The 'E' Type Forum

I’ve just driven the car today, for the first time in six and a half years after bare-metal re-build.

Mechanical gauge plumbed inside engine bay shows good oil pressure.

Electrical one on the dash is resolutely below zero.

BUT I have switched from positive to negative earth during the rebuild ……… and haven’t swapped the wires over at the back.

I can see that these Smiths gauges exist as two separate products, one for -ve and a different one for +ve.

Am I screwed ?

It’s not just possible to flip the wires ?

Am puzzled why that shouldn’t be possible.

And whilst I’m at it, is the water-temperature gauge voltage-polarity-sensitive too ?

It’s reading way off to cock as well.

I’m off to bed and am hoping to wake up to some Australian wisdom, not just to be told that i should go to the Knowledge Base even if I know that’s what I deserve.

Six and half years, that deserves a bit of indulgence.

The "australian way" would be to just do it, see what happens. Worst that could happen would be enlightenment. Imo shouldn't need to, the oem gauges and senders were resistance type; current in coils heats bimetallic strips, polarity irrelevant. Maybe, after 6 and a half years, yr gauge and or sender have died of boredom?
Check yr voltage regulator. Where the o/p gauge connects needs to be 12 V different to earth. Where the water temp gauge connects needs to be 10 V different to earth.

We've done a few + to - earth conversions and I haven't noticed this as a problem.

The oil gauge works (see 3.8 manual page P51) by having a bimetallic strip in the sender that is closed by oil pressure but which opens itself by heating.

It generates a signal which alternates between ground voltage and battery voltage in a ratio proportional to the oil pressure. The higher the pressure, the more the voltage flows as a ratio per unit time.

The gauge also works using a bimetallic strip attached to the needle. The longer a voltage is applied, the further the needle moves to the right, registering a higher oil pressure reading.

So looking at this, it shouldn't matter which direction the current is flowing.

After the + to - earth correction i forgot to check the fan behind the radiator.....!don't forget that but i am sure you did it!

I slept, and was not disappointed.

The oil-gauge is definitely "handed", as if you flip the wires it immediately goes off-scale beyond 60psi, and Smiths effectively list two separate models.

My sender unit is a single-wire PTR1001/06 type, so all it can be doing is creating a path to earth.

I read here

https://www.triumphclub.co.nz/wp-conten ... _2_2v0.pdf

that the PTR (Pressure Transducer Resistive ?) is of the resistive rather than the bi-metallic type.

I think this means that I do indeed have to buy a new oil-pressure gauge to suit - ve earth.

Bugger.

For the temperature gauge, I've flipped the wires and will go and try it.

PS - yes, Yves, good point - I had indeed checked the blowing/sucking direction.

Hi Rory....is your voltage regulator the original bi metal type..or a replacement electronic unit that could be polarity sensitive.......guages as mentioned in the posts above arnt polarity sensitive...but you could have a mismatched oil pressure sender/guage..this would just make it read incorrectly.....I would suggest getting the guages/senders on the bench to test rather than wired into the car....Steve

Rory, as no-one else has said it…… well done for getting your car back on the road. A few niggles along the way will be almost inevitable. But you’ve done it

Geoff

The Oil gauge runs off un-regulated 12v, it’s only the Fuel and the Water gauges that have the regulated 10v.

The the Fuel works fine, and the regulator is the solid-state type that I made in preparation for this switchover.

I did have to go and check the pictures to make sure I hadn’t made Positive earth ones

Geoff - a bit stunned to have got this far, more to follow.

I have a negative earth conversion on mine and didn’t have to change anything on oil pressure or water temp gauges. I have a replacement electronic 10V regulator and that is obviously polarity sensetive and wired accordingly. Hope that helps.

There is more than one type of oil pressure sender and they're housed in the same near identically looking bell shaped housings. To tell them apart, connect everything up but with an LED in series and observe:- the "traditional" bi-metallic sender will make the LED strobe as it turns the circuit on and off to make the average voltage 10v less its reduction for oil pressure voltage; the much less common type with a coil of wire inside (just like the ~240 to ~25 ohm fuel sender) will not strobe the LED.

kind regards
Marek

EDIT:- If it's a resistive gauge, them check the ohm reading and then recheck it with the test leads swapped around. If you get the same number, then you ought to be good to go. If the resistance becomes infinite after the swap, then it's obviously handed and they put a couple diodes in series to drop the 12v to 10v to make it run with a standard gauge.

Hi Rory..yes the oil guage runs on 12v..but as per the original wireing its connected to the voltage regulator...that's why I questioned if you have an electronic volt regulator and have the connections correct....
Steve

The sender-type I have is the PTR as detailed on Page.22 of that NZ publication I linked to.

The R is indeed the resistance type.

This ensemble gauge-sender was actually retailed as a pair by one of the UK Jaguar clubs, circa 2010 from memory, and it's the same pair of components that Caerbont/Smiths still retail today :

https://www.smiths-instruments.co.uk/pr ... -series-1/

THIS is the reason that you can't just flip the wires, or rather just ignore the polarity, like you can on the original set-up.

You can buy just the gauge, reference APE2300-02NEG, from them for £96 inc VAT, 4 week delay.

Which is a lot sweeter than paying £253 from SNGB for the kit that just includes the sender.

So it's not a "Smiths gauge" or a "Smiths sender" as most people would understand it - it's a modern aftermarket nominally look-alike repro part and totally different rules apply. One expects there's a 7810 in there and with a bit of innovative soldering work, a 7910 can have its legs bent around and be soldered back in to replace it.

kind regards
Marek

why not just fit a mechanical gauge in place of the electrical one? I did it 16 years ago and it has never caused a problem and always accurate: viewtopic.php?f=9&t=173&p=779

rfs1957 wrote: ↑

Fri Jul 12, 2024 11:18 am

My sender unit is a single-wire PTR1001/06 type

The PTR as you say is resistive, but is not what should be fitted to your car.

This article from the Forum KB details what to expect when changing polarity on an original car. Positive to negative earth conversion guide: https://www.jcna.com/positive-negative- ... nversion-0

One thing it doesn’t mention is to reverse the wires on the windscreen washer pump…..ask me how I know…

MarekH wrote: ↑

Fri Jul 12, 2024 3:09 pm

So it's not a "Smiths gauge" or a "Smiths sender" as most people would understand it - it's a modern aftermarket nominally look-alike repro part and totally different rules apply. One expects there's a 7810 in there and with a bit of innovative soldering work, a 7910 can have its legs bent around and be soldered back in to replace it.

This Caerbont/Smiths electrical gauge and associated sender is the only version that has been available from them for over 15 years, so we'd better get used to seeing this type around.

There are no electronics whatsoever inside it, rather a fiendishly delicate web of 0.3mm wires that are spot-welded to various terminal posts, and there's no simple way of tweaking the contents.

Currently (sic) awaiting a 100% mechanical gauge from Caerbont/Smiths £79 plus VAT.