1986 420 sel

How do you know definitly if the the distributor cap is bad as oppossed to rotor or coil or wires or spark plug. How can I defferentiate all these components.

Please enlighten me.

I am not an expert on the 420, but generally speaking here is how I do it:
1) pull the wire from the coil to the distributer and lay the end of the wire about a half inch from the metal of the engine. Crank the engine. You should see a hot blue color spark jump at least a half inch with a snap. That proves the coil and wire to distributer is ok.
2) Wires usually measure about 7K ohms each. If they measure 20K, there is a problem.
3) Reinsert the wire from coil into distributopr and pull spark wires one by one and repeat the initial test. Lay each wire one by one a half inch from engine block and crank engine. Observe that spark is hot, blue color and with snap. If all wires show weak spark you have bad distributor cap or rotor, or bad wires.

Thats how I do it.
Rich

Look for CRACKS, CARBON TRACKING and BURNT AREAS around the TERMINALS . . . Rotors tend to BURN behind the CENTER contact to GROUND . . .

Thanks for the info guys, I will test it tomorrow and let you know. How can I differentiate between cap and rotor and spark plug?

satkhalsa

Go back and re-read mpnye's post. Checking MB caps and rotors is mostly a visual inspection.

How many miles on your present cap and rotors? If > 50-75 k miles, pretty good chance you'll see a reason to replace them once you LOOK at them carefully.

What kind of running problem are you having? Rough idle? Poor acceleration? What is prompting you to think you have an ignition system problem?

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Steve
'93 400E

hey guys i have a 1993 400e i have 56,000 miles on it now i have what u call rough idle and even the dealer couldnt fix it they said there was a bad supressor switch but i guess it wasnt it doesnt do it all teh time some way around i notice is what should be the first thing i be checking and changing
any advise would help
thank s in advance
mani

The 1986 420 sel had stalled while on the highway. towed it back worked on. There is fuel in the chamber, spark very week. While testing spark I had another car hooked with booster cables, while checking the distributor to coil wire all of a sudden the starter stopped turning. checked under neath there is juice to the starter but wont turn.
Please advise on possible causes.

There is something connected to the trainy that sometimes wont let the car start unless it is in neutral that has happened a lot in the past. even in neutral it doesnt turn. Is there blown fuse? I checked what i believe is the ovp relay the fuse is in tack. is ovp relay located next to the fuel pump relay and ac realay?

I cant check the new spark plugs since no crank.

I need help fast.

I used to test for distributor cap leaks when I was hot rodding by revving the engine on a humid night with the hood open. Humid nights are common here on the Texas coast so I never had to wait for one. If you don't have the pleasure of humid nights, mist a little water around the cap, start the engine and rev it to 4,000 rpm or so. Watch your hands while you are messing about under the hood. The fan is running at night just the same as it runs in the day.

If you have a leak you will easily see it track down the side of the cap. You can see other ignition leaks along your wires and plug caps.