Unable to start car after fuel system maintenance

[farmboybob]

I have a '92 500E which had a slow start problem for a few months, so I decided to read the forums here. As a result, I tested the fuel pressure, which was spot on when running but dropped too quickly after shutdown. I have the early 2-pump arrangement with the replaceable check valves, but after reading more I decided to change out both pumps, filter, the tank check valve and all hoses at the tank and pumps. Used Bosch pumps and dealer parts where available for the rest.

Someone had likely changed most everything before as some of the spring retainers were broken around the tank vent hoses. I ended up not changing the supply line from the tank to the pump as it seemed in very good condition, and I couldn't get it off from below and didn't want to remove the gas tank or risk having to do so at this time. I had drained all the fuel out of the tank including what was in the return line from the FRP.

When done, I added about 3 gallons or so to the empty tank and tried to start the car, both with and without the accelerator pedal pressed down, and with and without gas cap on. No start. When the key is turned to the start position without cranking, there is no power at the pumps or audible pumping action, specifically, no sign of 2-second run mentioned in prior forum posts. When the engine is cranked, there is power to the pumps and I feel the pumps thump thumping slowing but I don't see a solid 12 volts of power, rather a fluctuating amount.

The fuel filter and pumps are all oriented correctly and there is no sign of any fuel leak at any of the newly changed components.

How would I proceed to diagnose the problem? Or am I not following the right procedure for re-starting?

 

[gsxr]

There is no special procedure needed for re-starting, after the work you describe. Should fire immediately, no need to press on the accelerator pedal either, nor have the tank cap removed. 3 gallons is plenty of gas in the tank. Dumb question, but what vendor did you purchase the Bosch pumps from? If not a reputable seller, there's a tiny chance they are counterfeits that may not perform properly. But, let's assume the pumps are genuine Bosch and all the work was done correctly.

First step would be to check all fuses, including the blade fuses on the BM/GM module, and also verify both nuts are tight on the positive battery terminal (in particular, the smaller/10mm nut). If the pumps do not run for 2 seconds every time the ignition is turned to position 2 (all dash lights on), the LH module needs replacement or repair (new capacitors may do the trick). If you have a spare LH module, swap that in for test purposes.

The odd part is the fluctuating power at the pumps when running. It should be a steady +12V with the relay engaged. Double check the relay, swap it out if you have a similar relay handy. As a test you can temporarily jump the 2 appropriate sockets with the relay removed to force the pumps on. Another faint possibility is the LH module is cycling the relay, when it should not... again pointing to an LH module issue.

 

[Alphasud40]

In addition to what Dave said I would place your meter on the pump pos. terminal (Bk/Rd/Wt) and the negative to a good chassis ground and check your voltage. Could be a bad ground as well. Both the fuel pump wires go through an electrical connector X36/3 in my book could be a poor connection there is bypassing the relay still shows low voltage. The ground for the fuel pumps is behind the center console from what my book says.

 

[farmboybob]

Thanks so much for the detailed instructions. I'll report back on what I find. On the parts used, I bought a kit from a Haus in AZ which included 2 Bosch pumps, filter, an MB strainer (for inside the tank).

 

[farmboybob]

Progress so far... Looked up the price of an LH Module and happily Mercedes still sells them. Unhappily though, they apparently sell only to those who can afford them. Time for careful diagnostic effort and perhaps prayers for those so inclined.

Checked the BM fuses, all 3 10A fuses were fine. I had put the battery on a trickle charger overnight to "top it up". It had only about 12.5 volts after being fully charged and I saw that it is a 6-year battery that's a couple weeks shy of 7 years old. I'll buy a new one right away before electrical gremlins begin to show up and haunt me. Checked the battery connections, especially the ground cable at both ends as this is usually where corrosion first sets in, I'm told. Nothing was loose or visibly corroded, but I tightened up all connections anyway. Pulled the Relay under the rear passenger seat behind the pneumatic pump, which looked new but had 05 M 91 stamped on it, so maybe original?

-With relay out, I put a voltmeter across sockets 85 and 86 and turned the key to position 2 a few times but got no 2-second or any other voltage reading.
-Even with the key off, socket 87 was live, with 12.11 volts.
-Used a wire with alligator clip ends to jump sockets 87 to 30 and heard a whirring sound so I slid my hand under the car and felt the pumps but felt nothing on either pump.
-Tested the Relay and it was dead. Voltage applied across pins 85 and 86 made no noise and showed no voltage across pins 87 and 30.
-Jumped sockets 87 and 30 again, which started the whirring, then turned the ignition key and the car instantly started up.

