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1973 450SL
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238 Posts
Discussion Starter · #1 ·
We're having trouble getting the SL to fire up and run.
  • Very weak / no spark at plugs.
  • Coil tests ok Ohms.
  • Spark plug wires have continuity.
  • Not sure how to test ballast resistors.
Can't find a good ignition diagram. Bright ideas?

Ed
 

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1973 450SL w/ 65,000 miles
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53 Posts
Here is the checklist I made a couple of months ago when I was trying to figure out if I had something wrong with my Pertronix install or switchgear (ended up being bad wire connection at .4ohm resistor).

Resistance checks:

Use voltmeter to check resistance of .4ohm resistor with no wires attached. Plus or minus .05 ohms acceptable.

Use voltmeter to check resistance of .6ohm resistor with no wires attached. Plus or minus .05 ohms acceptable.

Use voltmeter to check resistance of primary coil winding between terminals 1 and 15. .038-.45ohms are acceptable.

Use voltmeter to check resistance of secondary coil winding between terminals 1 and 4. 8k-11k ohms are acceptable

Voltage during cranking checks:

Use voltmeter to check for voltage at both sides .4ohm resistor – should get voltage from ignition switch and starter during cranking – result:

Use voltmeter to check for voltage at both sides of .6ohm resistor (this resistor is after switchgear) – result:

Use voltmeter to check for voltage at coil terminal #4 – result:
 

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'73 450SL, '83 300CD, '01 E320 4matic
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2,485 Posts
I'm in the same boat, and resisting the urge to start another thread on almost the same topic, but I don't want to hijack your thread. My 450SL is also a 1973.

My transistorized switchgear is bad. I ordered the pertronix kit to replace it, and after reading the directions, I see they want to retain the switchgear. I do not understand this. I have used pertronix products in other old cars, and they always fire the coil directly, so I'm back to square one.

I have installed new points, and have added a condenser. I am utilizing the points to fire the coil, and have connected the coil to B+ in the normal way, through a balast resistor which has full B+ during cranking.

This works well enough to get the car running, but it's not running properly.

The car is a one owner (used to belong to my uncle) that I've had in my possession since 2008, and it was always reliable, although never very poweful. Initially when I put the points in, it ran REALLY well without the switch gear, now it barely runs.

And about the Pertronix- why, if it retains the factory switchgear does it require resistor wires? This is in bold print. If the factory module is firing the coil, how can the pertronix unit even "see" the coil, or plug wires?

My car is running solid wires with resistor plugs, which I think is the OEM requirement.

Is there any full kit that omits the switchgear to get this right? I'm wondering if the OP is also suffering switchgear woes?
 

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1973 450SL w/ 65,000 miles
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53 Posts
Aaron - just wanted to make sure you were familiar with this thread: Pertronix Install with and without Switchgear

my understanding as it relates to Pertronix ignitor 1885 replacement for points is: if you wish to retain oem Bosch blue coil then you keep switchgear; if you wish to use flamethrower coil then you eliminate switchgear (but have to deal with tachometer via adding resistor).

What coil are you using. I'm a newbie but not sure I've seen anyone use Bosch blue without switchgear.
 

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'73 450SL, '83 300CD, '01 E320 4matic
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2,485 Posts
No, I had not seen that post, but I did manage to figure it out on my own over the last hour or so. I figured the pertronix unit should function as points do, since that's their purpose in every application.

I did purchase a set of resistor wires, but they are incompatible with the distributor cap that I have. I did find a resistor wire from another vehicle, and I set that up to function temporarily as the lead between the distributor and the coil. I figure that should be close enough for testing purposes, but I'll order a compatible distributor cap so I can utilize the correct wires.

It started right up and ran, not great, but I took it for a ride anyway. It acted like it was skipping and running terrible.

Once I got back with it, I realized it was acting like it had a missing cylinder. I pulled the plug wires one at a time to see which was the offending cylinder. All had some contribution, but the rearmost cylinder on the left side had the least. For the heck of it, I pulled the front and rear spark plug on the left bank. Both appeared to be burning ok. Compression on both cylinders tested at 165. I have not tested the whole engine, but that's encouraging to say the least.

