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Old 02-17-2009, 10:53 PM
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1976 450sl ignition switch

I am hoping some of you experts out there in forum land can help me determine if my ignition switch can cause no power to relays and other strange electrical issues. The car won't crank when the key is in the start position but I do get dash warning lights when the key is turned on. The fuel pump does not run when the key is switched on but I can jumper 12volts to the pump and it will run. If I apply 12volts to the fuel pump with the wires from the harnness connected to the pump...the starter engages and then disengages at about 5 second intervals...which I thought was very weird. With the harness disconnected and 12v applied to the pump...I can feel it running.

I had an extra relay and took the cover off so I can manually activate the relay and if I use this test relay in the upper left relay slot (passenger kick panel) and manually activate it, I can get the engine to crank. This tells me (I think) that the key is not sending 12 volts to activate the relay when in the start position. The wires all look to be good to the switch, nothing broken.

I checked for voltage at some of the other relays and it seemed some of them didn't have any power to them...which is probably why the fuel pump won't run from the key either or if I jumper the relay contacts to bypass the relay...it still doesn't get any power.

I think it's the ignition switch as I am sure all 6 relays are good and some are just not getting powered. Is it possible for the ignition switch to go bad in a way that it won't pass voltage to everything when turned on? I have checked the safety lockout switch and it doesn't seem to be the problem and even jumpered around the switch to see if that was the problem...no change.

I have started to remove the instrument cluster so I can remove the ignition switch...so far it's been lots of fun trying to figure this all out...NOT! I am thinking it points to the ignition switch as the problem seemed to get progressively worse over time...ie sometimes nothing would happen when turning the key to the start position but then it would start on the 2nd try. Then eventually...nothing happens when in the start position.

I've tried wiggeling the switch, relays, checked all fuses which seem to have 12v on both sides...I've noticed fuse #4 (4th one on the top row) keeps blowing though. Not sure if it's related and have not traced down what's shorting out but it's shorted somewhere as it blows as soon as soon I turn on the key after replacing it.

Hope all this makes sense to someone out there and maybe someone has had similar problems that were not the safety switch or the fuel pump relay which caused a no crank/no fuel condition. On the 1976 450SL I think the fuel pump relay is one of the 6 relays (5 pin, 219's) in the passenger kick panel. There's mention of some SL's having a different fuel pump relay with more pins...if my car has this...I sure can't find it, looked for hours. If it does...then what I am thinking is the fuel pump relay...might not be the fuel pump relay...? Does a 1976 have an OVP relay and if so where is it?

Any help would be apprecaited and sorry if my discription seems confusing or doesn't make sense...it's got me baffeled too. Normally I can fumble through things electrical in nature and figure things out but this problem doesn't make any sense to me. The more I try to reason out what is the best way to troubleshoot it...I wind up down a dead end scratching my noggin. I figure if nothing else I'll replace the switch as it's common wear item anyway...but would like a more systematic way to troubleshoot.

Thanks....
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Old 02-18-2009, 09:16 AM
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do you have the electrical schematic, I would start there , I think I recall a circuit to the starter that somehow must be in tact for other circuits to work. Grounding as simple as it seems could also be an issue.
I could PDF the schematic if you need it
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Old 02-18-2009, 11:39 AM
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Thanks

Any info you have (starting circuit diagram) would be handy. Its got me chasing my tail a bit. I'm not sure the ignition switch will cause all the weird symptoms but being 33 years old and a wear part, I ordered one today. At least I can rule that out and know I won't have to mess with it ever again. Considering the problem get progressively worse over time, I think the switch is a good prevenative measure and may fix the problem.

