Hello!

This is my first post, although I've been linked to the forum quite a bit in the past via google searches. I've owned a 1988 240DL for a few years and have been mostly working on it myself.

Recently the engine won't start. It happened abruptly, the last time I drove it there were no issues. It will turn over, and spark, but won't fire. The spark plugs are dry, so I'm thinking the engine is not getting fuel. There is no in-tank pump, it was faulty and we never replaced it, although we did replace the main pump a little over a year ago, as well as the pump relay, located inside the dash.

The main pump works, I ran 12 volts to fuse #4 and can hear the motor working. There does not seem to be a fuel leak, so I believe the system is still pressurized.

Can anyone point me to where I might search next? I'm assuming it's fuel because the electrical system seems fine and the spark plugs are dry. A little help here?

Thanks!

Andy in San Francisco.

 

you should verify the pump relay is working when you crank the car over. you could hook a volt meter or test light between fuse 4 and ground . If it lights/reads when you crank the car, then ECU is turning on, and seeing timing, and the fuel pump relay is working, thats all good. if that circuit doesn't get powered, then the fuel injectors aren't powered either, oops, no fuel.


you are putting a strain on your new main pump, and it will fail early if you dont replace the tank pump. you definitely should not run it below about 1/2 tank.

 

Thank you for the advice, I'll try that when I get home. If the test light proves the pump is engaging come on, what should I try next?

Should I try those in-car computer diagnostic tests I've read about?

 

1988 was the last year for LH2.2, It was LH2.4 in 1989+ that has the onboard diagnostics.

 

So I got home and hooked up my test light to the fuse #4 and ground. I figured ground was anywhere that was metal on the car that wasn't making my light go on (correct?). The light would not come on when I tried to start the car. I checked out the relay unit, which is under the passenger dash and everything 'looks' fine on it. Does this mean it's bad or does that just mean I'm grounding to the wrong spot?

Conversely, if I attach my light to fuse #4, and say, some other wires elsewhere on the car, the light goes on. Is that just because the circuits are connected?

I investigated my relay unit, located under the passenger dash, and it looks just fine, how do you know if it's faulty?

Thanks for the info on the diagnostics, I notice I don't have that box, ha.

 

to double check your ground, go from any fuse from 6-10 to that same ground, thats always-on power, so if it doesn't light up, find another ground...


btw, on any 240 with those old style euro 'ceramic' fuses, I *always* recommend getting a complete replacement set of the copper/brass real european fuses. google for 'mercedes w124 brass fuses', and you should find enough to do the whole car twice over for like $10-12. the american Buss/Littlefuse silver/tin ones are nasty, they corrode and turn dull grey, then rot out. also when you change them, use a soft pink pencil eraser to clean the inside of the spring contact fingers. your electrics will be 10X more reliable after this.

 

Thank you!

I've tried like you said, and the light goes bright from fuse #6 to ground, so that means I've got a good ground.

When I connect to fuse #4, then to this same ground, and try and start the car, no light.

Does this mean I need to replace my relay, or could it be something else?

 

THe relay is about the only thing in this circuit.... WHen you replaced the relay before, did you use a Volvo branded one, or a Stribel, or just whatever was cheapest? I highly recommend sticking with the Stribel OEM relays, had a lot of the noname ones (and KAE) go bad. They have a logo thats a small circle with an S that has like )( going through it (almost like a $ sign in a circle), and they are white.

Fuel Pump Relay 240 700 900

 

We have never replaced the relay. When we replaced the main fuel pump 1.5 years ago, our repeated running fuse 4 and 6 to test the fuel pressure broke one of the springs in the unit, which we replaced. The relay unit itself is probably original.

Is there a way to test the unit itself to see if it is faulty or does the fuse test you mentioned prove it needs to be replaced?

 

ah, I thought you said you'd replaced the pump relay. this is a rectangular relay behind the glovebox. you can access it by pulling the 'felt' kick panel above the passenger side foot well.

the fuel pump relay on that car is actually a double relay, one half powers the ECU and stuff, and the other half powers the fuel pumps and injectors. and stuff. its kind of hard to test 'out of circuit' as the fuel pump half is dependent on the ECU half. if you unplug the relay, and jumper pin 30 (unfused and unswitched power from battery) to pin 87/2, the fuel pump(s) should come on.

oh. have you checked the injection main power fuse thats near the power steering reservoir ? tihis is a 15A or 25A (I forget) 2 prong fuse. that fuse holder gets pretty cruddy in 25+ years, I've replaced mine with a marine style waterproof fuse holder... if the fuel pump relay socket pin 30 has power relative to ground, then that fuse is OK.

 

No. I pulled the relay box, took it apart and replaced a broken spring inside, but the box itself has not been replaced. I pulled it apart again and the circuit board looks fine, and pins look fine too.

I went home today and checked out that fuse under the hood, sure enough it was fried, so I replaced it and the car fired right up! Thanks!!