On a 1989 420 SEL, I need to re-install my distributor correctly. How does one set ignition TDC? The service CD says "in ignition TDC the rocker arms at no. 1 cylinder are relieved" look through the oil filler cap.

I have the front cam lobe pointing up exactly at the oil tube and the second cam lobe pointing towards the passenger side, about horizontal to the ground. The pointer on the water pump is directly over the 0 on the harmonic balancer.

Is this ignition TDC?

Should I keep rotating the crank shaft to try to get both cam lobes pointing up, say at 45 degree angles?

If this is ignition TDC, do I just insert the distributor and position the rotor to point at wire leading to cylinder 1?

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1989 420 SEL

At TDC of the compression stroke, neither the intake nor the exhaust valve are open. Therefore, the lobes will not pushing the valve stems, and will be pointing away from the stems to some degree or other.

At TDC of the exhaust stroke, the exhaust valve will be open, and the intake may be open to some degree, depending on cam grind.

The cam lobes may not be symmetrical at TDC, so use the cam lobes to find if you are on the compresson or exhaust stroke, and use the pointer to find 0 degrees.

Sounds like you have it right as it is.

Best Regards,
Jim

Most MB's have a mark on the side or top of the dist housing where #1 will go with the cap on that you use to align the rotor.

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Chuck Taylor
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe

Thanks a billion! Lined up the rotor dead centre on the line on the distributor housing. Fired up, ran well! Took her for a little run, no leaks. My water pump job is finally finished.

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1989 420 SEL