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MG MGB Technical - Pos ? Neg Earth

My 66 B has been coverted to negative earth for some time. Could I have switched it back to Pos Earth when jump starting and accidently arching/flashing the positive cable to the negitive ground body. My radio is blowing fuses and I can find no apparent short anywhere. Everything else seems to be working.
Thanks
Steve C.
Steve C

Steve, flashing the system will only affect the generator if that is what you have fitted. It cannot affect the basic polarity of the system as it is the battery connections that determine that. If it was neg earth before it is neg earth now.

I can't say with any certainty whether the generator was re-polarised by the jump start, but if it was it would simply not charge the battery. I'm not sure it would even work at all.

Tony
Tony Oliver

If the radio was on when you were jumping the car, it is quite possible that you fried the radio. Check the battery connection at the solenoid with a meter (positive lead to the battery cable, negative cable to ground). If the meter reads up scale (or positive on a digital meter) then the polarity is still negative ground. Cheers - Dave
David DuBois

I wouldn't expect jumping-starting with the radio on to have blown the radio, as the flat battery will absorb a huge amount of voltage spike. You *could* have jumped it in the wrong direction, even doing this momentarily is more than likely to blow the radio. If it is the type with a permanent voltage supply to keep the station memory then it wouldn't even have needed to be switched on at the time.
Paul Hunt

Thanks guys, pulling the radio this evening. Im thinking I did fry the radio. Also, should I show continuity with the ohm meter when testing from the positive lead of the Radio to the negative earth on the car? I guess what Im saying is does the ohm meter complete the circuit and you get a positive read. Sorry but Elec. is my weekness.
Steve C.
Steve C.

Steve - No you should not show continuity when reading from the positive lead of the radio to ground (unless you have the meter set to read high resistance - this test should be done with the meter set to RX1), also try reversing the meter leads when taking the reading and see if that makes a difference. Cheers - Dave
David DuBois

Steve

If it is not already. After I fried my alternator when someone jumped my 67 wrong, I bought two of the Stickers that say. This vehicle is NEGATIVE earth. I have one on the grill bracket and one on the battery box cover.
Bruce-C

Radio Fried, took it out so it wouldn't be grounded and it is a dead short in the radio. Didn't like it anyway, it skipped when playing CD'S at every little bump. Anyone have any suggestions on something reasonable that will tolerate the ride of a B?
Many Thanks
Steve C.
Steve C.

In regards to Bruce's suggestion, see the article, Negative Earth Stickers for part numbers for the stickers. In thinking about it, it certainly wouldn't hurt to put Positive earth stickers in the same places on cars that are positive ground - just might save some grief in the future. Cheers - Dave
David DuBois

I would consider a mp3 or ipod device instead of a CD player. I'm not current on available radios, but I'll bet there's some out there that take SD cards and will play mp3's from that. No skips. Or I'm sure you can get a radio with a plug for an ipod. The only problem I can see is choosing the songs while driving.
Tom

Steve
If in fact you have somehow flicked it back to positive earth with the jumperleads you would have fried the tacho as well. If your tacho is still operating normally you are still neg. earth and all is well and ok to fit your new neg. earth audio device. Willy
WilliamRevit TasmaniaAustralia

Thanks Folks, now another issue. I have a Amp/Volt meter and want to install it. What goes where and before or after the regulator and generator? 65-B Negative Earth with original Generator...Thanks Again.Steve C.
Steve C.

Steve-
Bad idea to install an ammeter, since you have to run all power in to the meter and back out to where it came from. That would be the big browN wire(s) off the solenoid. Dangerous and inefficient and takes really big wire. voltmeter is fine, connect to any browN and ground, or any White and ground if you only want it to read with IGN ON.

FRM
FR Millmore

I second not touching an ammeter with a barge-pole.

Thermal i.e. slow-acting voltmeters shouldn't really be connected to brown or purple as they take quite a bit more current than moving coil meters commonly found in analogue multi-meters. If you do connect it to brown it should be via a fuse close to where you tap into the brown circuit, purple (interior lights, headlamp flasher most years) would be better as that is already fused. Likewise if you use the white circuit you need a fuse, but again the green circuit is already fused if you only want the voltmeter registering with the ignition. However when you start getting into the white and especially green circuits you are starting to drop voltage through 30-year-old connections and are not seeing battery voltage, which is what is important. A half-way house is to connect it to the accessories position of the ignition switch, where it has one, again via a fuse if to the white/green directly off the switch. There is an inline-fuse off this feeding a green/black from 71 to 74, but this is likely to show fluctuations and lower voltage when the wipers are on, as is the white/green to some extent on these years.
Paul Hunt

Steve

I have a inexpensive sony unit in my B. Cost around $100. slips right in the dash. Works very well. Can;t say it never skips, but it takes a good bumpy road to get it to. Nice thing is if I want to. It only takes me a few minutes to pull it out and slip in a period radio (if I ever want to do another car show).
Bruce-C

This thread was discussed between 28/06/2009 and 07/07/2009

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