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Thread: RPMs drop when headlights on

  1. #11
    Again, any input welcome.

  2. #12
    Senior Member
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    In keeping this thread going for you: It is one of two things when you turn the headlights on and there is a 200 RPM drop on the tachometer-

    -The actual engine RPM's fall 200 RPM

    or

    -The electric tachometer displays a 200 RPM drop

    My guess, and only a guess, its the ground cluster connection(s) close to the headlight switch and tachometer. Have your checked those connections?

    Questions: Does your headlight switch work correctly and properly? For an example when you twist the know to dim or brighten your gauge lights, does it move smoothly?

    Haasman
    Haasman

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  3. #13
    I still think it is a ground problem as ret9191 and Haasman have said. One test you can try is to use a set of jumper cables that will let you go from the engine block to the battery ground post directly. Repeat your headlight test and see if there is any difference. Also remove the battery postive and negative cables and make certain that both have clean tight connections to the battery. The battery ground straps themselves must be good cables with no broken strands. Make sure the ground strap from the transmission to the bulkhead is tight. I've seen that left loose after an engine R&R .
    Early S Registry member #90
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  4. #14
    Quote Originally Posted by Haasman View Post
    In keeping this thread going for you: It is one of two things when you turn the headlights on and there is a 200 RPM drop on the tachometer-

    -The actual engine RPM's fall 200 RPM

    or

    -The electric tachometer displays a 200 RPM drop

    My guess, and only a guess, its the ground cluster connection(s) close to the headlight switch and tachometer. Have your checked those connections?

    Questions: Does your headlight switch work correctly and properly? For an example when you twist the know to dim or brighten your gauge lights, does it move smoothly?

    Haasman
    The actual engine RPMs drop. Can anyone post a picture of the ground cluster connections close to the headlight switch? Thanks!

  5. #15
    I can't imagine the alternator is putting out full 770 watts at 850 or so RPM.
    At 770 watts output the alternator could stall an idling engine. Our alternators don't come on strong until maybe 1500 (or more) RPM. That's why ambulance and police vehicles (and sailboat marine engines) have special high-output-at-idle alternators for extensive idle and low RPM service.

    Think of the battery as a big "compensator" at idle. The battery has reserve power beyond the accessory loads at idle and then buys it back from the alternator during cruise. Big changes in engine load causing 200 RPM drop might indicate a bad battery- yet the battery is strong enough to start the 2 liter? Seems puzzling-

    Remember the starter circuit is separate from the accessories circuit so engine start doesn't indicate big amps-on-tap available in the accessory circuit. It's defiantly an unusually high load to rob 200 RPM at that already low RPM (and output)- But it's very possible. It seems the battery isn't helping out as it should and you're relying on your alternator alone for your accessories circuit… On any other car you could feel the alternator HOT. A backhanded test would be to loosen the alternator belt and see if she slips when you turn on the accessories and lights.

    I just replaced the alternator and regulator in my '69S and I also cleaned every ground and all three fuse boxes. I used FLITZ polish on a felt Dremel at a low speed for the fuse spring tabs. I even replaced the bullet fuses with brand new genuine copper ones. Beware of cheap "lead" fuses and plastic bodies. The plastic bodies distort with normal operating over time and then the added resistance can cause more heat... The lead is a poor conductor compared to copper and the fuses heat up under load and can melt the fuse blocks! The glass ones look nice but I found them a little short (my usual problem!). Rather than screwing with the braided transmission ground I replaced it with a new one. Be sure to polish the undersides of the fasteners where they act as conductors or replace them. Ditto for the threaded parts they screw into. If it looks like it may block current- it will.

    Also check the individual screw clamping terminals at the fuse blocks and the 40 some year old wires from the harnesses clamped within. They can be a real mess and heat from resistance can even melt the panels allowing the terminals to shift— more resistance at the fuse-and-tab interface and even more heat. I cut back a few wires and re-stripped them to insure electrical continuity. BE CAREFUL this is a rare old harness (I dated her sister) and the insulation is BRITTLE. The two sets of twin brown wires are grounds for the optional accessories like rear window wiper and defrost. Why they chose to use twin conductors I'll never know.

    Trouble-shooting a supposed "new" electrical gremlin can often be a combination of a few smaller problems teamed together to cause a visible fault.

    With electrical problems come the possibility of FIRE.

    Remember to disconnect the negative leads to the batteries before wrenching.
    A big bad-ass fire extinguisher is key when working on our old Rods.

  6. #16
    Senior Member platas's Avatar
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    Quote Originally Posted by edmayo View Post
    I still think it is a ground problem as ret9191 and Haasman have said. One test you can try is to use a set of jumper cables that will let you go from the engine block to the battery ground post directly. Repeat your headlight test and see if there is any difference. Also remove the battery postive and negative cables and make certain that both have clean tight connections to the battery. The battery ground straps themselves must be good cables with no broken strands. Make sure the ground strap from the transmission to the bulkhead is tight. I've seen that left loose after an engine R&R .
    I am with Ed..
    San

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    Early Reg # 771

  7. #17
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    been here done this....

    1) run a rpm guage at the distributor end..
    and look through the rear windshield..
    2) turn on lights
    3) if tach drops and your RPM gauge in the back (using a snap on multi-guage digital gauge ) (i was the one turning on the lights for the husband) does not...its the guage cluster system..
    if its the multi guage droppes its on the charging system.

    turns out that our light switch was getting old..and sorting.


    best of luck
    marge


    CEO of the Best early 911 parts store in Southern California

    www.Schnellautosports.com

  8. #18
    Thanks for all the ideas. Whenever I have a bit of time will try them and report the results!

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