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MG MGB Technical - Alternator Replacement?

I could use some advice.

I have a 74.5 that had an engine fire back in 99. I bought the car last year and got it back on the road. The question I have is whether I need to replace the alternator.

The alternator light doesn't work. It does come on when it is grounded but not when you turn the ignition on. Everythign works fine until the headlights are on. then, the right one doesn't flash, only stays on and the left flashes but very slowly. Overall charging is fine.

I'm tempted to replace it just to know I have a new one.

Thanks for any recommendations.

Cheers,

Tim
Tim Henderson


Tim,
I'm thinking you're having some ground and connector problems. Many probably reside in the front wiring harness behind your grille. Poor grounds on a MGB (or any car) can lead to some crazy circuits. Electrical potential will seek a ground. If the proper path is blocked or of high resistance, current flow will divert to another path if available.

Just a few suggestions for the coming weekend. Others, I'm sure, will add to this.

(1) Remove the battery cover and drive your car to one of your local auto parts stores (e.g., Autozone, Advance, O'Reilly, NAPA, etc.) and have them check alternator output - it's free. No need to buy one if you don't need it.

(2) If you have one - great. If you don't - buy yourself a small Dremell (or like) tool and one of their small/soft wire brushes. In addition, pick up two or three 0.22 cal barrel brushes (brass). You can get these at your local WalMart. With the above in hand, remove your grille (its not that difficult), release harness from its securing clips (just to give yourself some slack) and attacking one connector at a time, start cleaning. The male bullet connectors are cleaned with the Dremell and the female connectors are easily cleaned using the 0.22 barrel brush. I remove the female connectors from their rubber covers before cleaning (WD40 makes removal easy) and holding one side with a pair of hemostats, clean the other. To hold the barrel brush, you can use a gun cleaning rod. My preference is a variable speed drill.

(3) Taking notice of where each connected wire(s) goes, remove fuse panel from its mounting and clean male side connectors and fuse securing clips. The Dremell tool can be used for this. The female connectors from your main wiring harness are a little more difficult to clean. Suggestions here would be welcomed.

(4) If you have time, do the same for the connectors in the trunk.

I went through much of the above about two week ago. Left front headlight was dim, turn signal lights weren't blinking, side marker lights were going on and off, horns didn't work, brake lights were dim, right side turn signal indicator on dash stayed on for no apparent reason, some bulbs blinked when they shouldn't and on and on. A couple of hours cleaning corrected all. Should add - the car was an '80 LE that hadn't been driven in over 10 years.

Good Luck
Steve Buchina

Tim - someone on another forum has a similar problem - the warning lights works if you ground the (disconnected from the alternator) brown/yellow but not when connected to the alternator. His also charges, could be a faulty voltage regulator in the alternator or possibly someone had put a diode inside (the wrong way round!) to 'correct' a running-on problem that can affect 77 and later models.

You mention charging voltage is 'fine', but what is it? You should get up to 14.7v on the alternator, battery, and brown at the fusebox, slifghtly less in practice on the white and green, which *will* drop as you increase electrical load i.e. turn on the headlights.

When you talk about non-flashing I assume you mean the turn signals. These are very sensitive to bad connections/low voltage anywhere in the circuit. The fact that one flashes slower when the headlights are on (which is relatively normal) and the other stops flashing means that there is a bad connection, or incorrect rating bulb, in the non-flashing side. There could be any number of these bad connections all acting in series, so don't expect a 'magic bullet'. You could equally have bad connections in the 'good' side making the slowing down more pronounced than it should be. If you have the correct voltage coming from the alternator, then the best approach is probably to bypass the flasher unit (on the right-hand cabin side of the firewall, with green and light-green/brown wires) so the lamps glow constantly, then plot the voltage through the circuit from the alternator, broewn at the fuseboc, white at the fusebiox, green at the fusebox, green at the (bypassed) flasher and light-green.brown at the turn switch, and the green/white and green/red wires, bulb casing, bulb-holder socket, and body looking for a sudden drop in voltage. On rubber bumper cars only the front indicators have a ground wire, all the rest relay on their physical mounting to the body. RB fronts share the ground wire with the headlights, so if you see *greater* than 0v on the bulb casing or bulb holder this implies the shared ground connection (4-way bullets by the headlights, ground connection by the fusebox) could be the problem. However with only one side affected this tends to rule out the ground connection by the fusebox, but not the bullets (black wires) by either headlight.
Paul Hunt 2

Paul and Steve,

I really appreciate your comments.

Unfortunately, before I started heeding your remarks, I replaced the alternator anyway. Given the engine fire, I didn't have confidence in it. In looking at it after removal I may have been right. It was very rusty and the bearings were on their way out.

However, I made matters worse. Now the right blinker does not blink at all and the left is very slow. Both get worse with the headlights on.

I wont't have time this weekend but next week I will go through the connectors and clean them up and doing the other things you both recommended. I'll let you know how I make out.

Thanks.

Cheers,

Tim
Tim Henderson

Tim - I would suggest replacing the female sleaves for the bullet connectors rather than trying to clean them. they are cheap and the old ones have often lost their temper and result in loose connections or break while trying to clean them. You can get new ones from British Wiring <http://www.britishwiring.com/> very reasonably (particularly when you consider the amount of time cleaning them requires). If you still have problems with individual circuits after cleaning everyting, start looking at the bullet connectors themselves. I have found the crimped connectors often develope corrosion inside, where it can't be seen, which in turn causes a high resistance. I just got through fighting that on our daughter's MGBGT. In that regard, you should also pick up a dozen or so solder on bullet connectors to have on hand when you order the sleeves from British wiring.

Both Paul and Steve mentioned grounds. Those are very mportand and can cause all sorts of weird problems. See my article on ground point preparation at: http://www.omgtr.ca/technical/General_Technical/grounding.htm for tips on how to prepare a ground point for a reliable and long lasting connection. Good luck - Dave
David DuBois

If just replacing the alternator has resulted in the slow or non-flashing of the indicators then I'd suspect it isn't charging. Does the warning light glow when you turn on the ignition? Does it go out when the car is started and revved to at least 900rpm? Does it stay out unless the revs are dropped (i.e. by dragging the clutch) to below 600rpm? When out in normal running does it come on when you turn the ignition off and flicker and die as the engine slows to a stop? It should do all these things. What is the voltage on the alternator brown and brown/yellow, battery, and fusebox brown, white and green when the engine is running at about 1500rpm with minimal electrical load? All these should be in the region of 14 to 14.7v. Whilst a difference between the sides does indicate a slight problem with connections somewhere it would be very unlikely for these to suddenly develop just by changing the alternator if the alternator is charging correctly.
Paul Hunt 2

This thread was discussed between 05/05/2006 and 12/05/2006

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