gauges are thermal they are 
very slow acting which means unless you look very carefully you can't see 
the pointer moving as the voltage goes on and off.

PaulH.

----- Original Message ----- 
> From looking at the wiring diagram of the MGA I see that it doesn't have a 
> voltage stabilizer to feed the gas gauge & sender;  it depends on the 
> output voltage (V) of a terminal on the control box. 

[email-masked]

---

From: Charley & Peggy Robinson via Mgs <[email-masked]>
Date: Sun, 19 Apr 2015 18:29:38 -0500

Paul,

Thanks for this info; you make it clear.   We kneed to archive this.

CR

On 4/18/2015 10:30 AM, PaulHunt73 via Mgs wrote:
> The MGA and MGB Jaeger gauges use a completely different method of 
> voltage regulation to the MGB Smiths gauges, and the system voltage 
> can vary significantly on both types - albeit a little less with 
> alternators. Effectively the earlier system uses a Wheatstone Bridge, 
> which is where with three known resistances you can arrange for a 
> voltmeter to display the resistance of a fourth unknown resistance, 
> and this is independent of supply voltage.  The three known 
> resistances are in the fuel gauge (two being magnetic coils), and the 
> unknown is the tank sender of course.
>
> MGB Smiths gauges use a thermal voltage stabiliser which outputs an 
> average of 10v over time regardless of system voltage, and thermal 
> gauges, both using heating coils wrapped round bi-metal strips.  
> Despite being called a 'stabiliser' the factory stabiliser is actually 
> switching system voltage on and off about once per second, once the 
> ignition has been on for a few seconds. At high system voltages you 
> get a relatively short 'on' time compared to the 'off' time, and at 
> low system voltage you get a longer 'on' time, always averaging about 
> 10v.  The tank sender simply varies the current through the gauge 
> heating coils. Because the gauges are thermal they are very slow 
> acting which means unless you look very carefully you can't see the 
> pointer moving as the voltage goes on and off.
>
> PaulH.
>
>
>> have a voltage stabilizer to feed the gas gauge & sender;  it depends 
>> on the output voltage (V) of a terminal on the control box. 
>
> Unsubscribe: http://autox.team.net/mailman/options/mgs/[email-masked]
>

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