positive ground

[1heybuddy]

I have a 1964 MGB roadster that has been converted to negative ground. The car works great. I installed a radio in it and it works ok as long as the radio does'nt touch the metal on the car. The radio is grounded correctley and no wires are crossed. I was wondering if there was something that wasn't changed when they did the conversion. The only other thing that does'nt work correctley is the gas gauge and the sending unit has been replaced, maybe the two are related to the conversion. any help would be appreciated. Thanks, Jon

[Flash75]

Jon, Do you get a spark when the radio chassis touches metal? You said the radio is grounded correctly. Do you have it grounded to the black wiring ground wires in the wiring harness or is it attached to the a metal ground in the car? If you have a multimeter check for a voltage on the radio chassis, attach the black multimeter lead to the battery negative post and the red multimeter lead to the radio chassis. Also measure at the radio ground. You should have 0 volts at both locations. If you have more than .1-.2 volts you have a poor ground somewhere. Start at the battery cable first. If everything is OK and you have the radio wired correctly then the problem is probably in the radio.

For the gas gauge remove the green/black wire from the sending unit. Temporarily connect it to ground with ignition on, if the gauge is good it will slowly go to full, providing you are using a good ground. If it doesn't move check for 12 volts on the disconnected G/B wire. If no voltage there check for 12 volts on the green wire attached to the gauge. I don't think your two problems are related since you say everything else works.

Clifton

[1heybuddy]

Originally Posted by Flash75

Jon, Do you get a spark when the radio chassis touches metal? You said the radio is grounded correctly. Do you have it grounded to the black wiring ground wires in the wiring harness or is it attached to the a metal ground in the car? If you have a multimeter check for a voltage on the radio chassis, attach the black multimeter lead to the battery negative post and the red multimeter lead to the radio chassis. Also measure at the radio ground. You should have 0 volts at both locations. If you have more than .1-.2 volts you have a poor ground somewhere. Start at the battery cable first. If everything is OK and you have the radio wired correctly then the problem is probably in the radio.

For the gas gauge remove the green/black wire from the sending unit. Temporarily connect it to ground with ignition on, if the gauge is good it will slowly go to full, providing you are using a good ground. If it doesn't move check for 12 volts on the disconnected G/B wire. If no voltage there check for 12 volts on the green wire attached to the gauge. I don't think your two problems are related since you say everything else works.

Clifton

Clifton I forgot to say I used two different radios and got the same result

[KimMG]

What do you mean by the radio doesn't work correctly? Electrical short? Noise, interference?

[MGBSS63]

Originally Posted by 1heybuddy

I have a 1964 MGB roadster that has been converted to negative ground. The car works great. I installed a radio in it and it works ok as long as the radio does'nt touch the metal on the car. The radio is grounded correctley and no wires are crossed. I was wondering if there was something that wasn't changed when they did the conversion. The only other thing that does'nt work correctley is the gas gauge and the sending unit has been replaced, maybe the two are related to the conversion. any help would be appreciated. Thanks, Jon

Is the engine grounded to the frame?If the car has been changed to negative ground the tachometer wouldn't work unless changed over to negative ground tach.The fuel gauge should work unless you have other problem with it sometimes the connections at the fuse box are a problem.If you are using a negative ground radio it shouldn't matter if it touches anywhere.Have you verified that the negative post on battery is going to ground? It almost sounds like a positive ground car with your radio problem.Check all grounds. mgbss63

[professor101]

One thing to look for on these cars is the ground strap from the engine block to the firewall. Many people will overlook this strap. It should be located at the rear of the engine on the right (passenger) side under one of the bolts holding the "hoghead" or clutch housing to the engine block and run up to the firewall. If this strap is disconnected or broken the only ground you will have is through the throttle cable which can cause the cable to overheat and seize. The battery is grounded to the body of the car but the alternator, starter, etc. are grounded to the engine block. Have had many customers come in complaining about electrical problems, and/or stuck throttle cables and this has been the problem.. Also has your alternator been changed to a negative ground one? There is a difference. And if you are using a factory radio for this particular car it may be a positive ground radio. Buy an aftermarket negative ground radio (most if not virtually all aftermarket radios are negative ground) If you insist on using the factory positive ground radio, you will have to isolate *all* connections to the ground, which includes the antenna connection. Feed the positive from the battery to the case and ground lead of the radio and ground the positive wire. You will have to use nylon screws/bolts to mount it with appropriate rubber washers between the radio case and the chassis/dash panel.
If the car has been correctly changed to negative ground, you will *not* be able to use the factory radio without isolating it from the ground.
Also if I am not mistaken, this car does not use an alternator, rather it uses a generator so there are no diodes in the generator to cause problems with polarity. (alternators have diodes that rectify the generated power and they must be wired properly to provide either a positive or negative ground)
Basically, if you are using a proper negative ground radio and your system has been properly converted to negative ground, you should not be having the problems you report. So either you are trying to use the factory positive ground radio, or there is some problem with your conversion. Recheck all connections and be double sure about the firewall to engine bonding strap.