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  #1  
Old 03-02-2010, 04:40 PM
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Fuel Pump Relay Blues

We have had our 88 560SL for a little more than six years now. It is my wife's daily driver. Today it died on the highway on her way to work. I managed to get it home, it would run for a couple of miles, die and restart again after cooling for 3-5 minutes. It has all the symtoms of a fuel pump relay failure. The thing is that this is the 3rd FPR to go bad. The one that lasted the longest is the one on the car when I bought it. Car would die when taking a right turn. With the other two the car would just die, they lasted about 0ne year/year and a half. Is this to be expected? I bought one FPR through the internet, the othe other from a local parts store. They both were the exactly like the original. Am I buying bad parts? Any suggestions befor I buy another one?

Thanks.

Jl
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  #2  
Old 03-02-2010, 04:53 PM
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My wild guess is that something else is causing the relay to go bad. Wrong voltage coming from somewhere. Maybe degrades the relay prematurely.
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  #3  
Old 03-02-2010, 05:06 PM
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Sounds reasonable and worth checking, but why would it only damamge the FPR?
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  #4  
Old 03-02-2010, 05:47 PM
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Have you measured the amperage the pumps are drawing???? If the pumps draw to much amperage the relay does not last for long. The main reason for frequent relay failure is clogged fuel filter.
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  #5  
Old 03-02-2010, 05:52 PM
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Not bad voltage.....too much current. The fuel pump is drawing more current than the relay can pass and the contacts get burned and eventully fail. Look for cause of fuel pump working too hard. Possible plugged fuel filter issue or perhaps the fuel pump itself is at the end of it's life.
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  #6  
Old 03-02-2010, 06:07 PM
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There is a fusible link under the hood, but this thing can pass a LOT of current. From what I've seen, the printed circuit board copper will probably vaporize or the solder melt off before the relay contacts go bad. They are not difficult to open and examine, they just sort of "squeeze and snap" together. I hope you didn't toss the old ones because they may be fixable if the current overdraw is the source of the problem.
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  #7  
Old 03-02-2010, 07:00 PM
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I still have them. What am I looking for? How would I fix them?
Too high current makes sense, this would explain why it starts after shut down for a while.
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Old 03-02-2010, 08:07 PM
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They come apart by wedging something thin along the bottom edge and getting it started, the cover and base snap together (at least the MB and KAE units I've seen do this).

Look for melting or discoloration at the parts of the circuit board where the big metal relay is soldered to the board, and where the socket pins are mounted. Do not adjust the potentiometers on the board unless you know what you are doing (assuming that you have MB relays).

Some people swear that the KAE units aren't any good, but they are actually of more modern construction and design (contain a microprocessor and no potentiometers and adjustments).
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  #9  
Old 03-02-2010, 08:08 PM
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It's open

Strife, you are right (as usual). It was very easy to open the relay. Unfortunately there is nothing obviously wrong. No Broken connections or burn marks. Any suggestions?
Thanks.

Jl
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  #10  
Old 03-02-2010, 08:11 PM
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It's POSSIBLE that overheating the relay can cause it to open, when it cools, it would work again.
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  #11  
Old 03-02-2010, 08:27 PM
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Nothing looks terribly bad on the board. There are two heavy duty sorders supplying the relay. They show a thin brown line where the sorder "bump" ends, but that is about it. Maybe I should touch that?
For the heck of it, I am thinking of chopping off the top part of the cover to improve cooling, what do you think? Dangerous?
My last two units were KAE, I am thinking of buying a couple of URO, they are pretty inexpensive and it may be worth a try. Have you heard anything about them?

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  #12  
Old 03-14-2010, 07:06 PM
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Not the FPR?

Well I installed a new FPR and the car started right up, took it for a test drive, everything fine. As I came into the driveway, it died. Now it will not even start. What should I check next? Fuel pumps? Current draw?
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  #13  
Old 03-16-2010, 12:16 AM
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I'd listen to the fuel pumps. Maybe something is getting sucked in somewhere, straining them (and increasing current draw). Also, be aware that the car probably won't run at all with one of the two pumps bad or blocked.
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  #14  
Old 03-16-2010, 11:08 PM
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This sounds like you have something stressing the FPR, drawing excessive current and taking out the relay. In addition to the previously mentioned fuel pump, filter, and strainer checks, you might want to look for connection or wiring problems.

The fuel pumps are underneath the car, and may be exposed to salt, chemicals, and assorted grime. You might have a damaged wire that is intermittently shorting to ground, or a connector that is corroded and causing resistance to be added to the circuit. (more resistance, more current)

Battery cable connectors are known to have corrosion creep back under the insulation, eating up the copper conductors. Something like that could happen to an exposed connection under the car.

Just a suggestion for something to look for while trouble shooting, along with the "usual suspects" in the fuel system.

Best wishes,
Scott
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  #15  
Old 03-17-2010, 10:29 PM
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It's running again ---- no it's not!

I finally got it running again,(tell you how later) test-run it the next two days 45 min one, 20 or so yesterday. This morning it started right up and died inmediately. Has refused to start since.
I am beginning to believe I have a loose wire - bad connection problem. Let me tell you why. This is how I got it running. Since I initially supected the FPR, I substituted the one that was on with a new one. No joy. I substituted it with and old one. Nothing. Then I put back the original one and it started right up and as mentioned run for the next two days.
Now let me back up and tell you what I am suspecting. This car is my wife's daily driver. She's been driving it for over 6 years now. Sometime last spring, (cant recall exactly when) leaving work,the car would not start so she called a mechanic nearby. By the time I got there they had remove the glove box, lower panel, broken one of the vacuum pods and who knows what else and finally replaced the FPR. I do not drive the car much, wife does not like that, but a couple of times that I did I noticed that the radio would work intermitently. Then she said it stopped working, then that it was working again.
When I removed the glove compartment to remove the FPR this time I noticed that the tray holding it moved quite a bit and seemed to be tied to something with a plastic tie-up. Finally, the turn signals have stopped working. It all may be a coincidence, but I cannot help thinking that I have an electrical problem in that area. What do you think?
I plan to check all fuses and then inspect the wiring as best I can, then I do not know.
This could be real hell, so I will appreciate your sugesstions.
Thanks.

Jl
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