I have an '86 560SL with 101,000 miles. It was running perfect until I
parked it this afternoon. Went back to drive it and it won't start. The
starter turns it over good but it never fires. So I tried a spark plug wire
and no spark. Then I pulled the coil wire off the distributor and no spark
What should I try / replace to get it going again?
How did you check for a spark at the coil wire? If you disconnect the
coil wire from the distributor and place it near a ground, you should
only get one spark (if that) right when you turn the ignition on. It
won't spark continously as you're cranking the engine.
First off, check the battery voltage. It should be close to 12.6V.
Even a reading as low as 12V means that the battery is
half-discharged. When you're cranking the engine, the voltage is
going to drop down even further, and some computer-controlled
ignitions won't produce a spark below a certain voltage, even if the
battery is still strong enough to turn the starter.
If the battery voltage isn't at least 12.5V, I'd put it on a charger
until it's fully-charged.
Assuming the battery's good, I would pull the cap and rotor and
examine the terminals inside, as well as the spring-loaded connection
between the rotor and the cap.
If that looks good, use an ohmmeter and measure the resistance of the
primary and secondary windings of the coil. (I'm not familiar with
your exact engine, but I'm assuming the coil has a positive terminal,
a negative terminal, and the connection for the coil-to-distributor
wire. While you're at it, measure the resistance of the wire from the
coil to the distributor - it should be significantly less than 1000
Disconnect all the connections to the coil, and measure the resistance
between the positive terminal and the negative terminal. You should
get a low reading (less than three Ohms). Make sure it's not zero,
though, because that would mean a shorted winding (bad coil).
Next, measure the resistance between the negative terminal and the
coil output terminal (the one that leads to the distributor). That
should be higher, somewhere between 8,000 and 20,000 Ohms. If it's
somewhere close to that range, the coil's probably good, but a zero
reading means the secondary winding is shorted, and an infinite
reading means that the secondary winding is open. Both of these
indicate a bad coil.
If the coil checks good, reconnect all of the wires to it, and turn
the ignition on. Check the voltage at the postive terminal of the
coil - it should be very close to battery voltage. If it's not, you
have a problem with the wiring to the coil. If the positive terminal
voltage is good, then you have a problem further upstream than the
coil - maybe a computerized ignition system?
I wouldn't hurt to check all of the fuses, too. Don't just look at
them to see if they're blown - pull each one out, check it with an
ohmmeter, and then put them back in.
Hope this helps,
"It may be that our role on this planet is not to worship God, but to create
Thanks for the reply.
I was expecting to see continuos sparking. I will try it for a single
spark. The battery has a good charge. Both of the coil terminals had 12v
with the ignition on. I'll try the other checks tomorrow.
Good luck - let us know. I wouldn't worry about trying for a spark
from the coil wire - if you have the wire too far from the ground when
you turn on the ignition, you can possibly damage the electronic
ignition. (Not likely, but there's no reason to take the chance when
the voltage and resistance checks will tell you everything you need to
know about the condition of the coil.
I'd still do the resistance check on the coil wire itself, though.
(Continue reading only if you're interested in how an ignition coil
An ignition coil is really two concentric coils wound around each
other, but not making physical contact. The primary winding has a low
resistance (I'll use 3 ohms as an example), and the secondary winding
has a high resistance (I'll use 10000 ohms). The 12V battery voltage
applied across the primary winding sets up a current of 4 amps (12V
divided by 3 ohms) in the primary winding. Since the two windings are
wound around one another, this causes a magnetic field that also sets
up a 4-amp current in the secondary winding.
When the distributor rotor hits one of the spark plug terminals inside
the cap, the secondary winding now has a path to ground through the
sparkplug. A coil resists any change in current, so it will try to
maintain the same 4-amp current it had before it was grounded. The
only way it can do this with its 10000-ohm resistance is to set up a
very large voltage (in this case, 40,000 volts, which is 4 amps
multiplied by 10000 ohms.) This is a high-enough voltage to bridge
the gap between the spark plug electrode and the spark plug body, so
you get a nice fat spark to ignite your air/fuel mixture.
As the rotor continues past the spark plug terminal, the circuit is
broken and the secondary winding has a few milliseconds to be "charged
back up" by the primary winding before the distributor rotor gets to
the next spark plug terminal and it all happens again.
So, anything that increases the resistance of the primary winding, or
decreases the resistance of the secondary winding, will decrease the
voltage from the secondary winding, and you may not get a spark. The
most-common problem is an open or shorted winding, which you'll detect
when you do the resistance checks.
"A child of five could understand this! Fetch me a child of five."
OK, I will take the coil into work to check the resistance from pos to neg.
My cheap meter looks like 0 ohms. It won't clearly resolve 3 ohms.
I expected some resistance from the spark out terminal to the coil body
ground / mount but it shows an open connection. Is this to be expected?
I don't know what the resistance should be between the negative
terminal of the coil and ground - it may even change depending on
whether the distributor rotor came to rest between two spark plug
terminals or in contact with one.
The only two values I'd worry about are the resistance between the
positive and negative terminals, and between the negative terminal and
the output terminal.
I'm wondering if your meter truly won't resolve a resistance of three
ohms, or if your primary winding is shorted - I'm looking forward to
hearing what you find out at work.
"What happens on cruise, stays on cruise." (Navy saying)
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