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1974 450 SL Won't Crank

8K views 24 replies 7 participants last post by  MBGraham 
#1 ·
Hello,

I pulled my brother-in-law's 1974 450 SL into my garage. When I went to start it to pull it out an hour later, the starter would not crank. I had been working on the flasher switch because the signals only light up, but do not flash. So, I only removed the center console switch cover, and the knee cover under the dash on the driver's side. I didn't unplug anything other than the window switches and hazard switch (that I know of).

When it wouldn't start, I checked the battery and it was slightly low, so I charged and load tested it. I has 12.6 volts and doesn't fall below 12 volts under load, so the battery is good. I checked voltage at the S terminal on the starter with my wife trying to crank the car, and it has 8.6 volts at the S terminal. I assume that it should have close to 12 volts at the S terminal. If anyone could point me in the right direction as far as what to look for regarding why the starter won't turn, I'd really appreciate it.

Thanks!
 
#2 ·
Hello,

I pulled my brother-in-law's 1974 450 SL into my garage. When I went to start it to pull it out an hour later, the starter would not crank.!
I have attached the electrical diagram. It may be of some help.

Otherwise:
- check/clean all battery and starter connections as well as ground from battery.
- Check that wiring to ignition switch was not disturbed.
- There are lockouts to prevent starting - maybe one of those was disturbed?


Maybe starter drive (bendix?) is stuck. Give starter a good solid blow with 2x4/hammer etc.

Good luck with it
 

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#9 ·
I found the wire junction block behind the coolant overflow tank and I put 12 volts to the starter solenoid with a jumper wire. The starter cranked immediately, so the problem isn't the starter. I also don't know what I was probing the other day when I got 8.6 volts by the starter solenoid. I can barely see anything on the solenoid while the starter is installed, so I guess it wasn't on the S wire that I was probing (Although it was switched power). I probed the S wire at the junction block with a volt meter and had no voltage, so the problem is before the junction block.

What is my next logical step? Should I bypass the neutral safety switch? Is it possible that the seat belt lock out is causing trouble, even though it's disconnected?

Again, thank you all for the help!
 
#10 ·
Check the Relay separating the AC from the Starter - number 4 in the diagram although in your car the wires going into it should be marked with a key code tag (4 on the white tag as seen below) which takes precedence over the position.

All the relays are the same so you can simply swap them to test. The worst part is removing and replacing all the trim pieces to gain access to the relay rack. Sometimes I pull the glovebox liner and go at it that way.
 

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#11 ·
I found the starter/ac relay and swapped in a different one. The car still did not crank. I also checked for power at the relay plug. One wire had power with the key on, but no other wire had power when I tried to crank the motor. So, it seems that the problem is before the starter/ac relay. Please let me know what the next thing to check would be.

Thanks!
 
#12 ·
In review, the starter works when you hot wire the junction block terminal 50 (purple wires). Your ignition switch works at postions 0, I, and II. You found Start/Run bus power (terminal 15) at one of the pin bushings in the "starter/AC relay" . . .

I also checked for power at the relay plug. One wire had power with the key on, but no other wire had power when I tried to crank the motor.
With the relay pulled out and the ignition switched to position II, you should have 12V at pin bushing 3 (PB3) of the relay receptacle; this connects to pin 30 on the relay. Is this the "wire that had power with the key on"?

You should be able to read 12V at PB4 of that same receptacle when the ignition is switched to the spring loaded III (start) position. Clip your ground test lead to the M1 ground point which is just aft of the fuse box where a big group of brown wires is bolted to the fender wall. If this test fails, make a continuity check between PB4 and terminal 50 on the starter switch.

If everything above checks out but your still can't start the engine normally, test for a valid ground at PB5 of that relay receptacle; should only occur when the gear shifter is in park or neutral. Worn shifter linkage bushings could present a problem here. If you don't read a valid ground, jiggle the shifter. If you can't get a valid ground, make sure the transmission is actually in park, create a short to ground at PB5 and attempt to start the engine.

If all the above works, plug a known good relay into the receptacle and see what happens. That relay has power running through it anytime the ignition is on whether it is open (rest state 1) or closed (powered state 2):

1. Ignition at II: Start/Run bus power is present at pin 30 and pin 87a which leads to fuse 9 powering the climate control temperature vacuum switch.

2. Ignition at III and Shifter in Park or Neutral: Terminal 50 power is present at pin 86 and ground is present at pin 85; the relay closes. Start/Run bus power is removed from pin 87a / fuse 9 / temp vac switch, power is delivered to pin 87 / junction block / starter solenoid.

Quite a lot to check on this circuit.
 
#17 ·
With the relay pulled out and the ignition switched to position II, you should have 12V at pin bushing 3 (PB3) of the relay receptacle; this connects to pin 30 on the relay. Is this the "wire that had power with the key on"?

