W111 3.5 ECU-fuel pump relay switching experts needed!

[sjefke]

So, I have traced the problem of my dead fuel pump to the ECU pin 19 and it has me baffled. The fuel pump relay works fine out of the car. It has voltage on the proper pins and clicks when ground circuit is closed. But if I put relay back in car, nothing happens. If I ground the black-white ground connection, fuel pump starts humming. I saw in wiring diagram that black-white relay ground cable goes to ECU brown-white wire in pin 19 on ECU. I have connectivity between relay plug and pin 19 on ECU (no resistance on Ohm meter). Measuring from pin 19 to ground (car body) show some resistance, but there is connectivity.

When I switch ignition on, ECU does not seem to switch to ground fuel pump relay. Actually, when I measure pin 19 (puttin a small nail on it), it shows positive voltage.

Anybody any ideas?

Again, it is not the relay, because I switched it with the injector relay above it (which works fine) and I get the same behavior.

My next step was to take apart the ECU to see what is going on.

I basically need a wiring diagram for the ECU. Could I have a bad ECU ground connection? Where is the ECU grounded?

Also, can I just ground the fuel pump relay pin "directly" instead of going over ECU?

Thanks,

Bert

[ctaylor738]

I'm no expert, but I'll take a shot at this.

If you have no continuity issues from the relay socket to the ECU, and the ECU has a perfect ground at Pin 11, and you can't get ground with the ignition on, then the ECU has gone south. I don't know your car, but on the 107's they bundle all the grounds for the system in the engine compartment at the right rear. So look around for a big bunch of brown wires.

Switching the ground connection to the fuel pump is the only way that the ECU has to control the pump. So as I read the diagram, if you give it a hard ground, the pump will run whenever the ignition is on. This will work, but is obviously less than ideal. It might cause minor flooding, but the real danger would be in the case of an accident since the pump would keep running even if the engine died.

[sjefke]

Chuck, thanks for taking a shot at it. I dug some deeper too and of course one of the ECU complications is that it only grounds the pump relay for 1/2 second when switching key on ignition. So, when I switch the key on, walk back and check ground, ECU is already done no matter what with its switching. It grounds the thing when starting and I have not checked that yet (arms not long enough ).

BUT, something interesting happened Saturday when I put a circular connector on the ground pin to "hard-ground" the relay: the pump started running for th 1/2 second on ignition position even though the extra ground cable was not connected to anything yet! (my plan is to have a manual operated switch under the dash so I can hard-ground the relay in case I need to) Wiggling the circular connector caused some change.

In fact, it was working fine for a while (even car started) so I definitely need to recheck the relay plug. I put everything back in place (incl. ecu), but now I have the same problem again. I will try this evening to wiggle again, but it seems to really be an intermittent wire connection issue either inside the relay plug or in the ECU plug (ECU top cover was off when it worked - now back on and it doesn't work). And maybe something is just not grounded properly and although I read connectivity, it isn't enough to trigger the relay.

The reason I suspected the ECU is that it may not get the proper signal from either ignition switch or whatever that the pump needs to be grounded for a moment. It seems that the default is + on pin 19 and - (ground) is the special state that is triggered. So, what inputs to the ECU trigger that state?

The ECU may be fine (I read they are basically bullet proof), but the input(s) to activate it may be flawed. Hence my interest in figuring out its internal working and input-output signals. Anybody care to pipe in?

Thanks,

Bert

[ctaylor738]

If you send me an email, I will send you the testing guide off the 107 CD. It has a step-by-step test of the relay.

[sjefke]

Just sent you an e-mail. Installed the grounding switch this evening and car starts again and runs with it. I do want to figure out where the bad connection is. Injection relay and fuel pump relay are identical and when I switched them, fuel pump still did not work, so I am thinking: relay plug, bad ground, or ecu wire loose. To be continued...

Bert

[GGR]

Keep us posted. I have a similar issue on mine and do not have time to look into it as I'm on a coast to coast trip. For now the car runs with a wire bypassing the relay. Pump runs al the time when contact is on.

[sjefke]

Originally Posted by GGR

Keep us posted. I have a similar issue on mine and do not have time to look into it as I'm on a coast to coast trip. For now the car runs with a wire bypassing the relay. Pump runs al the time when contact is on.

