[W126 Coupe] No spark - 500SEC

[Michael S Cosby]

And remember:
Most problems are electrical and even when it is fuel,
most fuel problems are electrical.
Mtnmike

(Yea, I know, real profound, but worth repeating)

>>> eelploot at hotmail.com 02/08/05 12:55PM >>>

Hi Ian;
The chance that your fuel system and the ignition system crapped at the
same time is remote.

Here's a way to test without special tools or having a shocking
experience.
Remove the air filter cannister. Hold the throttle plate down a bit and
squirt a bit of "quick start" fluid (ether) into the intake.  If you
don't have a spray can of such a substance, carefully dribble a couple
of spoonfuls of gas into the throat of the intake.   Try to start the
car.  If it runs for 3 seconds, the ignition system is working and the
fuel delivery system is suspect.
You can usually feel (see or hear) if the fuel pump is working.  The
fuel pump is visible from the back over the rear axle behind the right
rear wheel.  Put your finger on the pump and have someone activate the
ignition.  The pump should vibrate if it is working.
A fuel pump usually lets you know it is heading down the toilet.  It
starts to humm a tune for you as it gets weaker.  If a good pump
suddenly does not appear to be working at all, the most likely culprit
is the fuel pump relay.  
Intermittent failure is fairly common problem with older relays.  The
car doesn't start but then for no apparent reason does again for a
while.  Eventually, it won't work at all.
The best thing to test the relay is to plug in a known good one
instead.  Unfortunately your other car, the 1981 500SE, does NOT have
the same relay.  (Check the numbers... the '83 might say something like
a "01 545 53 05 Kickdown", and the '81 says "01 545 06 05".)
But if you feel that a relay is the most likely problem, there's a good
chance you may be able to fix it yourself.
IF, on the other hand, the engine DID NOT START or fire at all after
putting gas in the intake, likely the ignition is the problem.  You have
no spark (and the spark plugs are the LEAST likely culprit).  If it just
suddenly wouldn't start, the ignition module is high on the list of
suspects!
But try the fuel system check first.
-Bellamy
BY THE WAY... Maybe someone else has had the problem of water
accumulating in the fuel system.  Whether it is condensation in the tank
or bad gas from the pumps, I don't know.  But injectors sure don't like
water.  An occasional splash of methol alcohol in the tank cures it
instantly.  (I'd use the term "gas line antifreeze" but people in
Florida would think I mean Bourbon, which tastes about the same and
makes you go blind too.)

>From: "Ian Gatenby" <igatenby at optushome.com.au> 
>Reply-To: Mercedes Coupes Mailing Lists <mbcoupes at mbcoupes.com> 
>To: <mbcoupes at mbcoupes.com> 
>Subject: [W126 Coupe] No spark - 500SEC 
>Date: Tue, 8 Feb 2005 20:11:17 +1100 
> 
>My 1983 500SEC is in trouble. 
> 
>It has been a bit troublesome to start on our rare wet mornings - 
>and a few weeks ago refused to do so. 
> 
>So - new spark plugs - no go, but had spark - checked the fuel 
>filter - clogged (I have had problems with this recently - either a 
>local petrol station has dodgy fuel, or a hose on the car is 
>deteriorating).  Replaced fuel filter - car started and ran for a 
>couple of minutes - next time I went to start it - no go. 
> 
>Checked spark - no spark.  The water pump on the firewall for the 
>climate control is running when the ignition is on (haven't noticed 
>this before) and there is about 8 volts across the ignition coil 
>terminals with the ignition on.  My other car, a 1981 500SE has 
>negligible voltage here - so, swapped coils and the SE still runs 
>and the SEC doesn't. 
> 
>Can anyone help?  Which computer module(s) or sensors should I start 
>looking at? 
> 
>Ian 
> 
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