You should be able to read 12V at PB4 of that same receptacle when the ignition is switched to the spring loaded III (start) position. Clip your ground test lead to the M1 ground point which is just aft of the fuse box where a big group of brown wires is bolted to the fender wall. If this test fails, make a continuity check between PB4 and terminal 50 on the starter switch.
I was able to verify a few things today based on your above directions. After loosening the large wire tie on the bundle of wires going to the relays, I did manage to find #4. It's the 2nd one down on the left side and appears to have the correct color wires going to it. I verified the PB numbers on the relay receptacle. I can now say for sure that I have power on PB3 when the key is in position II. When I turn the key to crank (position III), I DO NOT have power on PB4. My next step is the continuity test you suggested between PB4 and terminal 50 on the starter switch. My question is, how much of the dash do I have to take apart to get at the starter switch plug? Do I have to take the cluster out or drop the column? Also, if I do end up having continuity between PB4 and terminal 50, does that point to a bad ignition switch? From what I can see in the wiring diagram, the ignition switch should send power out of terminal 50 to PB4, regardless of a bad ground.

Thanks again!
 
#15 ·
I didn't get to do much today, but I did notice something that I should mention. My relay bank in the passenger kick panel only has 6 relays installed that I can see. There are 2 empty slots in the bottom of the bracket that houses all of the relays. The picture you posted has 8 relays plugged into the bracket. I also started to move wires around to see if I could find any white decal numbers because I initially did not see any. I found that the #2 decal was around the bundle of wires going to what I thought was the starter/AC Relay (relay 4). Trying to take a close look, I didn't see that any of the 6 relay plugs had a combination of wires consisting of 2 violet, 1 brown, and 1 black. Is it possible that the starter/ac relay is in a different location on this car?

I'll take a picture of the relays and post it tomorrow.

Thanks!
 
#16 ·
Here's an image of the relays in the car. The bottom 2 slots are empty. I thought the 2nd one down on the top-right was relay 4 (Starter/AC), but I found a decal that said "2". I've tried to move the wires to reveal other decals, but can't see any. I also don't see that any of these 6 relays have the correct combination of wire colors to be the Starter/AC relay. Could the Starter/AC relay be somewhere else?

Sorry for all of the newbie questions, I really appreciate the help!


 
#18 · (Edited)
I just got home today - catching up on your last few posts. Interesting that your brother's car has only 6 relays. MB put the 107 in the Electronic Troubleshooting Manual for the first time in 1974. In the preamble, they introduced the key code concept giving installers latitude to use alternate positions in the relay rack, the primary identifier being that little tag you found. From the picture in post #16, I agree that the second down on the left side is your relay separating A/C from starter.

Since you are at the relay receptacle, I would pull it from the rack (clip in back), open it up and see if anything looks bad. This is an opportunity to verify wire position with the wiring diagram. It is also easy to make a continuity check from PB1 to the junction block (where you hot wired to make the starter turn).

The continuity check from PB4 to ignition switch receptacle PB50 is most easily done by removing the steering wheel and the instrument cluster. If it were me, I would probably remove the driver's side knee bolster and reach up to the ignition switch receptacle to see if it is loose; you might get lucky. You also might be able to do the continuity check from underneath. At some point, a closer look at the ignition switch means pulling the cluster. If you continuity is good, the next step is to verify 12V at pin 50 of the starter switch when the key is turned to position III.

Be careful with the oil line at the back of the instrument cluster. Use 2 wrenches 13mm to anchor fixture, 10mm to remove taper fitting.

Realize if you test the starter switch with the oil line disconnected, you get an oil shower in the driver's seat. It is easy to pull the oil pressure gauge from the cluster to use and prevent this. I have a spare one for this purpose.
 

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#20 ·
If the continuity checks are all good, and I don't have power at the ignition switch terminal 50 with the key in "crank" position, does that mean the ignition switch is no good?
My car does not have the seat belt system that Droidster was talking about - gotta study how that works. Prolly best to do that before you tear into the dash and ignition switch. I also agree with Graham that now is the time to re-trace your steps to see if something simple is causing a no start.

Look at 46-640 from post #18. It is possible to separate the ignition switch from the steering lock . . . lots to look at before we point at that switch.
 
#21 ·
Mystery solved! I took the steering wheel and gauge cluster out today and unplugged the ignition switch to examine the plug. What I discovered was that at some point, someone had cut the violet/white wire that closes the starter relay and spliced it directly into the AC switch on the dashboard. I have no idea why someone would do this other than to bypass the starter relay, and I'm not sure why they would want to do that. Due to this wiring modification, the AC switch has to be in the "On" position for this car to crank. This whole incident occurred because the switch was flipped to the "Off" position. Unbelievable! I'm relieved that the problem is solved, but angry over the time and effort that went into something that need not have happened.

Anyway, I wanted to thank all of you for your generous help with advice, schematics, and shop manual pages. You are all truly good people and I really appreciate it!
 
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