This car has had a rough life and I think she's just mad at me. I rescued it from previous owner(s) that had all really let it go. Needed: rear springs and shocks, water pump, battery, a new hood, a new trunk lid and a few of other issues...but it did start and run and it was complete. Has less that 100K on it but it was a hard road. Just about every body panel had many dings. We got most of them out before it was painted but didn't get them all out. The car looks real decent now as for the body but it has to be put back together. Lights all installed, new weather stripping, door handles, bumpers ect. Then I get to start on the interior...it's been a money pit that I have more than I'll ever get out of it but that's how it goes sometimes...I do a lot of cars and usually do well on them...not this one LOL but it'll be a nice car once I get it all back together so I may keep it awhile to remind me to never buy one in the rough again ;-)

Is there any chance having some things disconnected could cause the problem...like power window switches or lights not installed? I know sometimes circuits feed from other things you'd not expect...I hope I've not created my own gremlins and when I put everything back in it, the problem magically fixes itself...that would be my luck. Spend a week trouble shooting and it wasn't broken...just something that I forgot I disconnected but when the problem started, I had not messed with anything.

The joys of restoration after chimps have had their way with a car and time erases your brain cells...it'll be a challange, but I'll figure it out and hopefully know this car much better when I'm done. Any insight or ideas (even semi-crazy ones) will be given consideration...I'm grasping at straws LOL Thanks, MarkG
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  #4  
Old 02-18-2009, 12:25 PM
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Just wanted to add that if I manually activate the relay, top left relay (I took the cover off it so I could do so) the starter does crank the engine over...I think that means IF the key sends 12v to activate the relay...the engine would turn over.

I think that's the issue, or one of them anyway...the key is not sending 12V in the start position. This does not explain why the fuel pump isn't running unless it's not passing voltage on another pin that feeds the fuel pump relay and lockout circuit. Considering I don't see voltage on the socket the relay plugs into...if the ignition switch also passes the voltage to that relay socket...it again points to the ignition switch. I am kind of guessing as to what feeds what and how it gets there by traceing wires.

I also need to add the car was stored in a barn for a bit...I don't see any evidence of varmints chewing on my harness but can't rule that out although I have inspected the harnesses where I can see or feel it. Areas such as behind the fuse box ect that I can't get to...who knows if they have been in there. I don't see anything chewed though and usually you'll see shreads of things if they have been there. The barn had cats ;-) maybe that kept the meeces out...good kitty...not that I am a cat fan...and I hates meeces to pieces ;-) if they chew on my car.

MarkG
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  #5  
Old 02-18-2009, 08:07 PM
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I got to thinking your year had a seatbelt interlock system, if the seatbelt was not connected it would not start, if yours has that it should have an overide button that sits in front of the coil, not sure on Euros

fuse 4 could be headlamps or electric windows depending on your chassis number
fuse 14 should be fuel pump relay terminal 30 and electronics main relay 86 hot at position 2 on ignition switch
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2011 Porsche Cayman - Bond,James Bond
99 E320 THE Queen Mary
62 220b - Dolly - Finally my Finny!
72 450SL, Pearl-SOLD
16 F350 6.7 Diesel -THOR
07 Lexus RX 350 - Lexi
14 38HP John Deere 3038E Tractor -Mean Green
84 300SD, Benjamin -SOLD
71 220 - W115-Libby ( my first love) -SOLD
73 280 - W114 "Organspende" Rest in Peace
81 380 SL - Rest in Peace
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Old 02-18-2009, 09:28 PM
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Interlock

I'll check the seat belt interlock...the interior is all out of it so I know that is not connected. It would be great if that's all it is. At least I know all my relay contacts are now clean and I'll have a new ignition switch ;-) even if that wasn't the problem. Might save me aggrivation later.

Thanks for the info...hope your right...REALLY hope your right! Could this interlock cause the fuel pump not to run also? In addition to the no crank problem...the fuel pump isn't running either when the key is turned on. Wondering if the seat belt interlock shuts both functions off until the seat belt is fastened...?

