Is it safe to take a voltage reading at the fuel pumps?
 

On a 560sl?

Thanks.

JB

??????????
 

If you mean "same" - same as what?

If you mean "sane" - yes. You should get at least 11.5 volts and 6-10 amps.

Crap, meant to say safe....not same.
 

Having no start probs, think it's fuel relay related.

Thanks.

JB

Jboggs:

What yr. is your 560SL?

If it's a 1900 or 1991, it may be equipped with what's called a MAS. Silver box on drivers side near fire wall. It has a knob on top that you loosen counter clockwise. Once loosened, the whole unit comes out.

Ports 1 & 2 are jumpered to power fuel pump(s). These 2 ports are what's used by the fuel pump relay. The FPR is incorporated into the MAS on cars equipped with such.

If you jumper 1 & 2 and the car starts, the fuel pump relay within the MAS is bad and the MAS has to be replaced. If not, your pump(s) may be bad. The MAS is $500 + at the dealer.

I once read here that the MAS can be opened and soldered. I'm not an electrics expert, but would wonder just how a diyer would go about opening this unit?

Be careful when you jumper 1 & 2. One or both is wired hot and if you don't hit them both just right with the jumper wire, you'll get a spark.

Understand that you really cannot operate the car like this. Jumpering 1 & 2 will prove if the FPR is functional or not. If the car starts and runs jumpered, you'll get a Check Engine light. This will reset itself once the MAS is reinstalled.

http://www.peachparts.com/shopforum/tech-help/9535-mas-units-contain-fuel-pump-relay.html?highlight=MAS

If your MB does not have a MAS, locate the fuel pump relay and jumper the correct pins. I seem to recall they are 30 & 87, but a SEARCH would verify this. If you have a faulty "independent" FPR, your wallet won't get hit as hard.

If the weather holds up, I'll be replacing my MBs fuel pump this weekend.

86 560sl.
 

The fuel pump relay on mine is behind the glove box I believe. I'm gonna have to jiggle it some over the weekend or just replace the stupid thing.

Funny thing, been having the no starts for several months off and on, but a week or so ago, the alarm started going off by itself. I pulled the connections to the alarm which is really close to the relay. I have checked it twice to make sure I didn't knock it loose but somehow the two must be related. I hope it's just a relay or a dirty connection and now a computer related problem.


JB

JB:

I read in an old post here that MB factory alarm systems play no part in keeping the vehicle from starting. Fctory alarm merely provideds an audible alert.

If that's true, you could rule out the factory alarm as the culprit.

Not sure what type of alarm you have.

Yes, I'm aware...
 

but the possiblity remains that I knocked something loose or bent a relay or ? I was suprised how easy it was to defeat the alarm-a thiefs dream.

Thanks for the reply.

JB

Not really. Eliminating the alarm is the easy part. Breaking that case hardened ignition tumbler is the challenge. I'm suspecting you have one or both fuel pumps going out. You can check current flow and wave form to each pump. A scratchy waveform means you have a failing pump. A clean waveform means you might have a failing pump. If you have a pump that has been drawing too much current, you may as well replace the relay too.

Earlier this evening I posted some incorrect information - had my head up the wrong place. I suggested pulling a fuse from the main box and hooking up an amp meter in series at that spot.

There is no such "fuse". Looking back thru the archives, one would pull the fuel pump relay and do the amp meter hook up there to read current draw.

In an old post, M.B.Doc indicated that single pump systems should draw less than 8 amps; dual pump systems should draw 6 - 7.5 amps.

http://www.peachparts.com/shopforum/tech-help/20307-fuel-pump-fuel-p-relay-doc-benzmac-steveb.html?highlight=single+pump