easy to rebuild vacume pump on 80 300td? - PeachParts Mercedes-Benz Forum

mgerdes · #1 10-07-2001, 05:24 PM

i just bought a 300wagon thats been sitting for a while. this car has hard brakes and wont quit with the key. i think its the vacume pump. how hard is it to rebuild and are the parts available? i have a 85 sd for parts (weak trans) but the pumps dont seem to be the same. any ideas? i did a search and didnt find a direct answer. thanks for your help! mark

#2 10-07-2001, 07:07 PM

Mark,

I have a 1980 300D and i rebuilt the vac pump several times. The first time repaired the ripped diaphragm and I replaced the valves as well. The new valves failed < 10k miles later. Replaced those valves and the new ones also failed < 10k miles later. SEveral shop mechanix told me they never bothered with rebuilding vac pumps for this reason. I couldn't figure out the failure mode for the valves, but I put a used pump in I pulled out of a boneyard and it's been good for > 30k miles now.

I don't know what to tell you, other than give it a try and beware if the valves keep failing.

someguyfromMaryland

#3 10-07-2001, 07:09 PM

Mark,

Sorry for not answering the original question, the pump is really easy to rebuild on the bench. Many people have rebuilt them in the car, but the fan gets in the way unless you remove it.

someguyfromMaryland

mgerdes · #4 10-07-2001, 07:21 PM

thanks for the reply, how do the parts come? from the dealer? and about how much? if this fan comes out easily, the bolts look pretty rounded, ill try to get apart on the car. thanks. mark

can-do · #5 10-07-2001, 08:56 PM

Greetings Mark,

I would rebuild the vac pump before thinking of replacing it. I to own an '80 300TD that had a bad vac diaphragm which I purchased on fastlane for about $8.00. There was nothing to remove except the pump housing itself. No need to remove the fan or any other item if you have just a few more minutes to spare taking your time to undo the metric head bolts that hold the vac pump cover in place. The reason the valves seem to go bad is that folks forget that once the diaphragm gets a tear or hole in it, it immediately starts drawing oil into the diaphragm section as well as through the valves. If you use a carb or brake cleaner once the housing is off on both sides of the valves they will hold pressure as they were designed to. Ensure the remaining spray cleaner is totally dry before assembly to get rid of all the oil residue.

Charles

mgerdes · #6 10-07-2001, 09:20 PM

thanks i will try that, initially it looks like not enough room to pull the pump with out moving the fan, but if it can be done ill try it, by the way, what drives the pump? thanks, mark

can-do · #7 10-07-2001, 09:51 PM

Greetings Mark,

Your Vac pump is driven internally by an idler gear. I don't think your problem is internal but most likely a leaking diaphragm on the pump. Do a search on vac pumps and you'll see my thread on this topic on how to diagnose it being the diaphragm as your culprit.

Charles

mgerdes · #8 10-07-2001, 10:10 PM

ill tear it down tomorrow and let you know how it goes. by the way the pump is making some vacume, the door locks work very slowly and the brakes will work once, then after another mile or so, and ive pulled one of the small lines and there is a trace of vacume but not much. hey what do you expect , the rest of the car does everything right except for the a/c, and its been converted to 134a. one owner to boot. and all for 150 bucks at goodwill. thanks for your help. mark

mgerdes · #9 10-09-2001, 08:19 AM

rebuilt the pump and i got great vacume, got my brakes back but the engine wont die with the key. i can run vacume direct to the pump and it shuts off great. the vacume is going inside the car and not coming back out with enough umph to actuate the valve. any ideas??? the parts to fix the pump were 15 for the two valves and the bladder here in austin. thanks , mark.

can-do · #10 10-09-2001, 08:40 PM

Greetings mark,

There's a vacuum shutoff valve located under the ignition switch, small item held on with two screws and a pain to get off. Once removed spray it internally with brake or carb cleaner to rid it of engine oil that it's been sucking. Let it dry completely then spray a little silicon spray in the ports to lube the o-rings. Reinstal and your problem should disappear. If this is not the cause then the vacuum shutoff on the back of the injection pump is bad and needs to simply be replaced. This item can simply be checked with a mighty vac to see if it hold a vac, if it leaks down in a few minutes the diaphragm is shot on it. This guy cost around $14 - $22 on Fastlane.

Charles

psfred · #11 10-09-2001, 09:20 PM

You may have a bad diaphram, and you may just have a serious leak in the main hose. Check the metal end for tightness, then all the plastic and rubber parts for integrity and tightness.

Locks will work on very low vacuum, usually, even if the shutoff and brake booster don't. For you to lose the booster indicates a very serious leak (as in big cracks in the main line) or a bad diaphram.

All the rubber in the engine compartment goes bad with time, heat, and oil, so expect to replace all the little rubber ends on the vacuum hoses.

If you hear a funny droning noise and have lots of blue smoke (and high oil consumption) the diaphram is definitly shot. There will also be oil in the pressure side line that goes to the intake manifold.

Peter

can-do · #12 10-09-2001, 09:24 PM

Greetings Peter,

He's already fixed his major problem with the vac diaphragm, he's working on the residual problems caused by oil passing through the vac system now.

charles

mgerdes · #13 10-09-2001, 11:21 PM

all is well , guys thanks for the help. the culprit was the valve on the side of the ignition switch, when i removed it it fell apart in my hand. found a new one in town for 16 and that fixed the problem. thanks again for all the help. i think i can make a car out of this yet!! with only 293k., and she passed tx state inspection!. the car looks great for her age, we cleaned her up a little this morning and she shines like new money. thanks, mark

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