Slow crank and huge voltage drop

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ricks

Member
Joined
Feb 3, 2018
Messages
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Location
toronto
My Car
71 mach 1
—Hi guys. I’m having a heck of a time figuring this one out. Slow crank with huge voltage drop of 7-10 volts at positive side of the solenoid.

2 things I found. 

  1. Remove 7mm coil wire from distributor and ground to manifold. Cranks fine no voltage drop. Put coil wire back on distributor and have voltage drop, slow cranking.
  2. Remove negative battery cable, positive cable still attached. Volt meter black probe to negative battery post and red probe to any bare metal. Volt meter reads 11.8 volts constant. Open the drivers door and volts now show 12.6. Press the door switch in and volts drop back to 11.8.
Same test on my other car with negative battery cable removed attach meter the same way and the voltage has a reading of say 9 volts but immediately starts to dissipitate the voltage slowly down to 0 volts. Which appears to be normal I think.

Any ideas would be appreciated. Engine was removed painted, engine box painted recently. Ran fine before this restoration done. Tried booster cables to test grounds from block to different places on firewall and around chassis but same issue prevails.

 
Last edited by a moderator:
Where is your negative battery cable connected? It should run from the battery to the bottom screw on the voltage regulator and from there to a bolt on the engine block. Scrape the paint off the block where the negative cable connects. Make sure all grounds and cable are clean. Pull the battery cables off the battery terminals and use a battery terminal wire brush on the posts and the insides of the terminal clamps.

 
Battery post clean, negative bat cable runs from battery to front of engine block. Nothing goes to regulator just single cable bat to block.

 
The chassis also needs to be grounded, either directly from the battery, or from the block to the chassis.

Is the block clean where the cable connects to it? Paint is a good insulator.

 
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Hi guys thanks for suggestions, so I’ve cleaned battery posts. Double checked and cleaned to bare metal. New negative cable to block. Added ground from clean metal of rear block to firewall. Added a third ground from neg bat post to inner fender below solenoid.  Same issue slow crank 7-10 volts being dropped.. Ran a fourth ground to be sure I’m grounded, from manifold to chassis. Same issue. Hard to read my original post so will reiterate the issue. Car turns over perfectly only when distributor side of coil 7mm wire is grounded to manifold. Also still have voltage when neg cable removed from bat. Test neg post to ground 11.8 volts (only poitive cable attached), open door and jumps to 12.6 volts. Car cranked well before with old spark plug wire set, but am sure they were arching because I remember getting a wicked shock when I touched one of the wires while setting the carb as engine was running. Once again any help is appreciated.

 
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With negative battery cable disconnected, you should not see any voltage from battery negative post to chassis or engine ground. If you do, that says something in the chassis/engine/alternator is connected directly to the battery. Try disconnecting the voltage regulator first, then the alternator lines, and seeing if the system improves.

 
Thanks for suggestion. Disconnected voltage regulator, then removed wires off alternator. Also removed coil + and - wires. No change still shows voltage. But I did find the coil is leaking oil.

 
Try disconnecting the + wire from your coil, and then measure the negative terminal to ground voltage. Try cranking the engine with the + coil wire disconnected.

You definitely have something drawing voltage, a short, or something that is staying on. When you open the door the voltage reading goes up because it turns the dome light on, which is more of a draw than the short or other voltage draw.

Try disconnecting the starter side of the solenoid and the measure the negative terminal to ground voltage.

You'll have to systematically go through circuits until you find the one that is drawing current when it shouldn't. You can also disconnect the fuses one at a time. When you find the bad circuit you'll have to trace it and disconnect and/or replace whatever component it is connected to.

 
Thanks Don C. No changes to cranking or voltage readings to removing + coil or starter side of solenoid. Will try to identify the circuit this weekend. Added pic of solenoid as per 73pony request. Thanks again guys.



 
Before cranking at + solenoid. 12.6. During cranking both sides of solenoid bounce around reading 7 down to 2 volts. 3 new solenoids all do the sane thing. Solenoid is passing through the volts its getting. Coil mounted on top of engine. Pulled every fuse I could find no change to voltage reading of 11.8 volts still with neg cable removed. The only time I get a proper 0 reading is when I remove the + battery cable from the solenoid ( which obviously breaks the battery away from the system so 0 reading is expected) but the yellow fusible link off the + side solenoid is the only fusible link (of the three mounted on + side solenoid) that when removed give me an expected 0 reading. The only time it cranks strong and sends 12.6 to starter is when distributor side of coil 7mm wire is grounded to manifold.

 
This one has me scratching my head.

If the timing is too far advanced it will make cranking the engine difficult, which was the first thing I thought of when you said that removing and grounding the high voltage lead allowed the engine to crank better, no spark at the spark plugs. But, that was ruled out when you removed the ignition wire from the positive terminal on the coil.

You ruled out the solenoid being the problem.

You made sure everything is well grounded.

And, you still have the problems with a short circuit to ground or some component staying on. You are trying to isolate that circuit.

And, you still have the cranking problem. Which is why I suggested measuring the current draw when cranking, to rule out the starter as the problem. I can't make a connection between grounding the coil high voltage lead and the starter working better.

One more thought, make sure the insulation on the battery and starter cables is in good condition and there isn't any place where either of those could be shorting to ground.

 
Last edited by a moderator:
Have you tried using a remote starter switch to bypass the circuitry in the car during cranking? If you do that and still have the huge voltage drop, my thought is there's something up with the starter.

 
Guys thanks so much for all the answers and help. Don C you did suspect timing. Turns out timing was advanced. Unbelievable it was just a bit maybe less than an 1/8 of a turn on the distributor. Once again want to say thanks for all the support, you guys are awesome.

 
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