Hi Group. Enclosed is a diagnostic of starting problems from another
car group I belong to. I thought it was very through, and pass it on to
you for your information.
Steve, this may be a good post on the United site, and I am sure the
author would not mine putting it on the site.
Larry
The esteemed Williams, Lionel has said:
>
> Attempted to take my 250sl out for a drive this weekend, only to find
> the car wouldn't start. Checked the battery it is fully charged. Even
> so tried to give the battery a jump just to be sure. Still would not
> start.
>
> When I turn the key I hear one click. I assume coming from the starter
> solenoid ? and that's it the starter does not turn over. I do hear the
> fuel pump run, and if I turn on the lights they shine bright.
>
> I have not had an opportunity to check the connections to the starter
> (will be doing that this weekend. But before I get under the car and
> start to fumble about. Is there anything else that may cause this
> problem. Could it be a faulty solenoid ?
>
I think that an orderly diagnosis needs to happen in a case like this.
Turn on the headlights. Then operate the starter switch. If the
headlights stay bright, then your problem is in the starter circuit.
90% of the time, the lights will go out, indicating a need to clean
the battery terminals first. If this does not solve the problem, then
hang a voltmeter across the battery, making contact with the terminal
posts directly. Try it again. If voltage stays up, then you have
problems with termination's on the cables. If voltage at the battery
posts drops about 2 volts as a step function when your turn on the
headlights, then you have high internal resistance in one battery
cell.
Now, assuming you've got around 12 volts at the battery in the above,
connect the positive lead of your multimeter (15 volt scale) to the
engine
block, and connect the negative lead of the multimeter to the battery
negative post. Operate the starter switch. You should not observe
more than 1 volt when the starter is cranking the engine. If you see
more, walk through the battery ground circuit, cable to body attach
point, and braid from body to engine block. Clean and replace parts
as necessary to eliminate voltage indication when trying to crank the
engine.
Once you have assured that you have a good block-body-battery ground
circuit, connect the multimeter positive terminal to the small
terminal on the starter, and the multimeter negative terminal to the
block. Operate the starter switch. You should see, at the very
minimum, 9 volts, and more likely, around 11, and be rewarded with a
healthy "clack" (not just a "click") from the solenoid/switch.
Voltage, but no action implies either a sick solenoid or high
resistance inside the starter. If you are not getting 9-11 volts at
the starter switch (small) terminal, the switch circuit (starter
switch in ignition switch, neutral safety switch, associated wiring)
is at fault.
Now, connect the voltmeter positive lead to the positive battery post,
and the negative lead to the starter battery cable terminal (the post
on the starter solenoid). Operate the starter switch. There should
be well under 1 volt drop here. Excess drop suggests a bad
termination on the cable, or a damaged cable.
Finally, if it is accessible, connect the positive terminal to the
starter switch terminal on the solenoid (runs from the solenoid to the
starter itself) You should see a 12 volt indication when starter
circuit is at rest. No indication means internal open circuit in the
starter. Now attempt to operate the starter. No voltage drop
and no starter action (the solenoid may or may not click) while
attempting to operate the starter indicates high internal resistance
in the starter. Full 12 volt drop (even if the solenoid pulls in)
indicates defective switch in the solenoid.
Attempts to "jump" energize the battery circuit should be made only
AFTER verifying that the battery cable connections are making good
contact. You may get a start from jumping, but the car's charging
circuit is designed to operate with a low-resistance connection to the
battery to stabilize things, and running without this connection may
cause wide voltage swings in the car's electrical circuits.
--
Hank van Cleef
1986 420SEL
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