'95 E320 lost fuel pressure, pumps okay and new OVP relay.. - PeachParts Mercedes-Benz Forum

puzzler · #1 10-24-2016, 08:00 PM

So there is amazing depth of information here! The diagnostic summaries contain a wealth of information and Ive read thoroughly! Thanks especially to the experts giving specific information.

Now I found a lovely, neglected wagon and tuned it up, cleaned out the mouse nests and installed a heater fan and fan control module, got the AC properly charged and working and then reuilt the cylinder head - head gasket replacement..And have a wonderfully running trip car, doesnt leak a drop of oil or water.... I love it..But suddenly the car gave a no start problem although it cranked freely and would fire with starter fluid..At first it began a start/die pattern and would not run..It would start-run for a minute if allowed to sit, then it would not restart for half an hour but later it would run a minute again..Finally I began pulling the fuel system apart and seemed to find rust and clogs in the check valves (replaced the fuel filter) and ordered a new pump. But I could find nothing wrong with either fuel pump and when I put the system back together the issue resolved. Then it failed to start and before the tow could arrive, it did start and ran fine again several days. Now it has no fuel pressure although the OVP has been replaced and I jumped the FP relay to test for a faulty relay and there is power to the relay, but the fuel pumps do not respond..

So am I looking at a bad ECU? Im thinking the engine computer senses the inputs that turn off the fuel pumps due to the crank position sensor and other inputs..turning them off even when the relay closes the circuit And my CPS is okay because I have spark..it runs. and the pumps are good..This isnt a simple one..But the car is worth fixing!

So I see most of these troubleshooting threads dissapear at this point and none ever come to identify fault with the ECU..Now this is an ASR car which has wheel speed sensors and it works great..Ive checked the engine temp sensors and theyre good.. Everything I can think of..Car has a 2005 electronic throttle and new delphi wiring harness..The usual issues, fuses and so on are all okay.

Actually I got the electric cooling fans working and then fuse D began melting out the fuse box and I find this is common so I made a blade fuse holder and soldiered it into the spot which cured the issue and now the fans run well with no overheating of the connections..ON/Off frequently with the a/c system activation..

Thanks anyone and everyone for your thoughts..

Diseasel300 · #2 10-24-2016, 08:20 PM

If you jumped out the FP relay and still had no fuel pressure, but you know the pump is good, have you gone back to make sure that the FP's are actually getting power? Are you sure you jumpered the correct pins in the relay socket? If you've bypassed the FP relay, the fuel pumps should be running, regardless of the ECU commands. I'd imagine the ECU simply cycles the FP relay.

You may have faulty wiring. Is this car one of the models with the eco-wiring? I know a lot of the early 90s cars had wiring harnesses that would self-destruct, not sure of the cutoff date. The symptoms you describe of running, then not, then cooling down and running again seem to indicate a hot/poor connection somewhere. I've seen FP relays themselves cause this type of behavior, jumpering their pins will confirm if it's the relay.

puzzler · #3 10-25-2016, 11:43 AM

Thanks for the right on comments Deasiel..I found an excellent wiring diagram on another chat and I can see if you power the feed to the fuel pumps they will come on, its a direct connection. And this convinces me that the FPs must, one of them... be bad. You can go direct from the battery post to the FP feed to the pumps and they must come on or as you say, bad wiring somewhere..So I dont need to puzzle over the ECU which is not involved..

I will post the excellent link to that wiring diagram here on my next message..someone shared it in reply to another issue yrs ago..

Its strange..I DIS connected one FP and then the car ran, with one pump..then I tested that pump and could find nothing wrong with it and reinstalled and all was good, but I suspect its grounding out and stopping both pumps with intermittent failure..the meter showed continuity across the entire pump, both posts to the case and each other..yet it worked great when connected to a battery. The new pump does not show this pattern at all..So I will get the new pump installed and test again! Very hopeful this resolves it..

And yes its a 95 with the eco junk wiring but they say the chassis wiring doesnt crumble, yet in high heat areas it may..such as the fuel pumps if it goes near exhaust on the way..

Back with that diagram and to complete the story.. Still curious how the ECU "knows" to turn off the fuel pumps when the engine is off ...Looking at the wiring diagram may hold a clue or it could be a timer circuit..activated in the absence of the CPS signal..

puzzler · #4 10-25-2016, 02:02 PM

Web Site Blocked

hopefully this will work for users..shows two engine electrical diagrams one of which is the systems and the other is the ignition..

I will repeat the tests using a jumper to energize the pumps and prove whether its the pumps or wiring..possibly a funky relay..

These are complicated cars but each part is simple and if you correctly pursue each issue theyre wonderful..What seems like gremlins are reduced to a bad ground or corroded contacts or something simple and then youre good again! When I got this it was full of mice, had sparkplugs underwater in the cylinder head and rocks piled on the intake manifold I assume by squirrels! But it had a recent cylinder head gasket which was done poorly by Mercedes starmark..one bolt was not torqued at all and the gasket leaaked.They had ground only the intake valves and exhaust valve guides were totally gone..Now its all up to spec...But all the hoses were new along with the brake booster and a ton of other expensive parts..So its nearly 100%.

What else could one get for under 5k that compares in terms of comfort and quality and performance!

