Probably the bloody fuel pump relay but... - PeachParts Mercedes-Benz Forum

Stretch · #1 11-05-2016, 10:02 AM

G'day Folks,

Effing effing effing effffffffinnnnnnnnnggggggggg CIS - Sodding car!

I have a problem with the fuel pump not running on my 190E.

Put a jumper lead across the contacts with the fuel pump relay removed and the pump runs - so pump and wiring is fine.

(Phew - I've just replaced the fuel accumulator and do not want to drain the tank again - and yes I could clamp fuel lines but that buggers up old pipes which are seriously expensive to replace {like the best part of 500 euros})

Anyway:-

Put relay back in and turn the ignition key two clicks so the ignition is on and the pump does not run for the usual one second or so to prime the fuel pump.

I can hear clicking in the relay when the ignition is turned on though - so I thought may be low battery - but it ain't that 'cos the starter motor will crank the engine over.

So I now think the relay has probably decided to be a pain in the arse and die.

########

The only thing that is nagging in the back of my head is "TD signal". I'm 95% sure the TD signal is only used once the flywheel is turning - but it is one of the inputs that goes to the fuel pump relay...

...and I'm only thinking TD signal because I have just had the engine out again...

Stretch · #2 11-05-2016, 10:04 AM

Focus - yep - point is I'd like a bit of confirmation:

Is TD signal only important once flywheel turns or is it necessary to get the fuel pump to prime when the crank is still at a stand still?

Markus · #3 11-05-2016, 11:01 AM

I'm not entirely sure on whether the TDC signal activates the fuel pump relay on rotation, but I would think that on the CIS with the separate cold start valve you need constant fuel pressure already there, so when the starter engages the CSV sprays immediately, but like I said, not sure.
Here's a tip on hoses though that I am sure of. If your hoses are a bit dodgy and are ridiculously priced (if even available) take them to a hose rebuilding shop. Just had 2 "fuel cooler hoses" for a 90 300ce made up at a shop which saves the metal flared ends, or banjo fittings, replaces the hose with stuff certified to handle ethanol + what ever additives go into what is called fuel these days, and crimps with new ferrols. Viola! $40.00
bucks. Did the same thing on my 95 911 oil cooler hoses. One of them NLA, and the other $560.00 Same shop did both for $180.00. /:-)

Diseasel300 · #4 11-05-2016, 11:06 AM

I'm not sure about the newer cars, but on the 500SL the pumps don't run unless the engine is rolling. On the older CIS cars, there is not "purge" when the key is turned. At some point, it seems they changed that, but not sure when the cutoff date was.

If you can hear the relay "click" when you turn the key, but the pump doesn't run, the relay is very likely faulty. This is a very common issue, not only with Mercedes either. Over time the contacts wear out and the relay will "pull in" but not make connection.

Stretch · #5 11-05-2016, 12:17 PM

Thanks for the responses chaps - I've just got a second hand relay on order. Fingers crossed - always a gamble.

Merry dance with the part numbers too - manual transmission doesn't have the kick down functionality of course so I'm not sure if the automatic transmission relays will work in a vehicle with manual transmission...

...for others who are chasing down manual transmission W201 / W124 parts the following numbers are the correct bits =>

0035451905 (my original version) - now NLA - new number is

0035452005 - currently 240 euros at the dealership - sorry Mr Benz I'm just not paying that much! I don't mind being nickled and dimed to death so much but 240 euros for a crappy little plastic box is plain daft.

Tomorrow I might get a knife out and hack the old relay to bits to see if I can see what's gone wrong.

Diseasel300 · #6 11-05-2016, 02:00 PM

If it's anything like the relays I've taken apart, I'm willing to bet you'll find burnt points, a buildup of black tar-like grease, or heavy oxidation of the contacts.

I had a friend with an '86 BMW 318i that was nothing more than a SPST relay, was a dealer-only item, and was well over $100. Wasn't even fancy or containing any sort of protection! Just a coil and a set of contacts. Ridiculous.

