1972 fleetwood knocks and lights flash in synch



Can anyone help with with my 1972 Cadillac Fleetwood? I just bought it and as soon as I got home it started making an intermittent knocking sound that now became permanent. When I drive it at night with the lights on the lights flash (dim very briefly) in synch with the knock. I can hear the knock well from inside the car, but not under the hood. The frequency is a 2-3 times per second and only weakly related to the rpm of the engine and the same if the transmission is in neutral or drive. The car has 80,000 original miles.


Voltage regulator


It sure could be the electro mechanical voltage regulator. They used to be cheap to buy.


Great, thanks for the reply! I will check it out.


For what it’s worth, run it with the hood open and watch the alternator and alternator belt.


Thanks. I had done that and there is no audible knocking under the hood, at least I cannot hear it with the engine running. The alternator runs smooth and the belts look fine.
There was another case on another chat site with exactly the same problem. They checked alternator and regulator and claimed they were fine. They don’t say how the fixed it, though.
I will definitely try the regulator and also check the cables. I think the knocking sound is just a symptom of the problem, may-be a relais in the dashboard that ticks when the Voltage drops, similar to how the lights flash. So the root cause is probably in the charging system itself.
Thanks again everyone for the input.


A circuit breaker that automatically resets could make the noise. Sometimes the dimmer switch goes bad from rust and it could short out. The dimmer switch used to be the first thing to check in any headlight system. Any parts place should have one.



just wanted to update that it was indeed a bad voltage regulator that caused the pulsing.

In case someone else has the same problem, I found after some research that the regulator from Autozone is the best replacement part. It is fully electronic like from a modern car, while the other ones from Pep Boys or the original AC Delco are still electro-mechanical and not very reliable by design. The Autozone part Wells VR715 is also by far the cheapest, about $15, even though it is made or at least assembled in the USA.

Thanks again!