I read your coast to coast post and you definitely may have the wrong relay. 3.5 fuel pump relay is same as injector relay: rectangular boxy type mounted behind ECU (I put a MB picture in my relay location post). Not sure about the load capacity, but if you have a simple indicator relay, it could get fried over time by the load. I am also weary about heat being a cause of my relay's grounding malfunctioning and am curious in what climate yours failed. My car never overheats, but it was definitely in city traffic when it failed. I put the switch in (under the dash using an existing hole) so I can check whether relay works again by itself or not before switching it.

Bert

[sjefke]

OK, I feel I am getting warmer: when I wiggle the wire loom that connects to the main relay (the one above the fuel pump relay) I hear an fuel injector firing every time I wiggle it (with ignition on). That does not seem normal.

Shouldn't the injector fire only once if relay is energized?

How does this relate to the ECU connection with the fuel pump?

Is this an intermittent grounding?

I cleaned the plugs, but so far that did not have any effect. I may have to take the whole relay-ECU assembly out again for testing.

Anybody have any clues as to what the symptoms are of poor connections at the master relay?

Thanks,

Bert

[sjefke]

I am pretty sure I located the problem: it's the main relay above the fuel pump relay - either the relay itself or the connector. After my last post I decided to hard ground the car again and just ride it for a while. But when I did that, I noticed that it ran pretty bad - almost like it was not firing on a some cylinders. After some more wiggling and a glass of wine to help stimulate the brain cells, I decided to go for a test drive. But to my surprise the hard ground switch failed to engage the fuel pump! Nothing worked. I kept the switch on and started wiggling wire looms again. Suddenly pump started humming. A long story short, I was able to start the pump AND kill the engine too (!) by simply wiggling the master relay. So, in certain positions all works fine (just had the engine run smoothly for 5 minutes), but with just a touch it stops (stopped the engine like that).

I don't know whether it is a bad pin or circuit board in the relay or the socket. I suspect a bad internal relay contact. Either way, I think I am going to order a new relay and see what that does. The relay controls both some ground to ECU and some voltage to ECU, so it definitely can kill the engine by stopping voltage to ECU.

The annoying thing is that the fluctuation in contact is very sensitive (literally the touch of my finger sets it off). So I am pretty sure that when I measured voltages earlier with sockets in my hand, I had voltages and connectivity. Same with relay checking. But as soon as I put everything back in its original (impossible to get to) positions in the engine bay, I may have gotten the bad contact again.

OK, I am going to clean up and have another glass of wine to celebrate current level of succes. This puppy is staying on the road.

Bert

[GGR]

Hi,

my relay faild with very hot weather and engine. Previously, it failed in city traffic and everything was going back to normal when resting for a bit.

[sjefke]

Originally Posted by GGR

Hi,

my relay faild with very hot weather and engine. Previously, it failed in city traffic and everything was going back to normal when resting for a bit.

Sounds very much like your relay internals are warping too much due to thermal expansion. You could fix it by taking relay apart, cleaning contacts and bending the little metal contact arms a little closer together. Could also be broken solder joint that gets connectivity again when it (or the board) contracts in low temp and expands in high temp. All you need is a small gap and connectivity is lost. May be time to get a new relay.

I was hoping my problem was temp dependent, i.e., predictable, but it wasn't. Either way, pesky problems.

Bert

[sjefke]

Well, I got the new relay today and plopped it in for the master relay. To my disappointment, the problem persists. Relay clicks, but I still hear no fuel pump (although it worked this weekend and car ran beautifully - go figure) and wiggling red + wire still causes injector or so to fire repeatedly.

I will try the new relay also on the fuel pump connector just to see what that does.

If that does not fix it, it must be one of the connectors (or the wire loom but I doubt that). I start to suspect connectors 30 (red voltage wire to battery) or 87 (black to ECU and blue or so to fuel pump relay) on main relay. They seem to be controlling the whole enchilada.

Bert

[sjefke]

Progress update on my fuel pump saga. After days of on and off fiddling, here is my current status.

