MGB & GT Forum

Lately when driving at night, I have noticed a pulsing in the headlights and dash lights. It isn't as if they are gradually going dim, they pulsate. At first, I suspected a bad ground was becoming worse when driving over a bump but then I noticed that even when sitting still with the engine running, the same thing happens.

Any suggestions would be appreciated as I really hate troubleshooting electrical issues.

Dave

That's one symptom.

Others are whining, other strange noises, and perhaps an alt light that stays on, although all of those could be attributed to a loose or bad belt as well.

That sounds like a bad regulator. Easy enough to verify with a voltmeter. Other common issue is burned out diodes which drain the battery when sitting. Reason many upgrade to either a Bosch or GM alternator. Some will argue the Lukas unit is fine and dandy but I haven't had much joy with them.

In my experience, the pulsating usually means that the alternator is going out. try this for a home test. get yourself a voltmeter, start by hooking leads to your battery, start car, note the voltage reading. then start turning everything on, checking the voltage as you go. at idle, with everything on, headlights, heater, interior lights etc) you shouldn't really have a drop in voltage. normal is 13.5 volts, maybe a bit over 14. if voltage drops below 13 at idle, have alternator double checked at an auto parts store.
Bad internal regulator or diode rectifier.
I have had good luck with the Bosch alternator. $75 -80.00 if i remember correctly with your core.
Call around.

I haven't had much luck with Lucas alternators either.

I did the 83 amp Delco conversion on my '71 GT and it solved all my problems. I did the Saturn Delco conversion on my '70 GT.

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Jim K is a grease-stained wretch

If your red warning lamp is completely out above 1100 RPM, then your basic alternator is probably fine. It may then be a regulator issue.

I once had a regulator that pulsed exactly as you describe. I fixed it by putting a small electrolytic capacitor from ground to the sense terminal on the regulator. Smoothed it out completely. Say 50 uF, 22v - they are polarized so make sure the polarity is correct.

One thing you can do is put a voltmeter from ground to the field lead coming off the regulator. The voltage should drop smoothly as the load is increased (turn lights, fan, wipers on). If you see the voltage jumping about under constant load I would consider changing the regulator.

Before you go too far down this road though, make sure all of the connections to the alternator (and from the regulator to the various terminals) are sound. I have seen wires break inside the plastic connector that plugs into the alternator. The connector is well seated, but the wire isn't well connected. You may have to take the connector apart and make sure that the wires have broken. There are little tiny tabs on the spade connectors that can be depressed to extract the wire and connector.

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Terry Ingoldsby
terry.ingoldsby@DCExperts.com