[Tomakazi]

1980 SL 450 66K miles.

When I attempt to start the car, once the key is turned to the "on" or "start" position, the complete electrical system fails - does not work. The complete electrical system becomes inoperative.

In order for the electrical system to return to operation, I have to turn the positive Battery Terminal Disconnect Switch to off then back to on.

Then try to start car again, and the same problem occurs. This is a viscous circle.

Car has done this since I took possession from my decreased father-in-law, so I don't have any background information to help troubleshoot.

Anyone run into this problem? It might be as simple as a short to ground with the ignition switch.

Help.

Thanks

 

[rowdie]

"positive Battery Terminal Disconnect Switch"

I have no idea what that is.

 

[Tomakazi]

Hello Rowdie,

The "positive Battery Terminal Disconnect Switch" is a small disk that resides on top of the positive battery terminal. This one happens to be green in color on the 80 SL450.

The function is basically to disconnect the positive lead with turned to the left - Opens the circuit. So if the car was to be stored long term, the switch can be rotated to the left to open the circuit and keep the battery from draining. Actually kind of handy to have.

I don't know if this item is factory or if it was installed by father-in-law since he kept the car in storage for the majority of each year. He was a Vietnam Fighter Pilot and a big techy, so he probably installed it. But since you were not sure what it is, I am going the direction that it was not factory.

Either way, the problem has me playing the game of "stump the technician".

 

[Tomakazi]

Picture of the Disconnect Switch for the Battery Terminal

 

[rockcrusher]

Have you ever gotten the car to run since you have owned it???

the disconnect is not original, but not unordinary for a theft/storage use...

could be an anti theft device the colonel installed, and will be fun to figure out

 

[rowdie]

It is not factory. I disconnect negative but guess it is a choice. I would remove it. Could be corroded inside. The symptoms certainly sound like corroded cable contacts. Positive or negative.

 

[Tomakazi]

The car has not ran since I took possession a few months ago. Just now having time to look into the work to be done to the car. The fuel system was contaminated and I will need to replace the fuel tank before actually starting the car. Will begin the fuel system work in mid January.

I cannot run the car since the fuel in the tank is bad. I was just curious to see if the car would even turn over for just a second to see if I had any starting/starter issues to look into. My question was answered.

 

[rockcrusher]

the 73SLC has a disconnect on the negative pole, but I did make sure the battery connections were good on both ends................

But what I get out of his description is that is never gets to turn the starter motor, it just turns all the indicator lights on, then when he turns the key, they all die....

That was the question, has it ever started... lets wait till he reply's I guess..

 

[Tomakazi]

Yes with your last reply. The starter never engages. No mechanical sound/noise at all. Turn the key to "accessory" all marker lights come on, headlights, interior lights, gauges lights. Turn the key "start" all electrical items died immediately in unison.

When I turn the disconnect battery terminal switch from "closed or engaged contacts" to "open or disconnect" then turn back to the "closed" position, everything basically 'resets". Light come on and it seems like nothing every happened. Then turn the key to start and the situation occurs again.

 

[rowdie]

New battery?

 

[Tomakazi]

Yes. 3 weeks old.

 

[Tomakazi]

Since the situation appears to be a single point failure and consistent, in so much that I can duplicate and reproduce at will, I am thinking it is a grounding issue somewhere, maybe the key cylinder. Since all electrical power is lost instantly and the Bendix is not even engaging to the flywheel it must be a single point failure at the root of the electrical system which leads me to believe the key cylinder.

What are your (as in everyone) thoughts?

 

[petesmga]

As stated above, dirty terminals and or low battery would be my first guess.

 

[rockcrusher]

sounds like there is an anti theft device installed.................

I am not familiar with what systems were available, maybe someone else will chime in and have a location for you to look.

I will try to look at the wiring diagram for your car and see if there is anywhere to jumper to override the system.

be patient and I am sure we can at least get the starter to fire,...

 

[rockcrusher]

Can you take a picture of your battery area with the wiring...

 

[Tomakazi]

I will do so tonight and post by the morning. After the pictures, I will disconnect the after market positive post assembly and try to get to a factory configuration, then try the exercise again.

Will be taking a logical - systematic troubleshooting approach. Eliminate one item at a time then see what the results are.

 

[rockcrusher]

Your battery is in the boot, trunk, area correct???

If that is so, the positive cable runs up to the starter area so says the wire diagram.

Off the starter , a big wire should run up to a junction block. that would be the main feed for your electrical system.... if you loose all electrical power when you hit the key, that is where I think the bypass system might be connected. maybe someone else has an inpute to how they used to cut off power.

all my vehicles have the battery up under the hood,,(bonnet), so I cant go out and look, sorry.

 

[Tomakazi]

Yes the battery is in the boot. No damage or any type or corrosion, very clean. No appearance of any distress in the area (including two terminal cables) and no color change from high temp. But the cables are very difficult to move around, could be from age or just the gauge of cable MB used.

The battery is running longitudinally and the negative post is closest to the rear bumper.

Where is the junction box located - under the bonnet passenger side by the firewall? Can you forward the website or location that you are viewing the wiring diagram.

 

[rockcrusher]

I'll try to scan it at work today and post it. It is in the hard copy service manual, volume 1. at the back end of the book..

 

[Tomakazi]

Last night I removed the battery terminal switch and attempted to start the car.

Gauges did move once I turned the key to the accessory position. All electrical items appeared to work (lights, gages, buzzer, etc.). The only thing different that occurred last night is that it sounded like the starter tried to engage the flywheel (solenoid seemed to work/move), but as soon as I engaged the starter the electrical system immediately failed. I was able to duplicate the problem at will.

I am thinking that the starter is shorting the electrical system?

Will look at it again tonight if time permits. Moving into a new house.

BTW - I did take a few pictures last night and attached them to this thread.