1 - Switching relays did not provide consistency. Twice I got fuel pump working for the required 1.5 sec when turning key but a couple of days it would not work anymore. Cleaning main plug connection between master relay and ECU also made no difference. Grounding the relay consistently gets the pump running. So, I started to suspect the ECU internals & pin connections again.

2 - Thanks to an old post from Tony H, I found diagrams for the ECU and spent time studying. Anybody interested in reading how the Djet and its ECU work, the 914 guys have really done some great web posting. Paul Anders' site is phenominal and has the electrical schematics I was looking for, see http://members.rennlist.com/pbanders/ecu.htm

I found the ECU plug diagram / pin readout at Kjell Nelin's 914 site.

Fuel pump relay circuit works as follows (see Paul B. Anders' site and ECU diagram 2):

"The FPC operates the FPR during starting and normal running. The FPR is controlled by T552, which when on, provides ground to the FPR. When the key is turned to the "on" position, +12V is supplied to the ECU, and the FPR is activated for 1.5 seconds. At power-on, T101 is on and T102 is off because C101 has not charged to 1.4 V yet. While T102 is off C102 is charging and T103 is On. Eventually C101 charges and turns T102 on which pulses T103 to off via the discharge of C102. The pulse charges C551, turning the FPR on, then C551 discharges (taking about 1.5 sec), turning the FPR off. Turning the key to the "start" position sends the Start signal to the base of T552, activating the FPR. If the car starts, and the engine speed is greater than 100 rpm, pulses from the ES pump up capacitor C551, turning T551 and T552 on, activating the FPR . If the engine speed drops below 100 rpm, there is insufficient pumping to keep the voltage on C551 high enough to keep T551 on, shutting off the FPR."

3 - So I took my ECU apart. In attached picture you can see the opened up ECU and in the upper right corner you see the circuit for T552 that controls the pump. I also attached a close up from that corner. Opened up ECU even more, tried figuring out whether I saw burn marks from shorts, but nothing out of the ordinary. Just lower post of main board seemed to be missing the nuts. Opened up the auxiliary board, flipped it over, and also nothing except a pinched wire, but no shaving anywhere.

4 - I put it all back together after many measurements in and out of the car to no avail. But one thing has changed: if I want to start the car, I need to ground the pump relay. Once the car is running, I can turn off the ground and the car and pump) keep running. Which means that the 100+rpm ES circuit keeps the pump going. Interestingly, after immediately switching off the car, the fuel pump start up circuit also works for the required 1.5 sec. After a while standing, it refuses and I need to start the car again with the relay ground bypass. Something maybe going on with some capacitors somewhere...

Next step: I need to try to find an ECU to see whether that will give me proper fuel pump engagement at contact. If anybody has one, please let me know. I just want to see whether plopping in another ECU will make the fuel pump circuit work again - which will tell me whether (or not) my ECU's fuel pump circuit is faulty. If ECU circuit is faulty, I may start looking into a MegaSquirt conversion versus a replacement ECU.

As always, any suggestions and advice are welcome and appreciated

Thanks,

Bert

[sjefke]

Drove the car today in HotLanta city traffic. Had to start pump & car by manual grounding relay. Tried switching off bypass ground occasionally but rpm immediately went up, telling me that ecu was not switching pump and engine starving of fuel.

After shopping, on way back, idle seemed to run better. I switched the ground off and car kept running -- whole fuel pump circuit was fine again.

I am curious why idling would be worse (lower rpm) when hardgrounding fuel relay vs normal (correct) operation? Could the fuel pressure regulator cause ECU input & control problems?

Bert

[sjefke]

I know this is an ancient thread, but given the complexity of the D-Jet system I thought I'd let you know that this weird fuel pump problem is solved.

It turned out to be the ECU.

I bought a spare ECU from a 350SL a while ago (thanks Al) and finally put it in today to test whether it made a difference in fuel pump operation, and it did. It gave the correct short squirt when on contact. So, it looks like my old ECU has a grounding problem in the fuel pump circuitry. It was a remanufactured unit that was installed in the laet 1990s in the car. My car runs fine on new ECU (same ECU number, so it should anyway). Have not done road test yet due to oil leak in oil gauge hose (annoying...)

Bert