Hi all,

New to this forum, but maybe some of you have ideas on how to solve this little problem: My 1973 450SL all of a sudden would not start and had no electrical. I thought the battery may have discharged, so charged it. No luck.

The battery voltage measures 12,42v.

I can measure voltage between + and engine block. I measure 0 resistance between engine block and -, so ground should be ok?

As in the title, the only thing that still works is the clock, so there is some current, but no interior lights, etc.

Have started to dismantle the ground connection despite measuring connection, and it is a bit rusty. No effect from cleaning/polishing it, and when trying to connect the battery to "alternate ground" (engine block) by holding the cable manually, there are still no headlights/response.

Any ideas?

Thank you in advance,
Thomas

 

... in my head, not in the car. Although the battery to chassis may be ok, the ground on "the other side" may of course be faulty... Anyone know exactly where it is located? Behind the fuse panel?

 

All, limited response here, but have measured very limited voltage at the fuses with good ground. So I am beginning to suspect the "+-side", very possibly the main relay electronics or electronic feeder. Presume the relay is one of those right above the fuses - anyone know exactly which one? Where is the electronic feeder located?

 

Hello, Try this, I use a long jumper wire with a 20 amp fuse and connect it to the negative of the battery and attach the negative of my test meter to it. 1st measure across battery to assure myself that I am connected correctly. Further that my meter is working correctly. Then I start following the main hot wire. Most modern automobiles path is from battery positive to the lug on the starter solenoid, then to the back of the fuse panel.
Fuse panel rear connection to fuse panel distribution bar. From there to various relays as well to some fuses. Most individual fuses are connected to the distribution bars but not necessarily all.
I first check for power at my fuses. I have found that eliminates steps. If you have power there , your issue is probably lack of grounding somewhere. You say not, so the next place to have a hard look is the battery positive connection bolt. I have dealt with a number of these due to salt water corrosion. Next careful inspection is the battery main positive cable connector. They fail here due to acid off gassing or salt water. Follow with removing the nut on the starter solenoid. Seldom a trouble spot but it happens. If you have full power there , the connection at the back of the fuse box may have failed. Good luck Allan

 

Thanks, Allan. As you point out, it seems that my ground is ok. The battery + terminal looked a bit dodgy (although I could measure ok power at the end of it), so I replaced it - with no effect.

According to the Haynes-schematics the main power feed to electroncis first goes to a relay and from there to several other relays before ending up in the fuse box.

I have not figured out exactly what relays are what, and have not measured them all, but found 0,8v at one compared to 0,14v on (some of) the fuses. So my guess is that there is something fishy between the + terminal and the relays/fuses - just have to figure out where...

Does the low voltage sound like a relay is gone?

BTW; does it seem plausible that the clock can actually tick away even if it is fed with very limited voltage? I think that it working is the most odd thing...

Thanks again,
Thomas

 

Ok, so after crawling around upside down in the car for a while: The main relay electronics is #3 (second relay from top on left hand side) and I have nice 12,4v power there. So now I am a bit lost.

One curious observation: The clock seems to run because there is 8v of power on terminal 30 (fuses 1/2/4) when the ignition is OFF. When it is turned on, the power drops to 0,1v and the clock of course stops.

Terminal 30 is apparently fed from the starter cable sleeve, but I think locating that is a bit beyond my reach, so about to throw in the towel here.

 

Hi,
I'm new too. I have a 74 450SL. Are both the ceramic main fuses on driver inner fender OK? There should be a blue band on one and white one.

Also, bought a car once where bad positive connection was "fixed" with zinc coated steel star washers made for fender bolts were sandwiched in between electrical ring connectors....check +'s like grounds.

Alternatively, my car is for sale...runs great 53K miles, $4K needs paint.
Cheers

 

Thank you for the suggestions.

By ceramic fuses on inner fender, do you mean any other than the ones inside the foot well? All these seem ok.

Investigated some more yesterday, figured out that there seems to be good power to the starter solenoid as well as on the 6sq red cable connected to it. However, from what I can see this cable runs back to some sort of distribution coupling (sorry for not being more technically advanced) that distributes onwards - for instance to the fusebox and it seems to the alternator.

After poking a bit on the solenoid cable, I actually increased the voltage in the car to the level that when turning the ignition I got a "door buzz" for a second. Then the voltage dropped again. Seems that the voltage builds up a bit (in some sort of condenser?) but is discharged fairly immediately upon connecting any that demands more power than the clock... The same happens if I turn the light switch - the clock stops.

I tried to measure the resistance between the +-connector and the distribution coupling, seemed a bit high. If I did it correctly I got ~750ohm. Should have been virtually zero? So maybe the cable between the solenoid and the distribution is shot...

Keep calm and carry on.

Thomas

 

Hello, I wish I was close and could drop by. The reason I first suggested a long fused jumper wire, coupled between your batteries negative post and your test wire is this. With it , you have a known good quality mobile ground for your meter. Think of it as your best friend. You need not guess if you have a ground or not anywhere on the car. You can connect it to the battery, turn your tail lights on, walk to the back and get a yes/ no answer if you have power or not If your battery is at 12 volts, the answer at the tail light should be 12 volts.
I had to deal with a dead, 2012 Ford truck , in a remote gold mining camp that had all of your symptoms. I used my fused jumper wire from the positive post, through the drivers side window, then under the dash to the feed side of one of the fuses. That bypassed the failed wire connection or wire that fed the fuse box. We were able to start the truck , and the owner drove it 450 miles to a dealership. They replaced the entire fuse box and wiring harness under warrantee. You might try it if it helps troubleshoot. It wont fix the problem, just help find the fault.
The clue you have is you have full power at your starter. I do not have your car, mines a baby merc. However some manufacturers run the power to that solenoid and have a second cable attached under that same nut that powers the rest of the car. If so disconnect your battery, then pop the 13 mm nut off the starter solenoid . Clean both the wires and the nut, re asseble and see if that is the bad connection. Allan

 

Thanks for the offer to pop by, but being a trans atlantic flight I think it might be a bit challenging... Taking your advise I have progressed a bit.

Been busy at work, but finally got around to putting in a second +-feed (wire A in the chart) with a 30 amp fuse. Alt the systems in the car lighted up, but when trying to start the car everything died instantly. The fuse blew. I _think_ I heard the starter turn briefly.

I then replaced the fuse and "cut" the 6 sq mm rt wire (wire B in the chart) to only give the starter power through the thick 35 wire. The car was back to normal, but when trying to start I only got a humming sound. I think that sound is more or less the same as when the battery is very low.

So, I am thinking that the 35 sq mm wire (main feed to the starter) is at fault? Or is it something within the starter/solenoid?

Thinking this is a bit out of my league here...

Thomas