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30-09-2020, 02:51 PM
(This post was last modified: 30-09-2020, 03:08 PM by JonE.)
I've gone through the Cornwall A7 site diagnosis. I have got good earth to central pole, and supply from fused source. The gauge immediately flicks to full when ignition turned on.
When the tank (rheostat) wire is removed, no change.
Oddly, I've swapped the gauge with another and... it does exactly the same.
This suggests that either BOTH gauges are faulty - or something else!
My question - how do I check or fix the gauge itself. It seems to be dismantle-able... but not sure how.
original gauge...
24 ohms T terminal to earth
42 ohms supply terminal to earth
and the other gauge is the same... hmmm
one is a x42139 8HP 6V "petrol" marked
other x49522/S "Fuel" marked
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Location: On a hill in Wiltshire
What happens if you swap the connections on the ammeter for the power feed and the sender unit over?
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30-09-2020, 04:08 PM
(This post was last modified: 30-09-2020, 04:14 PM by JonE.)
The terminals were definitely the correctly marked ones.
Simon - I'll have to do that on Saturday morning to test! Is it something to do with earlier versus later gauges? The late gauge is interacting with a 31 tank sender if it makes any difference.
...
woah... hold on... ammeter?.... is that a typo, or is this getting deeper?
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30-09-2020, 04:57 PM
(This post was last modified: 30-09-2020, 05:07 PM by John Cornforth.)
Hi
Internally there is a coil from B to T and another from T to ground. Both are about 25 ohms so those measured resistances sound about right. The supply goes to B (Battery), the sender to T (Tank) and Ground to the body of the gauge. it is not polarity conscious but needs 6V not 12V.
Are you certain that the tank sender is working OK ? The tank sender will vary from a few ohms to about 30 ohms, but some are max resistance when full and others are max resistance when empty. If the sender and gauge are mismatched you will get a backwards reading.
To simulate a sender unit, apply an open and then a short circuit between T and ground, you should see the gauge move. Connecting a resistor will give an intermediate reading for example 15 ohms will give about a half tank reading. Small resistors used to be available from Maplin, but these days you need to go online.
Oh, and to answer your question, yes you can dismantle the gauge but I strongly advise against it unless you are absolutely certain that it is faulty and you have the means to remove and rewind fine wire coils and then recalibrate it !
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Sorry, meter/gauge, not ammeter - mind elsewhere, I'm afraid
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John - thanks. So, if the two gauges 'similarities' support the fact that they may likely be functioning fine ...if the sender is "mismatched", can it be somehow reversed for normalised action?
Do we have any information about when the changeover occurred with senders? I know that dash panel is very late, and the front oblong (sendered) van tank - not sure what the range of use was for this.
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Hi Jon
Not sure if it helps but recently had a problem with fuel gauge / sender unit which stopped working. Checked off the car worked ok, checked tank earthed ok etc. Fitted back the car/tank (tank not fitted temp earth) all ok. Put tank back in car not working. Long process as stated before lockdown and couldn’t access until after lockdown but eventually worked out the tank was earthed but the sender unit was not earthed to the tank so had to add a wire to the top of the sender unit. Obvious now but a big pain especially as no access to sender unit and not easy to remove tank.
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ah - good thinking, thankyou. Hadn't though about sender earth whatsoever.
So for security/test, put a wire from one of the outer setscrews to bulkhead? (Mine is easy - right under bonnet!:-)
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Hi
The changeover happened some time in the mid 1930's, possibly when the Ruby changed from ARQ to ARR but that's only my guess. Early senders have an oval cover plate and a side terminal, later ones a rounded rectangle shape and an end terminal.
Over the intervening years many senders and gauges must have been swapped of course !
I don't know of an easy way to change the sense of the sender, other than dismantling and modifying it.
One thing that goes wrong is that the end of the sender's resistance wire is earthed by simply trapping it between the outside of the insulating bush of the terminal and the hole in the mazak body (on the ones I've seen anyway). You eventually get corrosion and poor contact. Dismantle and clean to cure.
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Location: Haverfordwest
Car type: RP Deluxe Saloon 1933
I found this a very good illustration of a petrol gauge construction.
MGA
Roger
Location:- Haverfordwest, Pembrokeshire.
