Mr Shifty Sidney

Had an issue with my 70 Cutlass tonight and I think the problem is a bad connection on negative battery cable. I installed a disconnect between the negative battery cable and battery a while back. Anyway, while driving
the radio started turning off and on, then when I turned on the turn signal or hit the brakes the car wanted to die. In fact it did die one time but started back up without any issues. All this started after I turned the headlights on and the fan blower on med.
speed. With some quick troubleshooting I found the engine would die when the disconnect on neg battery cable was opened. Is this right? I was under the assumption that as long as the alt was working the engine would continue to run without a battery. Does the negative cable have to be connected for the engine to run? This is a car that for the most part is unmolested. It still has the original battery cables.
Thanks
Don W

BILL DEMMER

smh, i'll say it again, it is not good practice(or a good idea) to disconnect the battery on a running vehicle. semiconductors don't like voltage spikes beyond their ratings at all. as long as the alternator is putting out enough current to power the load(output varies vs. engine/alternator rpm) it should keep running. if the load exceeds the output at any rpm, the chances that the engine will die increase greatly. the load doesn't have to exceed the output much, as the system voltage drops rapidly. the ignition system is particularly sensitive to low system voltage.


bill

nsnarsk65cutlass

X2.Yes it will unless there is not enough voltage to run the ignition system, ie alt not working or very low battery voltage.

MDchanic

By the way, your symptoms sound like those of a bad ground strap from the back of the engine to the firewall.

- Eric

oldcutlass

If you are disconnecting the negative battery cable and the engine stops running, then either the alternator or voltage regulator is not functioning properly. When an alternator is not charging and the car is still connected to the battery, everything will be powered from the battery. It will run until the loads are not getting enough voltage to operate at which time they turn off on their own and the engine will stop.

Mr Shifty Sidney

I suspected the engine ground strap. Will need to check it out. Alt/reg seems fine as the headlights never started dimming. Just wanted some different opinions because I am thinking about driving the car on a 120 mile trip to a family reunion today.
Don W

brown7373

In the 90s, I was on a company vacation trip to the Island of Margarita a 1/2 hour plane ride from Venezuela. All the mid 70s 4 doors were taxi cabs. They constantly had dead batteries. No one used jumper cables. They would ask to borrow a battery to restart their cab.. They removed the battery, inserted the good battery, started the cab, then switched them back. I bet in the week we were there, it was done a dozen times in various cabs. Maybe that is why all of them had crappy electrical systems!

Mr Shifty Sidney

As a follow up...the problem was the engine ground strap as Eric suggested. It was broken at the connector that bolts to the head. I plan on replacing original engine ground strap and probably adding another somwhere out of sight. It is not fun trying to stop and/or steer when the engine dies. Thanks for everyones input. We made the trip without any problems.
Don W

MDchanic

Glad to hear you got it going!

Another (non-concours) solution is to connect a wire (10ga is probably best) from the negative battery terminal to a handy screw of bolt on the core support, a few inches away.
No reason you can't also do the ground strap, but the connection to the battery is often simpler and more direct.

- Eric