Apparently, these relays aren't popular, and I had to order one from Mercedes, supplemental part # 001 542 96 19, $43 plus tax. Even a Vemo aftermarket one couldn't be had locally (San Jose, CA area) and would take as long or longer to obtain than the dealership part.

I'm not sure I'm out of the woods yet with the LH Module. I'll install the new relay when it arrives and then see if my pumps run for 2 seconds when the key is turned to position 2.

 

[maw1124]

Your symptoms point more to a failed relay than a failed LH module (to me at least). I've had a failed LH on a fairly low mile '92 car, but yours seems like a simple failed relay. Fingers crossed.

Cheers,

maw

 

[Alphasud40]

While we are on the topic of a suspect LH control unit now would be a good time to replace the capacitors. I was going to order caps for my LH and spare. Was thinking a group buy if others are interested. They would be 105 degree C high temp caps from Vishay or Panasonic.

 

[sheward]

I am interested.

 

[gsxr]

More info on caps here:

No fuel pump and buzzing relay | Electrical, CAN and Wiring

I need some advice please... When I turn the ignition on I have a buzz coming from the "central" rectangular relay (1/92) in the relay box behind the fuses; also the fuel pump does not run. The engine cranks but does not fire up. I have cleaned all the fuses but there does not seem to be a fuel...

www.500eboard.co

 

[farmboybob]

The new fuel pump relay I ordered from the dealer came in. I drove to the big City to pick it up and saw hundreds of new Mercedes, not a single one with a hood ornament. Is that really true? Hood ornaments are NLA on new MB's? I know I digress, but geez.

With the new relay in the car it started right up. I was happy. Really happy, then....

-About 5 seconds or less later, the relay started rapidly clicking.
-with relay pulled, measured voltage across sockets 85 & 86 when the key was turned from off to position 2: a constant 11.2 volts, no 2 seconds of voltage then off.
-with pumps hot-wired, started the car and got a fairly constant 14.3v across sockets 85 & 86. I let the car run this way over 30 seconds (10+ seconds after the automatic fast idle slowed down). Seemed to run fine.
-Repeated 1st step: relay in, car started, ran fine, but after about 5 sec the relay again started clicking, then the clicking slowed a little and almost stopped, but quickly started again with variation in speed and tempo. Not so happy anymore.

@gsxr had posted above with a link to another similar post(s), in which it was said that the constant clicking of the relay is a sign of a failed LH Module.

So, do I need a replacement module? And if so, I don't have skills to repair caps, and I know that the usual fixed is then to pop in a used one at less than a 10th the price of new. But how reliable are the old ones? Can they go for 5 or 15 years? Or is this a likely swap out every-few-years proposition? How about the reman units, which it seems MBZ sells as well as new ones?

I'm planning to keep the car as a daily driver until the end of the 2nd age of the dinosaurs, i.e., when fossil fuels are NLA for practical purposes. Especially now that replacing it with a new MBZ in future may not be possible on account of the hood ornament thing.

 

[nocfn]

You can send the unit for new caps to Programa, or buy a used one that will also need to have new caps at some point.

 

[gsxr]

So, do I need a replacement module?

Yes - or, repair the existing module. Note that your 1992 500E should use a 1992-specific LH module to retain the WOT enrichment function (this was deleted as of USA 1993 model year).

And if so, I dont have skills to repair caps, and I know that the usual fixed is then to pop in a used one at less than a 10th the price of new. But how reliable are the old ones? Can they go for 5 or 15 years?

Yes.

Or is this a likely swap out every-few-years proposition?

Nope, just get a module refurbished with new capacitors. This isn't rocket surgery, any local electronics shop that knows how to handle soldering equipment on PCB's can handle the repair. Paying more doesn't guarantee a better repair: I think Beckmann/BlueLink wanted like $1500 (?) to "refurbish" an LH module, but that's just boosting their kid's college fund, not providing a 'better' repair.

How about the reman units, which it seems MBZ sells as well as new ones?

AFAICT all the E500E modules are NLA from the dealer, per MB Classic (click here and here), despite what dealer websites may claim. Besides, $1500-$2500 is insane money when you can get your existing module re-capped locally for probably $150-$250. Note that any NOS module will still have capacitors that are 20-30 years old and there's no guarantee you won't have the same problem after your wallet is lightened by nearly three grand.

Im planning to keep the car as a daily driver until the end of the 2nd age of the dinosaurs, i.e., when fossil fuels are NLA for practical purposes. Especially now that replacing it with a new MBZ in future may not be possible on account of the hood ornament thing.

Same here. Guess I could tape an old hood ornament on a new car but it might look funny.