Frank If I put a timing light on it and turn it over with the starter what should the timing be reading?

actually if the check valve on the pump operates ,,,,,,,,,,running the pump backwards should not do anything

Re: Ign. Timing
At cranking speed I would like to see 5-8 (even 10) deg. BEFORE TDC.
This is somewhat advanced from conventional US spec, however it is about where I have usually run my M110's when using a distributor w/o vacuum advance, and total advance limited to about 36 deg. BTDC.

I should not think that there would be any pump damage. If you open a fuel line do you have flow, or do you show pressure at the control port?

Provided that you can create a situation in which you can command the pump to run at your discretion, you can then proceed from the problem of SUPPLY to the problem of METERING. While you are into the ignition system, have a look at a few of the spark plugs. Are the plugs dry? wet with fuel? wet with water (at the top of the thread you mentioned a car wash!)?

If dry, is the control regulator stuck closed (full lean)?
If wet with fuel, is the cold start injector staying on?
If wet with water, PUNT.

Although US wiring diagrams usually show the power to the pump relay to be un-fused, my non-US diagrams show that circuit to be fused. Hence, I think it could be either.

If you've dealt with M110's for a while you know how much crud can accumulate in the spark plug wells. Be careful, clean the wells completely before removing plugs (there are drain holes in the wells that exit under the exhaust manifolds). It's a pain, but necessary.

this is what I know now..based on a snippet of a wiring diagram obtained from BenzWorld.
1. contact 30 is a dry contact of the fuel pump relay and 9V shows up when
the key engages the starter. I believe the 9V-12V is supplied by a relay in
the starter? There is also a A/C starter relay....
2. contact 87 is a dry contact and supplies the 12V from the frp to the fuel pump
3. contact 87A is a dry contact and supplies the 12V to the fuel pump once
the car starts and the safety switch switches. I do not know yet where this
12V comes from. One would assume the fuse box. I know this works
because when the key is in the run position removing the safety switch
connector turns the fuel pump on.
4. contact 85 goes to the safety switch and is part of the relay latch. The
other side of the relay latch is 86. Contact 86 gets its 12V by piggybacking
off of contact 87A.

Attached Thumbnails

 

Here is what I see in the diagram:
1. "A/C Starter Relay" This is a relay that simultaneously powers the starter
motor solenoid and removes power from the A/C compressor when
cranking.
2. The 9V (hand written) on the line to 30 on the FPR is the result of
system voltage being pulled down to that voltage by the load of the
starter motor.
3. Voltage is present in the above mentioned circuit only during cranking.
4. The contact for 87 in the FPR is made to 30 when the engine is off or
during cranking. After engine start the sensor plate in the air flow meter
deflects and opens the safety switch, thereby de-energizing the latching
coil in the FPR and allowing the contact arm to move to 87A.
5. 87A and 86 have a common 12V fused supply.
6. The line to 30 is not connected to 87A/86, it jumps over that connection.

With the FPR removed you should be able to measure approx. 9-10V in the pin socket for 30 during cranking.

Frank.....agreed!
In this car, key in run position, safety switch disconnected the fuel pump operates. So the 12V to 87A is there and the latch must work.

Also confirmed the 9V at contact 30 when key is turned to start. I will need to confirm the 9V shows up at the pump?
Will check this evening and post results. thanks

okay tonight confirmed that dry contact 30 supplies the 12V to the fuel pump.
Two Functions of the fuel pump relay>
First function is 12V from the AC/starter relay goes to contact 87 when the key is in the start position. Also confirmed that contact 85 and 86 are hot, the relay is in the latched mode when the key is in the start positon. We already know where the 12V for the latch comes from, see previous thread.
Second function is when the car starts and the key returns to the run position, the latch circuit (contacts 85 and 86), which we know was in the latched mode while the key was in the start position, now becomes an open circuit when the throttle plate opens, opening the safety switch, because the motor has started.
This unlatched position feeds the 12V source from contact 87a to the fuel pump while the throttle plate is open.

I confirmed on my car that the 9V is present on the relay socket contact 87. I disassembled the relay and touched the contact points but when installed only .2V shows up at the fuel pump. I removed the relay and ran 12V thru wire 30 out to the fuel pump and it was fine. I think this is a faulty relay latch. Need to get replacement and test.

