I have a 1989 300E that has been sitting for a little while; I got a new battery and the car doesn't seem to start. It turns over but doesn't start. I have replaced the fuel filter and now think that the problem is either the fuel pump or the fuel pump relay. I am pretty sure that the fuel pump SHOULD be heard when the key is turned; however, I just don't hear anything when I turn the key other than the engine trying to start. Is there any way I can figure out whether I need a fuel pump or a relay? Is there any other problem it could be? Should I hear the fuel pump? Thanks to all who answer.

 

some mercedes fuel pumps can be noisy for a few different reasons.
1. If the fuel filter is plugged it can make the pump noisy.
2. If the fuel pump is in the tank incorrectly to were the pump can touch metal in the tank to make noise.
3. The fuel pump is giving you signs that6 it is about to fail and needs replacing.

Try replacing the fuel pump relay, recheck system.

 

Hi LongliveBenz,

Hopefully it something simple like anoverload relay with oxidized connections, or the system has lost its prime due to evaporation or a tiny pressure loss.

How long has it been sitting? This is important for a number of reasons. "Just sitting" presents an entirely different set of problems thanwear.

Are you getting electricity at the fuel pump terminals, when you turn the igntion on?

Blue

 

yes, you should be able to hear the fuel pump. in fact, there are actually 2 fuel pumps, one in the tank and one outside the tank, assuming you have a vehicle using a cis fuel injection system, identified by the lack of electrical connectors on the fuel injectors. If so, and you want to verify fuel pressure without the use of a guage, just to see if the pumps are even working, remove the air filter housing (3 nuts) and you will see the brass colored fuel metering actuator (disc shaped). If you press down on this disc you should feel moderate resistance and you will hear a sound similar to pumping an old worn out jack as the control plunger travels in its tight fitting cylinder immersed in gas. Btw, you should be able to check this with the car off, just make sure to cycle the key first to build pressure, assuming the pump is working.

If the pump is not working, try bypassing the solid state fuel pump relay located behind the battery. I can look up the exact terminals to jumper if you find this is the problem. Good luck.

 

I have an '88 300E. I've had a similar problem in the past. My car does in fact have dual fuel pumps. There is a voltage test you can do if you pick up a Hanes repair manual that you can test the pumps. There has to be a certain ammount of voltage. What my problem ended up being is under the hood, near the battery box, there is a small black box. Being it late at night, I cannot remember the name of the box. I believe its called fuel delivery control or something along those lines. Mine was bad. It wasn't a terribly spendy part, but it was surely an annoyance. Try checking that as well. What I did, since I found out the 2.8 box (for a smaller liter engine) would actually give me better performance, I simply went with that instead of the standard 3.0 box. Didn't give me a ton more performance, but a little more.

 

I have a 88 260E that had a similar problem. Your car should have two fuel pumps in front of the rear axle on the passenger side. Remove the plastic cover to access the fuel pump/filter assembly. Attach a voltmeter to both of one of the fuel pumps power terminals and check for voltage. When the ignition switch hits the RUN position you should see a short pulse of 12 VDC to pressurize the fuel system. When the ignition is in the START position you should see 12 VDC constant or until you stop cranking.

If you never see power at the pump i'd suspect the fuel pump relay which is located behind the battery. There are two large black relays and one small one just behind the ABS computer. Just lift the ABS computer out of the holder and lay it aside. No need to disconnect the wiring harness just lay it out of the way being careful not to short the battery leads. The fuel pump relay will be the one closest to the battery and will look identical to the one behind it but they are not the same as the pinouts are very different. The smaller one mounted to the right is the overload protection relay.

From here you have several options. First check that the relay is fully inserted into the base. A loose connection here can cause multipule problems as the fuel pump relay is a multi contact device controlling more than just the fuel pumps. Push down firmly to insure the relay hasn't simply worked loose. Give it a good thump after you've seated it properly. Mine would go intermittent and only required a good thump to get it going again.

Second remove the relay and use a jumper wire to manually power the pumps. Check the wiring harness for your particular model and obtain the correct pins to jump. The holes that the relay plugs into are numbered to make identification easier. Unless you know exactly what pins to jump don't attempt this as you may cause a short and fry your ECU or worse. Use a short lead of wire to power the pumps just long enough to verify operation. You should not attempt to run or drive the car in this state this is for diagnosis only. Your only attempting to get 12 VDC at the pump long enough to verify a malfunctioning fuel pump relay not diagnose other issues. If you have power with the jumper in place the relay is the likely culprit.

Be sure to attach both leads directly to the positive and negative side of the fuel pump. Alot of people will use the chassis as a negative ground (-) which is fine in some situations but there are times when you lose the ground instead of the positive hot lead from the fuel pump relay. If you reference the chassis instead of the negative terminal on the pump you will get a good 12 VDC reading indicating a bad fuel pump when in fact it simply lost its ground. It's a good rule of thumb to reference the negative (-) to the device your testing no matter the scenario.

Hope this helps. If you can't find your models pinout for the fuel pump relay email model/ year and i'll find them for you. If you do need a relay don't get ripped buying local. Every store around here wanted $190 for the relay.....ouch! Try www.autohausaz.com instead. Replaced mine for $90 since shipping is free for any item over $50. Most items are Bosch/Mercedes OEM parts not the reman junk at the local shops. Fuel pump local (reman off name) $236-Fuel pump from Autohaus (BOSCH brand new) $130. You do the math.

Good luck!

 

Saw your message and post about your 300E. Got a similiar problem with a '88 190E. No voltage across the two terminals at the Fuel Pump. I've made the assumption that the terminals are + & - down under the car rather than going to a chassis ground.There are a number of numbered terminalsby the battery which I'll assume based on your note bypass some of the relays located by the battery. Any idea which terminals to cross to bypass the fuel pump relay?

This 88 190E with almost 200K on it had a whinning fuel pump which has suddenly gone silent. Not sure if it is the pump or relay. Car will not start.

Jim Gruber
Dayton, OH