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12 Volt positive ground

My fuel pump on my 1959 Morris Minor which is 12 volt positive ground has died. When I went to Autozone the salesman assured me that I could use a universal 12 fuel pump. When I mentioned again that the car is positive ground he said that all I had to do is switch polarity. It doesn’t sound right to me since the package indicates that it is negative ground. Will it work without causing trouble?

It may work but it may change the direction the gas flows. In may become out.

It should work, after you change polarity on the pump connections. Just make sure that the metal parts on the pump aren’t connected to anything (positive or negative).

No, you cannot simply ‘switch polarity,’ whatever that means. The pump has an electric terminal that must receive +12 volts. Negative voltage is supplied by the bolts that attach it to the car, which is assumed to have negative grounding. You cannot do this in your Morris.

I faced the same problem in my 1963 Sunbeam Alpine. I used a crude workaround. The pump was brought inside the car, resting on the carpet for electrical insulation. Fuel hoses and wires were similarly brought up to the pump for correct attachment, positive voltage (ground) to the proper terminal. This was a crude solution, but it worked.

Unless you can order a universal pump intended for positive grounding, you may have to do something similar.

This one scares me because I do not understand how an electric fuel pump inside a gas tank can be made intrinsically safe. Hopefully, someone who does understand that will comment.

With regard to the polarity, to pump fuel the correct direction, you will need to connect the positive and negative connections exactly as the pump is marked. You will not reverse polarity as the store clerk suggested.

I would do as RemcoW suggested, and take an ohm meter to make sure the pump housing is not electrically connected to either wire. Gasoline is a non-conductor, so I think this will work, but I would not bet your life on it without a better understanding of fuel pumps. Please don’t do this until more information comes in.

That’s why you don’t want anything metallic attached to either + or - of the pump.
It should be electrically isolated in that respect.

I believe if you mount the pump in an isolated fashion (where nothing on the pump body or mounting holes have any electrical path to the car’s ground), and you also reverse the wires, it should work.

If the body of the pump does need to be grounded, then you would ground it to the +12v circuit.

I’m open to hearing where this approach may not work.

Does this Morris have a carburator? Is the fuel pump a solenoid operated diaphragm type?

If it is a pulse type diaphragm pump with mechanical contacts, the polarity does not matter. The plunger will be drawn into the solenoid for each cycle no matter the terminal polarity.

If the pump is a solid state pulse type diaphragm unit, polarity has to be maintained.

What carburator(s) do you have on the Morris? Just curious.

Hope this helps.

You should be dealing with a British car parts supplier, not Autozone. Here’s one in the US you could contact:

The pump may work fine with the reversed polarity. It’s a pulse type pump and the internal electro-magnet that operates the pump piston probably is not polarity sensitive…If they sold you a rotary pump as a replacement, that WILL NOT work (it will run backwards) unless you isolate the pump body from ground (+) and run two wires of the correct polarity to feed the pump.

You best solution is to return the “universal” pump and obtain a Lucas positive ground pump…

It will be ok. Electrons are just electrons. They just want to go home. Just switch the wires and you are good to go. This is not magic. As long as fuel goes to the carb the engine will drive.

You are just dead wrong. The only thing stopping a positive dc motor from a negative dc motor would be a diode that you just re-attach going the other way. But most automotive dc motors really do not care. You just do not understand the basics of electronics. Since I build them I thought you might want to know.

Oh! I was assuming that the fuel pump was inside the fuel tank. Poor assumption on my part. If the pump is external to the tank, my worrys are unfounded.

Reversing the polarity would work – provided the pump has the proper capacity for the car that is – but it may be difficult to effect that voltage reversal, esp if the fuel pump ground is connected to the body of the fuel pump. If you aren’t skilled in auto-electrical, probably best to consult someone local who is. They can tell you one way or the other and shouldn’t need much time to do so if you take the new pump to them at their shop.

There is usually only one wire on an electric fuel pump. Hard to “switch” them…The pump is grounded through it’s housing which is bolted to the frame of the car…Airtex makes fuel pumps with the electrical’s isolated, they have two wires so they can be connected either way…About $50…

Good advice from @Caddyman. Reversing the polarity on a simple DC motor also reverses the rotation of the armature. As was also mentioned, if the case of the pump is connected to the negative lead then reversing the polarity to it would be a real issue if it touches the chassis of the car.

Euryvale, you chose the wrong time and the wrong person to try to pass yourself off as the site’s know-it-all expert. My post was not theoretical. I installed the fuel pump as described in a 1963 Sunbeam Alpine and the system worked. So did all of the homemade radios I have built. Find someone else to insult.

As long as it is a plunger type , it doesn’t matter whether it is grounded + or -. I have changed many, many of those cars from positive ground to negative ground, it takes less than 15 minutes. You have to, if you change the generator to an alternator.

By the way, have you tried to clean the points in the old one?. You can also buy a new set of points, way cheaper than a new pump. You just have to check if it is with single point or double points before you buy. Try, with the pump connected and ignition on, and give it a whack - though not on the bakelite cover - does it start or give a few “clicks” the points are the culprit, cheap and easy to fix. Do you have a pic of that little beauty?.The car, I mean. I have had nothing but Minors for the last 34 years.

Though it pains me to agree with anyone from Texas, texases is correct.
The engine appears to use the standard SU carb and external Lucas fuel pump that was used on almost all the Brit cars of that vintage. The Lucas pumps and repair kits are easily available in the US from Moss Motors BUT IN ADDITION TO POLARITY YOU NEED TO KNOW IF YOU NEED A HIGH OR LOW PRESSURE PUMP.
The carb float in most of these cars is located directly over the hot exhaust manifold offering a wonderful opportunity for excess gas overflow to give you a dramatic “warning light” so check the web first to get the correct part.