You've done in essence what I was to next suggest, that is, a bench test of the relay.
12V(+) to 86
12V(-) to 85
relay clicks? yes? there should be approx. 0 resistance between 30 and 87
no? discard relay

From your description, the relay is activating and closing the 30/87 connection but there is substantial resistance present. The resistance can be in the relay, i.e., the contact points, OR it can be in the pin 30/socket 30 connection (loose fit).

In your last paragraph I think you have interchanged the #s 30 and 87 in your typing. 30 would have 9V from the starter relay when cranking.
87 is the supply socket TO the pump. When you jumpered 12V to the pump it was thru 87. Yes?

Bottom line: replace relay, or clean contact surfaces (pin and socket) at 30

Yes I interchanged 30 and 87 on purpose. The original comment that contact 87 supplied the fuel pump was wrong. It is indeed contact 30 that that supplies 12V to the fuel pump and contact 87 gets the 9-10V from the starter relay. I did remove relay cover...inspected....cleaned contacts. This didnt improve the .2V at fuel pump. I believe I confirmed the relay is bad by removing relay and putting 12V on relay socket contact 30 and the fuel pump operated. Yes I functionally tested the entire relay on the bench and it appeared good....continuity only. I did switch the latch relay with 12V. I didnt resistance check the relay. It appears that when amperage is required across 87 to 30 (under load) it fails. The relay has no pcb board. It is all mechanical parts (brackets, springs) and there are some soldered connections. I will not be able to post final results with new relay since a new one needs to be obtained. I have my fingers crossed. But thinking about the cars symptom it all makes sense. If The fuel pump doesnt come on when key is turned to start then whatever residual gas that is in the system allows the car to fire but there wouldnt be enough gas to allow the motor to run and open the contacts of the safety switch.

The stars seen to be in alignment. Does the relay bear the part # 001 542 02 19? The diagram stamped on the top of that part corresponds with your circuit description/numbering. They are a common item with many applications. I have five on the shelf that I have pulled out of departing cars over the years.

Back from vacation. Changed the subject of the thread as Bob requested in a PM. You guys have done a good job unraveling this.

I just looked at a wiring diagram that I got from George Murphy a while ago. The pump is indeed wired to 30, and the power comes from 87. Electrically, I guess it doesn't make a difference, but why Mercedes would reverse the convention is a mystery to me.

Is the car running now?

__________________
Chuck Taylor
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe

Chuck thanks for changing the thread subject. There is alot of detailed information in the thread. If you have any wiring diagrams specific to a 1978 280SL I would appreciate a copy. Can pay via PayPal. I got the Mercedes factory manuals for the 380/450 USA Chassis volumes 1 & 2 with the car. There has to be at least 10 different schematics. I also left for a 4 day vacation. I purchased a Meyle relay from a parts house up here in Manchester NH. Somewhat surprised they got it within 24 hrs for $20. It has been picked up by my dad, waiting to acquire the relay and install. I will post results as soon as I get them.

New relay installed. Didnt confirm 12V at fuel pump by putting voltmeter on the line but the car now starts and runs for 20-30 seconds at a low idle and then dies. I only had time last night to pop the relay in to the socket and try to start the car. I think the relay died while I was troubleshooting. It seems I am back at the same scenario after I cleaned the distributor cap contacts and rotor. I am back to being suspicious of the cold enrichment not being right. I want to remove the auxillary air valve and ensure that it is not stuck in the closed position. Does anybody know if the symptom I now have could be caused by the aux valve not opening and is it possible to remove the aux valve without removing the entire intake manifold/fuel assembly? I also will reconfirm the system pressure and control pressure and check timing.

Although it is a bit of an inconvenience, please have a look at the spark plugs. The condition of the plugs will give a good indication of starting air/fuel mixture. (see post of 8/14, above) The aux air valve simply bypasses the throttle plate; the air passing through the aux valve is metered air. The function of the aux valve can be simulated by slightly opening the throttle. If the aux valve were stuck in the closed position, the resulting condition will be one of too little air and fuel, rather than one of a mixture error. Does the air flow sensor plate move smoothly off the rest position? Does the plate move off the rest position with engine start? Does the safety switch open after engine start? (If not, there will not be power to the pump).

At the risk of belaboring the point, please check the plugs.

It's easy to remove the valve. A couple of screws hold it to the manifold. But the screws will almost certainly break off. You can check the position and movement of the valve by removing the hose and looking at the slider.

Agree with Frank - you should try to get the engine up to temp by holding the throttle open a bit, and have a look at the plugs.

__________________
Chuck Taylor
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe