[JoshL600]

I have a 1991 560SEL that has a no start condition that I am struggling with to diagnose and any advice would be appreciated.
Car will start and drive (very minimal throttle response though) and then stall and will crank but not restart. This happened for about a week and now it will not start at all, just cranks over.
Replaced the fuel pump relay, no change in condition. Before going further, I removed the cover from the fuel pump relay and pushed the contact while having someone crank the vehicle over and it fires right up with no issue. As soon as I take my finger off the contact in the relay, the car dies. So I know I have spark and fuel, but no activation signal going to the fuel pump relay.
Any thoughts on where to go with this?

 

[csquared4290]

You gave a key detail that tells all. If you "push down" on the relay, all works. WHen you're not, it doesn't, which means the contacts aren't seating correctly.

Clean them and see what it does.

Also note that if you replaced the FPR with a used one, they all suffer from cracked soldering joints from age and constant heat cycles. If you know how to work a soldering gun, you can pop it open, look for hairline cracks on the solder joints, and just remelt the solder and let cool, to close the cracks.

Otherwise, get a new FPR, which you can get off ebay easily and cheap.

 

[JoshL600]

You gave a key detail that tells all. If you "push down" on the relay, all works. WHen you're not, it doesn't, which means the contacts aren't seating correctly.

Clean them and see what it does.

Also note that if you replaced the FPR with a used one, they all suffer from cracked soldering joints from age and constant heat cycles. If you know how to work a soldering gun, you can pop it open, look for hairline cracks on the solder joints, and just remelt the solder and let cool, to close the cracks.

Otherwise, get a new FPR, which you can get off ebay easily and cheap.

The fuel pump relay is a new one. I also swapped and tested it with a known good fuel pump relay from another car and all had the same issue.

 

[csquared4290]

so...refer to my first part. Check the contacts of the connector and clean them if needed. Check too that the cylindrical wire leads aren't too loose.

 

[Heikkif]

When it runs only by manipulating fuel pump relay or when it cranks but doesn't start does taco show rpm?

 

[ianrandom]

Welcome @JoshL600

Can I recommend filling in your vehicle details here please? Model and year are helpful - Complete your Profile

 

[John350]

When you push the relay to get the pump to run, are the contacts already closed? I.e. are you closing the contacts or just putting more pressure on them? My bet is it's the circuitry that tells the relay to turn on. When you first turn the key to "run" does the fuel pump run for a second? It should, but shuts off after a second absent an RPM input.

 

[Gogi]

Some good advice. To add can, you remove the socket from the plastic socket for the FPR. After cleaning the FPR relay and the FPR socket with contact cleaner. If the problem still persists. The female socket holder needs to be pulled up by releasing one or two black tabs in the socket/holder or connector. contacts in the socket need need to be tweaked just a tad just, so the the male ends of the relay still fit but snug. Use contact cleaner in an aerosol can from any office Depo or PC Richard’s.

Dave @[email protected]

 

[tusabes]

Everyone has pointed you in the right direction, the fuel pump relay will shut off if it’s not sensing RPMs. So that’s why you need to verify whether your tachometer is working

and if the fuel pump relay works while it’s being physically pushed on, it’s simply a matter of not making a good connection. But, as stated above, you may need to adjust the female receptacle ends for the relay, not just clean the prongs of the relay.

 

[csquared4290]

Some good advice. To add can, you remove the socket from the plastic socket for the FPR.
YOU SHOULD BE ABLE TO SQUEEZE. The female contacts in the socket need need to be tweaked just a tad just, so the the male ends of the relay still fit but snug. Use contact cleaner in an aerosol can from any office Depo or PC Richard’s.

Dave @d[email protected]

If I may add to this, if you do take apart the plastic socket for the FPR, take plenty of pictures or even, record a video of disassembly so you remember the pin order when reassambling. If you forget, members will supply the diagram and wiring order--but it'll save you a ton of time to keep track of your work. Otherwise, if you mess up the pin order you'll have a whole new set of headaches!

It's fairly easy to expand the pin connectors too much as the FPR is very snug in its connector. Careless people tend to wiggle the FPR out of its socket wayyyyyy too aggressively, which causes the pins to spread and not create a good contact when reconnected.

 

[JoshL600]

Thanks for all the replies so quickly. I had a few other vehicles I was tied up with today and haven't gotten back to that one yet. Yes, the taco is working and Yes I am pushing the contacts closed in the relay, not just putting additional pressure on them. I will go through the female end of it tomorrow and follow up with what I find.

 

[John350]

Take a look during that first second after you turn the key on (even if you need a second person). If the points close then open you are not getting the RPM signal. If they don't close in the first place the relay isn't getting power to the energizing circuit. (The switched power for the pumps is a separate, un-fused circuit.)

 

[Former Repair Manager]

without diagnosing with the equipment to me, it sounds like a computer related problem. You may be able to locate a good used computer, but it will have to be married to the vehicle by the dealership.

 

[Former Repair Manager]

If the engine management computer has a short circuit it can cause the relay in question to blow out. I’m pretty confident that you should get a Re manufactured computer or a good used computer

 

[DerPhantom]

As stated in an earlier reply, the problem may very well be the circitry that controls the relay. If you close the contacts manually and the car runs you've eliminated spark, air and fuel as the culprits.
I suggest removing the ECU connector and inspect the connections. Although unlikely, there might be some corrosion that needs to be addressed. Also do a continuity check on the harness between the ECU and FPR control circuit. Also make sure the FPR relay ground circuit is good as well.

 

[John350]

the problem may very well be the circitry that controls the relay

When I mentioned that I was referring to the logic inside the relay and its related inputs, not the ecu. Attached is the wiring diagram for the fuel pump relay. We have determined that if the relay closes the engine runs, so the question is "why is it not closing on its own." We've also replaced the relay, so it must be something external to the relay, i.e. one of the inputs.

 

[alexstorm]

Resolder the large contacts inside the FPR might help. Sometimes these parts sit for years and the solder gets brittle.

 

[Sven Gally]

Also just a minor point. It is possible in the heat of the battle to accidentally put the FPR in backwards. Make sure you have it in the socket correctly. I matched up the numbers on the socket to the numbers on the FPR diagram to make sure they aligned correctly. Also Kent Bergsma at Mercedes Source has designed a nice jumper with a switch to test the fuel pumps. It’s worth the time not having to create a jumper. It’s nice because the cables are long and the on off switch gives you a moment to set up, Also you don’t need to have the engine running to test the pressure with the switch.

 

[Former Repair Manager]

Catalytic converter is plugged up

 

[fitzcarraldo]

@ Former Repair Manager
The CIS-E computer does NOT activate the fuel pump relay.
The FPR needs terminal 30, it needs terminal 15 and when you crank the motor it gets power on the terminal 50 pin.
You can completely disconnect the CIS-E computer and the engine should start - although it will take longer, especially when the ambient temperature is low because the necessary mixture enrichment is missing.
The fuel pump relay also needs the TD signal (time division) from the EZL unit after you release the key when you stop cranking. If the FPR does not receive a "clean" TD signal the relay contact in the FPR will open as soon as you let the key go after cranking.
So forget getting another CIS-E computer - it won't fix the problem.

Just my few dimes from down under.