Starting Relays
 

I have a 75 450SL. The starter quit and I replaced it. However, the problem appears to be the relay or the switch. The Haynes Manual drawings do not have relay locations. I replaced the relay that comes on with the key in the "on" position, but I have no idea which is the relay that triggers the starter and simultaneously drops the air-compressor. Does anyone have a relay location guide?

Tony:

There is no starter relay. The two that click up there are the power window relay and the AC cutout relay. You also have a vac retard relay, but that shouldn't click when you start.

The problem is very likely the neutral safety switch on the transmission. It fits around the shift lever going into the tranny. You should be able to start it in N rather than P -- wriggle the lever back and forth a bit. Could be as simple as a screw loose so the switch isn't operated like it should be, could be a bad switch. Not too hard to replace.

Test by moving the lever around and checking for backup lights (same switch, different contact). You can jumper the switch temporarily.

You may also have a bad ignition switch, too.

The best test method is to pull the switch connection at the tranny and check for 12V at the wire for the starter when you have the key in "start" position. I think the two wires are labled "50". One should be hot. In park or neutral, you should have continuity through the switch.

If all the above are OK, the purple wire to the starter is broken.

Peter

starter safety switch
 

On my 450sl (1977) I had the same prblem (if it is the safety switch) Try some WD 40 and spray switch (attatched to transmission at shift levers)drivers side. This got under some corrrosion and dirt) and helped the switch operate normally. If this works, you will save time and money. good luck, Abe G

Replace the switch if it is bad, I've never seen one raised from the dead successfully.

Peter

starter safety switch redux
 

Did you give the switch a autopsy? Before replacing same, try WD 40 nothing to lose. Abe G

Yes. Burned off contacts.

Peter

Still have the problem
 

I really appreciate your replies. I recently changed out the tranny with a used Factory Rebuilt that had 5,000 miles on it from Potomac Auto. So the neutral safety switch is basically new. It tested perfect, along with the wire from the switch to the the starter, and I can start the car by jumping the solenoid wire to the main terminal with a screw driver (new starter). So I pulled the steering wheel and the cluster and took out the ignition switch. Contacts were clean and un-burnt and checked OK with the ohm meter. The Haynes manual shows a bunch of stuff hanging off the start system on pages 170 and 171, including a resister, an override switch, and a starter logic relay. I have no idea where this stuff is located, or if the Haynes manual is even accurate. I live at the beach in Florida, so if there is for example a resistor in the loop, and its under the hood, it could be the culprit from corrosion. Any other ideas??

Oh uh. Do you have the dreaded seat belt interlock on your car, that lovely system around for a couple years that prevented starting the car unless there was a driver in the driver's seat and the seatbelt was latched?

Seems that was 74-75, somewhere along in there.

What you need to do is see where the voltage is in the starting system -- from key switch to whereever it goes. I don't suppose the thought police will lock you up if you defeat the system by wiring direct from starter key switch to neutral safety switch!

Do check that you get continuity through the switch. Burned or corroded contacts aren't the only thing that will keep a switch from working...

Peter

Dug up the book (Chiltons) I have on this vintage car and it indeed shows a starter relay. Should be in the ECU box in the passenger side kick panel, but I don't know which one. It's not the top one on the rear bank, nor one of the top two on the front bank (those are the EFI relays). You should get voltage on pin 85 of the correct relay when the key is in start position. Jumper pins 87 and 30 and the starter should engage.

Now the fun part. I have no idea if that relay is actually used in you car!

Peter

relay location
 

The starter relay is the top rear relay in the relay panel. Repacing it fixed my problem.;)