MarkG
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Old 02-18-2009, 10:25 PM
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Well Mark , its a shot, there are actually five versions of the seat belt warning system which started in 72, your car was between version 3 and 5, 5 only buzzes, 3 had the interlock ,not sure if V4 had interlock, in any respect PM me your email and I will PDF the electrical schematics to you
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2011 Porsche Cayman - Bond,James Bond
99 E320 THE Queen Mary
62 220b - Dolly - Finally my Finny!
72 450SL, Pearl-SOLD
16 F350 6.7 Diesel -THOR
07 Lexus RX 350 - Lexi
14 38HP John Deere 3038E Tractor -Mean Green
84 300SD, Benjamin -SOLD
71 220 - W115-Libby ( my first love) -SOLD
73 280 - W114 "Organspende" Rest in Peace
81 380 SL - Rest in Peace
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  #8  
Old 02-18-2009, 11:12 PM
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Thanks again

Hey it's a long shot but still could be the problem...I'd be happy it fixed the problem and mad if it did as I've had to push this heavy car all over my garage for the past 2 months while painting it...not to mentioned hauling it from the 1st bodyman's house that had it for 2 years and never hit a lick on it. Getting a non runner on a tralier is lots of fun especially when you know the darn thing runs...

As I said it seemed to get gradually worse until the key would no longer start it which is why I keep going back to ignition switch...but that's a total guess. More than likely it won't fix the problem and I am just going to go over the car again, connect everything I see that's not connected and start tracing down circuits...I'll start at the pump and work my way backwards. I tried that once already and got lost where the fuel pump wires ran to the front of the car. Looks like it goes to a big connector on the upper right passenger firewall and that feeds the fuse box cables. My fuel pump wires are brown and white at the pump but couldn't tell exactly where they went up front. I assume (as I have no diagram) the + wire would have to go the fuel pump relay and the ground goes where all the other grounds are under the dash. The fuel pump relay should activate when the key is turned on and sends power to the pump...mine doesn't do this though. If the ignition switch isn't turning on the ignition relay, which I think sends the 12v to the fuel pump relay "hot" pin...then I'd get no cranking, no ignition and no fuel pump....but that's a guess. I know it's going to be something stupid or the ignition switch just gave out in a weird way.

Another clue is that makes me think it's the ignition switch is if I feed power to the fuel pump wires...the starter engages and about every 5 seconds the starter will kick the engine over. This should not happen unless maybe the ignition switch is in a positon internally allowing the power to backfeed to the starter....again I am guessing as to why it does this but am pretty sure this isn't "normal". I should be able to put 12V on the fuel pump and not have the starter kick in...maybe it's a benz thing but I think I have done that before when suspecting a pump was bad on a 450SEL and it didn't do that.

My switch will be in next Wednesday...I will be glad to rule that out...they can cause some really strange problems. Mine might be a combination of things and have nothing to do with the switch...which is why I want to know how to trace it all down...that's what I am expecting ;-(

MarkG
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Old 02-21-2009, 11:11 AM
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Try the seatbelt thing. My uncle had a 1976 Cadilac that someone peeled the column, but couldn't start the car because of the seatblet thing,,,
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Old 02-26-2009, 09:56 PM
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I have the engine cranking now with the key. Replaced the ignition switch but that wasn't the problem. I found the wires had correded up inside the insulation at the starter lockout/safety switch connector. Had to strip the wires back about a foot before I got good copper again. The connector that goes on the safety switch was also cracked. I basically jumper the wires once I got to good wire and the that fixed the "no cranking" from the key problem. Luckily someone in the forum has the connector with some wire on it so I can splice it in.

However, the fuel pump is still not getting power at the pump. I ran out of time to mess with it further but I'm closer than I was. I'll check voltages at the relays and see if I can track it down.

MarkG
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Old 02-27-2009, 08:53 AM
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good ! now then onto to tracing the electrical connections, since you have a good pump, you should be able to find the issue. If there was corosion on the safety switch , could that be issues other places, bad grounds on these ol gals causes a lot of bizaare behavior
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2011 Porsche Cayman - Bond,James Bond
99 E320 THE Queen Mary
62 220b - Dolly - Finally my Finny!
72 450SL, Pearl-SOLD
16 F350 6.7 Diesel -THOR
07 Lexus RX 350 - Lexi
14 38HP John Deere 3038E Tractor -Mean Green
84 300SD, Benjamin -SOLD
71 220 - W115-Libby ( my first love) -SOLD
73 280 - W114 "Organspende" Rest in Peace
81 380 SL - Rest in Peace
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Old 02-27-2009, 06:31 PM
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I have the wiring diagram in a .pdf but it is too big to post here. Send me your e-mail address and I will send it to you.
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