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post #1 of 11 (permalink) Old 06-17-2019, 06:40 PM Thread Starter
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500SL 1984 Help/General Question

I recently purchased a 1984 500SL from a friend of mine.

I've been working on it gradually trying to get it back in running order, however I seem to have hit a snag.

A few weeks ago a friend of mine and I attempted to turn the car over. The engine would crank, however would not ignite. We injected ether into engine and did get the engine to turn over but it would not run when we cut off the ether supply. We checked the fuel supply line to the injector housing and discovered that there was "old" gas as far up the fuel system as the injector housing. (The car was improperly stored and as a result there was roughly 7 US gallons of gas left in the tank while the car was stored, I have since emptied the tank, added in fresh gas and some B12 Chem-Tool to break down any lacquering in the tank as recommended by my mechanic.)

However upon checking the fuel pump, when turning the key we noticed that the fuel pump would not turn on. We checked the voltage and got no reading on the positive terminal.

I've changed the fuses and the battery. And ordered a new replacement fuel pump, filter and fuel pump relay.

I know from looking around under the car where the fuel pump, filter and accumulator are located, however I am having trouble locating where the relay control unit is located. I've seen conflicting answers and photos online, some have said behind the glove box, someone else told me somewhere under the hood, near or around the firewall.

Any assistance or answers that anyone could provide would be greatly appreciated.
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post #2 of 11 (permalink) Old 06-18-2019, 07:53 AM
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The pump only gets voltage momentarily when the ignition if first turned on. Try applying 12V directly to the pump to see if it runs. The relay is behind the glove box. Liner is pretty east to remove. There are four reusable drive rivets. Pry the center up to remove. Pop the light lens of and unplug.
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post #3 of 11 (permalink) Old 06-18-2019, 08:06 AM Thread Starter
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The pump only gets voltage momentarily when the ignition if first turned on. Try applying 12V directly to the pump to see if it runs. The relay is behind the glove box. Liner is pretty east to remove. There are four reusable drive rivets. Pry the center up to remove. Pop the light lens of and unplug.
I'll attempt to remove the lower glove box/dashboard assembly this weekend when I have more time. However, when we were checking everything we weren't getting any voltage to the fuel pump when turning the key to any position, or when attempting to turn the car over, nor did we hear the fuel pump spin/power up. We even had someone monitoring a voltmeter while we moved the key to all positions to make sure.
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post #4 of 11 (permalink) Old 06-18-2019, 09:28 AM
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Easier to remove the glove box liner.
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post #5 of 11 (permalink) Old 06-18-2019, 10:42 PM Thread Starter
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Easier to remove the glove box liner.
Actually I would argue that, to me it seems easier to change the fuel pump rather than disassemble portions of the dash.

Regardless, given that A) Fuel was left in the tank when the car was stored and B ) I have read from several sources that Mercedes fuel pumps of certain makes/models/vintages, tend to fail around 30k-40k miles, it probably needs to be replaced anyway either from age, lacquering or just general failure.

I'll attempt to pry the glove box liner up over the weekend when I do the fuel pump change.
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post #6 of 11 (permalink) Old 06-19-2019, 05:41 AM
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Actually I would argue that, to me it seems easier to change the fuel pump rather than disassemble portions of the dash.

Regardless, given that A) Fuel was left in the tank when the car was stored and B ) I have read from several sources that Mercedes fuel pumps of certain makes/models/vintages, tend to fail around 30k-40k miles, it probably needs to be replaced anyway either from age, lacquering or just general failure.

I'll attempt to pry the glove box liner up over the weekend when I do the fuel pump change.
My comment was in reference to your statement you were going to remove the lower dash panel. Glove box liner takes about five minutes or less to remove.

With that scenario the pump probably needs replacing. Never heard of OE or OEM 107 pumps only lasting 30-40K in general.
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post #7 of 11 (permalink) Old 06-19-2019, 07:48 AM Thread Starter
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My comment was in reference to your statement you were going to remove the lower dash panel. Glove box liner takes about five minutes or less to remove.

With that scenario the pump probably needs replacing. Never heard of OE or OEM 107 pumps only lasting 30-40K in general.
I misspoke, I consider the glove box to be part of the dash assembly and after working on several cars with various family members over the years, I've become averse to disassembling the portions of the dash as it usually becomes more trouble than its worth, and I will admit in my cursory scan of the glovebox I didn't even notice the four retention "clips" inside.

With regards to the fuel pump, I figured after there was gas left in the tank (mentioned in my initial post) and the fact that the aforementioned gas was in there for some considerable amount of time; there was going to be damage on some level to the pump, filter and possibly even the accumulator. (I haven't been able to diagnose the latter yet.)

To clarify my statement about the fuel pump, I've spoken to several mechanics and a few MB techs I work with (both worked for MB Europe). They've all stated to me that fuel pumps of this age are more prone to failing than modern ones, some take longer than others but their personal feelings/averages was roughly around 30k-40k miles based upon drive time, normal wear and tear, the placement behind the tires etc. The pumps are not built to fail, but those parts do tend to fail more often than other components.
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post #8 of 11 (permalink) Old 06-19-2019, 09:54 AM
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The fuel pump only gets voltage for a second or two when you turn on the accessories and then crank. You'll need a helper holding a multimeter onto the pumps poles to see the momentary spike of voltage. Most likely, you have horribly clogged fuel lines.

The fuel pump should be replaced, not because of age but because the old gas has probably ruined it .

You will probably want to consider removing the entire fuel delivery system in the rear to replace every component and hose.

I'd also consider removing the tank and cleaning it. A radiator shop will happily boil these for you. Don't get it coat.

Jyuma has a story where he disconnected the fuel inlet and outlet let hoses at the engine and then pushed compressed air towards the tank (components removed).

He unclogged a ton of crap doing so. I would consider this as your next step.

Once those lines are all clear and the fuel system has been refreshed , get two cans of techron and some gas and fill the tank up. See if you can start the car.
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post #9 of 11 (permalink) Old 06-19-2019, 12:17 PM
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And hope that the Fuel Distributor isn't gummed up. Lots of threads on this PITA problem.
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post #10 of 11 (permalink) Old 06-19-2019, 12:34 PM
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And hope that the Fuel Distributor isn't gummed up. Lots of threads on this PITA problem.
Yeah definitely include that. I'd also put the warm up regulator and the frequency valve too if the car was federalized at any point.

If the fuel distributor is gummed up, that can be fixed by soaking it once disassembled and replacing some seals. The warm up regulator is a bit more of a b!tch. I'd send it to the Kjet.biz in Australia or cis flowtech in Alabama since the effort involved in rebuilding those is strangely harder than the distributor.

Tackle these two after you've done due diligence in cleaning the tank and lines to the engine and the no start remains.
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