Recently, the smog pump on my 1986 420SEL died. The local dealer wanted to charge me $1200 for the new part alone. I managed to find one at an auto recycler for $350. It looks very simple to replace. There are only two bolts that hold it in place. One on the top and the one on the bottom that slides to provide belt tension. After that the two hoses slip on on and you plug it in.
My problem is getting the old one off. The bolt on the top of the unit looks more like a screw. I tried fitting various types of screw diver and hex keys into what looks like a wide slot with a hole in the middle. Nothing I have comes close to fitting in it. Is there a special tool, or tric to getting this off, or is it just a pin and when I undo the bolt on the bottom will it just slide off (unlikely)?

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Jason Priest
1986 420SEL

I think the top bolt is a 13 and the bottom is a 13 with a 19mm adjuster.

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Benzmac:
Donnie Drummonds
300E
ASE CERTIFIED MASTER AUTO TECHNICIAN
MERCEDES SPECIALIST 8 YRS
PARTNER IN MERCEDESSHOP.COM
OWNER OF MB AUTOWERKS .COM

Unfortunately the top fastener on my '88 is an allen head (10mm, I think) sunken into the fastener and the Official MB-Approved tool has a handle about 4 feet long (which the boys at the dealership like to crank on) so it is very hard to get loose.

I ended up completely removing the 13mm/19mm adjusting bolt on the bottom and was then just able to slip the replacement belt on...

Why do they do this kind of stuff? It is exactly the same situation with the a/c idler pully tension: they have this 4 foot long convoluted wrench and then tighten it so tight you have to use a hammer on a modified box end wrench to break it loose!

p.s. There is plenty of room for a conventional bolt for the top fastener.

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'91 420 SEL @ 199K, '92 SVX @ 181K, '93 SC400 @ 86K, '93 Kaw ZX-11 @ 30K, '87 F250 @ 181K , 2001 Valkyrie Interstate @ 6K, Y2K Honda NightHawk 250 with 1.5K, '88 420SEL I.@ 179K & the 2nd latest, an '88 420SEL II.@ 210K runnin' parts car, '85 F150 300/NP435