Its only with forums like this and online parts suppliers with occassional dealer help we can pull these old cars together but well worth it!

puzzler · #5 10-25-2016, 08:13 PM

So yes, running a hot lead directly to the FP lead on the relay produces nothing so it must be a broken wire..Pin 87 I believe. I tried jumpig 30 to 87 and then just took a hot lead from the battery post to be sure, still nothing. Two of the relay pins are hot so its being powered..So yes ... either bad pumps or broken wire..failed connection. Most likely a bad wire Im thinking currently but without a hoist and facility I will let the local indy sort it out..He has a good following with lots of high quality hobby cars..I was fearing a complicated control module issue but seems its just electromechanical so Im happy with that..

They typically want to replace both pumps but Im feeling we should power them and replace only if they dont spin..so easily tested for operation..

Rick76 · #6 10-25-2016, 08:47 PM

There is a connector under the rear seat bottom that has power and ground for the fuel pumps. You can do a check there to see if you have power before the wires go out to the pumps. It's on the right side near the vacuum pump for the door locks.

puzzler · #7 10-25-2016, 09:39 PM

Thanks so much Rick76! I know where the vacuum pump is on the sedan. I will bet the mice got in there! And sorry my link didnt work to the diagrams. I found it here on an old 2001 post..fortunately got them downloaded.

Im hoping this under seat location is where the one lead splits to the two fuel pumps and there may be a faulty Y connector..Now this is a wagon..but if I cant find it this tip will get me on the right track! Very helpful, great pointer!

puzzler · #8 10-25-2016, 11:13 PM

So Rick76, yes this is the wagon and the back seat is solidly carpeted under..Now the driver and passenger footwell both have tons of wiring and components. There is a wiring compartment with a snap on cover under the left of the drivers footwell.. and it looks like a LOT of wiring in the passenger rear fender well by the CD player and some in the drivers fender well by the spare tire..Nothing opens up under the rear floor at all or back seat..where the midget seats are...Everything very tightly bundled..

May be able to reverse trace from below to where it enters the chassis interior and up to the relays..

Rick76 · #9 10-26-2016, 03:43 AM

I don't know where the connector is located in the wagon but it will have 2 pins with a black/red/white wire and a brown wire on one half and 2 black/red/white wires and 2 brown wires on the other half of the connector. The connector acts as a splice or Y to connect the 2 pumps so it would be a convenient spot to check without having to get underneath if you can find it without ripping up the carpet.
I'll check to see if I have any info on where it is located. It is labled as X36/3.

puzzler · #10 10-26-2016, 10:51 PM

So Ive torn the interior apart to reveal any possible location for the x36 or possibly x63 junction connector..Which supplies ground and power to the pair of fuel pumps..from this junction.

Found lots of components..wheel speed sensor junction and a set of junction/relay boxes..big black one with plugs labled A, B, C, D and another yellow one..Lots of audio wiring for radio and CD player, vacuum pump for the locks.. and from below the Fuel Pump wires dissapear into the frame through grommets and I can find nowhere near that they come out inside the interior after pulling up carpet and opening up wheelwells, removing wheelwell covers and opening up the works..

Anyone with expereince or resources to locate this junction, 95 E320 wqgon, I would be eternally grateful for the location..

puzzler · #11 10-27-2016, 12:52 AM

So hey! I found a w124 wiring connections locator chart over on the 124 zone website in PDF and it shows connection X36/3 although its the sedan and not the wagon and the index for that connection is missing..its a line drawing and not too clear but its a clue!

Still happy to hear from anyone..Thanks all.

puzzler · #12 10-27-2016, 02:11 PM

OKAY, pulled out the vacuum pump for the door locks and its foam nest..lots of wiring stuffed undereath this in the chassis cavity under the back seat, and Im pretty sure I got it! X36/3 to the fuel pumps..To be continued, there is a pair of wires one headed back and another where it could feed the mid pump and a pin plug. Nothing is labeled..

puzzler · #13 10-27-2016, 08:48 PM

CONCLUSION: Okay the pumpss were failing progressively..I was mistaken in concluding there was no power to the pumps because I removed a pump and it tested solidly just a few days before....couldnt imagine both failing at once but its likely I was running on just one pump..The x36/3 connection does not appear to exist in my car but the vacuum pump conatined a mouse nest and some of the wires were chewed..Thank god I pulled it all apart and taped up the wires..they ate a little space in the foam box..

fuel pump wires come right through the well in the chassis opposite the pump where one can check and test..

So Ive got to order another fuel pump..possibly will now have full pressure operating since I purchased the car, its always felt weak by comparison to my previous E320, 94 sedan and the mileage is low which also may result from weak pressure..

Rick76 · #14 10-28-2016, 06:32 AM

Sorry for the wild goose chase looking for X36/3 but I see it wasn't a waste after finding the mouse nest.

puzzler · #15 10-28-2016, 11:49 AM

Not a waste at all, I located lots of components and cleaned rotten fabric wrapping out of the drivers side wiring channel..verified all that is stil sound. It also had a mouse trail..those guys can get through a hole the size of a nickle! Im so impressed this old car still has working grounds to all circuits and the connectors are all good..its truly a quality vehicle.. And the entire interior is made to snap out with the proper release clips pressed and pried. Very easy once you see the assembly methods..