RP tank sender unit

[Nick Salmon]

It being fine and warm - if a little windy - I have tackled the non-working petrol gauge in the RP. It has only ever moved a fraction when the ignition is turned on/off. Following Malcolm Watts (Cornwall A7 site) tests I determined that the sender unit is at fault and sure enough I found the float full of fuel. So far, so straightforward - but...

When I had sorted the float I wired everything back up to test the unit before putting it back on the tank and I find it reads in reverse! With the float up the gauge shows empty. With the float down it shows full. The gauge moves with the float going up and down, just in reverse.

I strongly suspect the sender is not for an A7 (from my MG knowledge I know that full and zero resistance can be at 'full up' or 'full down' depending on the type of sender).

Unless someone can tell me I have done something daft with the wiring to cause the reverse reading I guess I am looking for a replacement sender unit.

[philippe bacot-sevestre]

Hi Nick excuse my english I,m french maybe you make a mistake during the sender test don,t have you make an inversion of polarity is your RP positive earth ?? you have to test the sender outside of the tank and put the positive on the body of the sender and connect the loom on it that you explain is the symptom I had the same problem on my Austin A40 sport Jensen with positve earth regards Philippe

[Geoff101]

RPs should be negative earth, or at least mine is and so are the wiring diagrams I found.

[Nick Salmon]

Philippe/Geoff Many thanks for your swift replies. 

I just checked and the car is wired positive earth. I can of course flash it over to negative. It's a while since I did this on a 7. Aside from swapping over the battery, ammeter connections and the coil connections is there anything else I need to change?

Just to be certain - reversing the polarity will cause the sender unit to 'reverse' from its present upside down reading?

[Andy Bennett]

my RP is positive earth.
why not just reverse the connections on the sender since my understanding is that it is simply a variable resistor with a wiper?
Andy

[Nick Turley]

My RP is also positive earth always has been !

[Nick Salmon]

Thanks Andy. I tried reversing the connections on the sender but the gauge did not respond.

[John Cornforth]

Hi

The backwards reading is nothing to do with Positive or Negative earth.  These moving iron type gauges will work just as well either way round.  Smiths Industries changed their gauges and senders from Max resistance equals Empty to Max resistance equals Full some time in the mid 1930's.  Morris 8's also had this changeover, and sometimes Morris units have been substituted by owners over the years.

The early Max resistance equals Empty senders have an oval shaped cover plate with a side terminal, whereas the later ones have a round cornered rectangular shaped plate and the terminal on the end.

The correct sender (of either sense) has a maximum resistance of about 25 to 30 ohms.

Be aware that the gauge has 3 terminals, B for battery, T for Tank Unit and E for earth.  I can't remember if there is a dedicated earth wire or if the metal gauge body is simply earthed via the instrument panel metalwork. If the earth isn't effective the gauge won't read properly.

Your options are to change either the gauge or sender so that you have a compatible pairing, or learn to read the fuel level backwards !

Cheers,  John

[Nick Salmon]

Thanks John - You have reminded me that I had forgotten about the need to match the gauge with the sender. Duh!

[Bruce Nicholls]

Hi


The early Max resistance equals Empty senders have an oval shaped cover plate with a side terminal, whereas the later ones have a round cornered rectangular shaped plate and the terminal on the end.

How do you tell which gauge is which?