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Temp gauge reading affecting fuel tank reading?

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70tbolt Geoff Ullmann
Morgan Hill, CA, USA   USA
Still sorting things out my 75 Spit after total restoration.

I started noticing that my fuel gauge would increase (to full) only after my temp gauge settles in to its running temp (at around 3/4 gauge reading).
When the ignition is on (engine not running) the fuel gauge (with full tank of gas) goes up to just past half tank and the temp gauge is all the way to the left. After I start the car and the temp gauge rises to running temp you can see the fuel gauge also increase to full tank reading.

I have done the following:

New temp sender

Checked fuel sender after removing from tank. When hooked up to the fuel gauge it will it will show empty to full when moving the float from the bottom to top of its range.

Replaced stock voltage stabilizer with new one and it made no real difference.

Checked the wiring at the gauges and it appears to be correct based on the wiring diagram.

Any ideas?

Thanks!
Geoff

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Doug in Vegas Avatar
Doug in Vegas Douglas D
Las Vegas, NV, USA   USA
Replace the voltage stabilizer.

It works like a Christmas tree blinker by providing 12v then 0v, 12v then 0v over and over but over time it averages to 10v.

New voltage stabilizers are solid state and provide a reliable 10v.

70tbolt Geoff Ullmann
Morgan Hill, CA, USA   USA
Thanks for the reply. I did replace it with a solid state one. No real change.

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Doug in Vegas Avatar
Doug in Vegas Douglas D
Las Vegas, NV, USA   USA
Modern gauges react as soon as you turn the key. Smiths seems to take a while. Are they accurate when they finally finish?

It's all about power and sending unit to ground on both. When you ground the sending wire are the gauges pegging?

Tonyfixit Avatar
Tonyfixit Tony M
Duncan, BC, Canada   CAN
Typical solid state (50 cent) voltage stabilizers may not work properly below 12v, so disregard the readings with just the ignition on.

You can upgrade on the VS but that likely means building it yourself, or using somthing like a 'buck converter'

(Which will cost about $2)

70tbolt Geoff Ullmann
Morgan Hill, CA, USA   USA
In reply to # 1533384 by Doug in Vegas Modern gauges react as soon as you turn the key. Smiths seems to take a while. Are they accurate when they finally finish?

It's all about power and sending unit to ground on both. When you ground the sending wire are the gauges pegging?

Yes, they appear to be accurate. However the fuel gauge only shows full if the temp gauge has settled in at operating temp. Anytime the temp gauge drops the fuel gauge will also drop.

I know the fuel gauge will peg, because it did so when I tested the fuel sender. Will need to check the temp gauge.

Thanks
Geoff

70tbolt Geoff Ullmann
Morgan Hill, CA, USA   USA
In reply to # 1533395 by Tonyfixit Typical solid state (50 cent) voltage stabilizers may not work properly below 12v, so disregard the readings with just the ignition on.

You can upgrade on the VS but that likely means building it yourself, or using somthing like a 'buck converter'

(Which will cost about $2)

Got my VS from Victoria British which appears to be the same that is used by other venders i.e. Moss (they were out of stock).

Geoff

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Falkon Avatar
Falkon Al Martin
Appleton, WI, USA   USA
There's less voltage when idling, as headlights dim for example. Guages probably are affected too I'd think.

Yellowhawk Valley Avatar
walla walla, WA, USA   USA
1969 Triumph Spitfire "Walla Walla"
1969 Triumph Spitfire "Portland"
1972 Triumph Spitfire MkIV "Spokane"
1975 Triumph Spitfire 1500 "Dayton"    & more
I have to say I don't think there is any way that the fuel reading has any connection directly to the engine temp unless they are wired incorrectly. There is no normal connection through the temp switch that would affect the gas gauge when it gauge moves up or down. Is the fuel gauge power wire connected to the input side wire of the temp gauge or the other side? Make sure it is wired as in the photo. Less voltage out of the VS would just make both of them read low or not at all.
Dan

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1976spitpr-crop.jpg    22.1 KB
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Doug in Vegas Avatar
Doug in Vegas Douglas D
Las Vegas, NV, USA   USA
Maybe the solid state voltage stabilizer is faulty.

I mean,...it couldn't be the 30 year old gauges. Could it?

Tonyfixit Avatar
Tonyfixit Tony M
Duncan, BC, Canada   CAN
Typical solid state VS's have a drop out voltage at about 12v. ie. The can regulate 14v or so (running voltage) down to a stabil and consistant 10v, but below (perhaps) 11v they drop out and give an inconsistant voltage.

It could be your VS is on it's 'way out' also they are not buffered from voltage spikes from the alterator, and should be fitted with a capacitor on the input side (at least), but that would cost another 10 cents ;-(

This is what is inside the typical solid state VS
https://www.ebay.com/itm/10pcs-New-L7810CV-L7810-LM7810-ST-TO-220-Voltage-Regulator-10V-1-5A/271045058247?hash=item3f1b8b5ec7:g:fDYAAOxykVNRwEuU

Also chek you alterator and battery voltage, your gauges could just be an indication of something else.



Edited 1 time(s). Last edit at 2018-05-16 12:29 AM by Tonyfixit.

lef2wander Avatar
lef2wander Gold Member James Thomas
Hatfield, MA, USA   USA
Vs poorly grounded. Or vs bad.



Edited 1 time(s). Last edit at 2018-05-16 06:15 AM by lef2wander.

mkivmarty Avatar
mkivmarty Marty Yanik
N.E.Ohio, USA   USA
If you think the Temp gauge is affecting the Fuel gauge, just disconnect it. See how the fuel reacts.

Marty

Tonyfixit Avatar
Tonyfixit Tony M
Duncan, BC, Canada   CAN
In reply to # 1533464 by mkivmarty If you think the Temp gauge is affecting the Fuel gauge, just disconnect it. See how the fuel reacts.

Marty


Good idea thumbs up

70tbolt Geoff Ullmann
Morgan Hill, CA, USA   USA
Wiring is as the diagram indicates. I agree, don't see how it could effect the fuel gauge. Will try disconnecting the temp gauge and see if fuel gauge acts the same as Marty suggested.

Geoff

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