1987 MB 560 SL cuts off

I have a 1087 MB 560 SL that has been giving me problems. This is a daily driver or was before these incidents happened. Three times while driving my car just cut off. There is no hesitation or sputtering…which is sad cause I would like to make a sound like… I can drive for a week and nothing then it does it again. If I wait a minute it starts right back up with no problems. I took it to my mechanic and he said he tested the compression??? Couldn’t find anything. My boyfriend says it sounds like it is electrical. I haven’t a clue so I have nothing to add. I love this little car but am now afraid to drive it for fear it will leave me on the side of the road. HELP!!!

I have a 1975, 107044, 450SL, with about 80k that was also a daily driver. No hesitation, no warning. The motor just stopped. Awhile later it started, then stopped. It finally ran me home, where it started, but cut off after about ten minutes. This vehicle is well maintained, likes to go, and started first time every time. So, I replaced the rotor under the distributor cap. (Lined it up with the groove.) Good for about ten minutes. I replaced the coil, and the distributor cap. Good for about ten minutes. I replaced the electronic ignition unit, which replaced the points on this vehicle, with a newer model of the same make. Again, good for about ten minutes. Now I’m thinking the problem may be the fuel pump; more likely the fuel pump relay. I pulled off the kick panel on the passenger side, exposed the fuse box and the six, silver box-like relays. Which one is the fuel pump relay? Although the fuses appear sound, it seems like an electrical problem. The locking gas cap cover (pin) on the right rear quarter panel isn’t working either. Any help will be greatly appreciated.

I’m no Benz expert but this car likely has CIS injection, to which I extend my condolences.
The mechanic is a bit offbase by even considering compression as being behind this problem.

Without knowing what is missing (spark or fuel) about all I can do is list some of the likely suspects.
The fuel pump. These CIS pumps really take a beating due to the higher pressures they operate at and they’re always suspect.

Based on the no sputtering it does sound like electrical more than anything else so I agree with your boyfriend.
The likely suspects here would be the electrical part of the ignition switch, a poor connection on an ignition or switch fuse, or a fault with the electronic ignition.

There are other things that it could be but those would be the most likely.

Did more research and found the the most prevelant answers on this problem was to *tighten the positive battery cable connections. *Crankshaft (crank) position sensor. Is the crankshaft and the crank the same thing? *the Ignition control module .
This was from a poster with the user name Benz Expert Mechanic.

Replace the fuel pump relay. It’s on it’s way out, and it will leave you stranded.


Here’s a picture and price of the fuel pump relay. It’s behind the right (passenger) side kick panel.



Thanks pheish & Benzman; I’ve located a fuel pump relay for reasonable. Although on the 450SL, the fuel pump relay is, I am told, one of six identical silver box-like plugins, inside the kick panel, above the fuse box. I have no idea which of the six silver boxes is the pump relay, so I’ll plug and unplug until I get the correct one. Trial and error. I’m wondering though, what the five other silver boxes do. Could all six boxes be fuel pump relays? The single fuel pump relay I priced wasn’t too terribly expensive. Makes me wonder. I have some literature for the various SLs, but which relay is which on the 450 remains a mystery. The patient liked having her positive battery cable connection tightened. Thanks again. Will keep you posted.

The repair has me stymied. I purchased the fuel pump relay, pulled the passenger kick panel exposing the fuse box etc., and low and behold there are four vertical silver boxes per side, instead of three, plus another silver box on top. Now this wouldn’t be so bad, except that there’s also a slotted steel barrier protecting all but the bottom two silver boxes and the top one from being removed. So, I switched out the three boxes that I could. None seemed to have been the fuel pump relay. I used a mirror to see how he metal housing is affixed. I saw at least one phillips head screw (way up there). The silver boxes are visible, but they’re protected. I’m stumped. How to remove the metal obstacle and access the remaining six boxes? What tool fits into a narrow space like that? Is there a way in besides a ground-up restoration? I cleaned connections, and made sure all the fuses are correct and where they should be in the fuse box. I’m open to any suggestion. I got nothin.’

As I stated, I’m not a Benz expert and cannot tell you exactly where that relay is located but if the relay is bad you have to ask the question; WHY is it bad?

Contact points in the relay may burn and cause erratic operation of the fuel pump but burned contacts are usually caused by long term operation of a worn fuel pump.
Replacing the relay is curing the symptom of the disease, not the cause.

Thanks ok4450. While I cleaned the contacts on the three relays, they showed minimal wear for a 35-year old vehicle. However, if I can’t access the fuel pump relay because of that retainer, it would seem that I can’t even cure the symptom. Conversely, if I replace the fuel pump for $1000.00, and it runs that’s good. Let’s say it cuts out again though. (The fuel pump relay is under $20.00.) Now it could be the fuel pump. In which case, one of the three relays I did switch out may have been the actual fuel pump relay. Although there’d be no way to know for sure, because if the fuel pump itself were bad, the pump relay wouldn’t solve the problem. So, why might the relay be bad? My guess: it’s old. I don’t know that the fuel pump isn’t that old as well. It’s a learning experience. I looked at the metal barrier again today. The plastic panel under the glove box may also have to be removed. These machines are over engineered. It makes me know I need help. Thanks again for your input ok4450.

The price on that pump sounds way high unless this is a dealer or high end shop price.
You should be able to find a Bosch or Airtex pump for a couple of hundred dollars and they’re not that difficult to change.

If you can access the pump wiring at the back and connect an ammeter a current draw test could be performed. If the current draw (or amount) seems higher than normal a faulty pump could be suspected. Current draw should be under 10 amps after an initial higher surge when the pump is first started.

As I said, I’m not a Benz expert but have messed around with Bosch fuel injection on SAABs and VWs quite a bit. (same system and parts) The pumps really take a beating and every used and running pump I’ve cut open for inspection had a badly worn commutator. (this is the part of the armature where the brushes ride)

If you’ve ever heard about a starter motor getting goofy on an old Chevy and it works after someone leans underneath and whacks it with a hammer, now you know the situation with the fuel pumps.

But this still could come down to an electrical fault in the ignition (distributor, module, etc.) or a faulty ignition switch.
Without knowing what’s missing (spark or fuel) when it quits the guessing is a lot tougher.

I stand corrected. After some double checking, it seems that fuel pump does indeed run around a couple of hundred dollars. My parts lady says she can’t get one though, because she thinks her sources aren’t making them anymore. She’s also confused as to whether the 1975 SL has EFI, or whether it has CIS fuel injection. And, the problem I still have, is that I don’t want to tear the interior apart to access the relays, if there’s an easier softer way. Not only am I not very electrical and don’t have an ammeter, but my right rear tire goes flat daily. I’m draining the battery to keep it inflated with an electric air pump. Up until now the car never gave me any problems. This has got to be the repair from … What to do?



To net it all out, Buy 2 Bosch Fuel Pumps and a Bosch Fuel Filter from AutoHausAZ.com They are reputable and for the most part the LOW PRICE supplier of parts. They also check to see if the part is correct for your car.