The important thing here, was that I knocked the vacuum line loose to the MAP sensor while working on trying to get the spark plug out. The end of the hose was in terrible condition, so I cut it a little shorter and reconnected. When I started the engine it seemed to be running smoothly.

...Actually, it turns out the car can do 100 in second gear...

Much improved!

I think the MAP sensor was my biggest issue. I'll sort through all the vacuum lines and replace as needed once I get the new distributor cap.

To answer your question about the coil, it's an Accel coil, I forget which one. I'm sure I probably bought it for some GM car that I had. As stated, the Bosch coil did not function well without the switchgear.

I put some resistors together to get around 8K ohms, the tach seems to be running a bit slowly, but I have yet to connect an external tach and check it.

At least now I feel like it'll be worth my time to put all of the new suspension parts on the car that I ordered. Earlier in the season I was wondering if I was going to need to think about trying to find an engine... and I hadn't really had any time to spend on it.
 

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1973 450SL
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238 Posts
Discussion Starter · #10 ·
Very good stuff, guys... I just finished installing a Megasquirt to replace the D-Jetronic fuel injection system. Then, discovered the bad spark issue... Now will install a Pertronix 1885, along with the Flamethrower coil to fundamentally replace the stock ignition system. I know my car has good, even compression. Now it will have good spark & fuel delivery...
 
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'73 450SL, '83 300CD, '01 E320 4matic
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2,485 Posts
So mine's running crappy again.


I'm going to be paying attention- I'd love to hear details on the megasquirt system.
 

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1973 450SL
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238 Posts
Discussion Starter · #12 ·
Vexing! We still can't get the car to start... Installed Pertronix (removed switchgear). Spark tester shows sparkage. Installed Megasquirt, and it appears that's functioning. Good compression. I have a gut feeling that timing is off. We've tried to set it at 0 degrees with the car off. Sound about right to start with?
 

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'72 450SL, 107.044-12-000422
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618 Posts
5-8 BTDC would be better but I would think it would still run at 0.

Sorry I can’t help with the MS-I’ll be learning from your experience.


Sent from my iPhone using Tapatalk
 

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1973 450SL
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238 Posts
Discussion Starter · #14 ·
We're backtracing all the wiring connections, etc. We've touched so much on this, likely to have something misconnected, fallen off, poorly grounded, etc.
 

· Premium Member
'72 350SL, '85 300D, '98 E320, '19 Subaru Outback (sold '14 GLK250)
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14,172 Posts
Same here. Know nothing about MS. Is there not a site or local shop that can help you?

Re timing - make sure #1 cyl is at TDC, balancer pointer at TDC and rotor pointing to mark on dist at about 1 o'clock.if you have fuel and spark it should run.
 

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1973 450SL
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238 Posts
Discussion Starter · #16 ·
It just seems like we're missing something simple... To get TDC, we're cranking engine until pressure blows my finger off of #1 plug hole. Then rotating crankshaft (via damper) to get pin & mark lined up at zero. any other good way to get TDC?
 

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1973 450 SL
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2,509 Posts
#1 valve cam lobes should be pointed up (compression stroke). It is possible to line everything up as Graham described and be 360 degrees off at the crank. Apologies if you have considered this.
 

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1973 450SL
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238 Posts
Discussion Starter · #18 ·
All thoughts are welcome, Brad. We did turn the rotor 180 thinking that might be the problem. No joy...
 

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'72 350SL, '85 300D, '98 E320, '19 Subaru Outback (sold '14 GLK250)
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14,172 Posts
All thoughts are welcome, Brad. We did turn the rotor 180 thinking that might be the problem. No joy...
I was remiss in not including that #1cyl should be on compression stroke (your finger method should be OK, but also easy to look through oil cap and see cam lobes of #1 sticking up as Brad suggested)

I revised my earlier post. If you follow that exactly, the distributor should be in a position for engine to run.
 

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1973 450SL
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238 Posts
Discussion Starter · #20 ·
Not to be a complete idiot, but when I pulled the oil cap, could only see one cam lobe facing up.
 
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