Frank Reiner · #7 11-05-2016, 02:55 PM

Quote:

Originally Posted by Stretch

Tomorrow I might get a knife out and hack the old relay to bits to see if I can see what's gone wrong.

Sman:

Pop the cover off the relay, tap the contacts on and off a couple of times, reinstall the relay without the cover, and see if it will cycle the timer.

Stretch · #8 11-06-2016, 07:59 AM

Quote:

Originally Posted by Frank Reiner

Sman:

Pop the cover off the relay, tap the contacts on and off a couple of times, reinstall the relay without the cover, and see if it will cycle the timer.

Almost did that - but put the cover back on so I can get the thing back out of the plug! (It is very tight in there as well as being a bit difficult to reach with big hands)

I can't see any burning or any other give away marks of it being buggered.

I can still hear the thing clicking and connecting something - but the pump still doesn't run.

Fault finding that is totally beyond my current electrickery abilities...

...I know a man who can though but it will be easier to wait until the second hand part arrives.

#######

The one thing I can do is use a jumper lead to get the fuel pump running and then measure the current that draws at the relay socket (as I have a gucci ammeter clamp) - not sure if that's gonna get me any further as I don't think there's a limit in the FSM for that...

Diseasel300 · #9 11-06-2016, 08:43 AM

Looking up the schematic for that "relay" (I don't understand why they don't call it a control module), there are 2 actual relays in there. The larger one is for the fuel pump, the smaller is for the transmission kickdown cutout. The only one that should be clicking should be the large relay.

Another possibility that could be going on here is poor solder joints or a cracked trace on the PCB. Dealt with that enough in the SL that when flaky behavior happens, that's usually my first place to look. If you're handy with a soldering iron, you might try reflowing every point on the card with good quality solder with an aggressive flux (be sure to clean up well when you're done). I've had several issues with the SL where the solder joints "looked" good and would even test "good" with a multimeter, but were cold joints and wouldn't pass any current.

If you're not that handy, or if you're impatient and don't want to resolder the whole card (can't blame you either), try reflowing the terminals to and from the relay on the PCB (coil side and contact side, including the pins on the module).

Stretch · #10 11-06-2016, 09:08 AM

Quote:

Originally Posted by Diseasel300

Looking up the schematic for that "relay" (I don't understand why they don't call it a control module), there are 2 actual relays in there. The larger one is for the fuel pump, the smaller is for the transmission kickdown cutout. The only one that should be clicking should be the large relay.

...

I've got a feeling that the other relay is for the intake manifold electric heater. I guess if I had an "automatic transmission" fuel pump relay there'd be another relay in there for the kick down.

Stretch · #11 11-06-2016, 09:09 AM

Quote:

Originally Posted by Diseasel300

... If you're handy with a soldering iron, ...

(Mad giggling in the background!)

Diseasel300 · #12 11-06-2016, 09:12 AM

The schematic shows that MT vehicles simply don't use Pin 6 (which is the kickdown cutout).

Rick76 · #13 11-06-2016, 11:02 AM

It looks like you have burnt traces going to the fuel pump relay (the larger of the two). It isn't evident at first because it is covered by the green solder mask. Scape it off and I think you will see that the copper has vapourized. The open trace(s) can be bridged with a wire after scraping down to shiny copper.

For this trace to blow, you must have had a short or excessive current drawn by the pumps.

Use only electronic grade resin core solder. You don't need an aggressive flux whatever that is.

Stretch · #14 11-06-2016, 11:27 AM

Quote:

Originally Posted by Rick76

It looks like you have burnt traces going to the fuel pump relay (the larger of the two). It isn't evident at first because it is covered by the green solder mask. Scape it off and I think you will see that the copper has vapourized. The open trace(s) can be bridged with a wire after scraping down to shiny copper.

For this trace to blow, you must have had a short or excessive current drawn by the pumps.

Use only electronic grade resin core solder. You don't need an aggressive flux whatever that is.

Thanks Rick,

I haven't a clue what to look for - learning all the time!

I'll pull it back out tomorrow and have a bit of a scrape.

Note to others:

No matter how useless you might feel when it comes to this stuff - always take a picture and post it up! Who knows eagle eyes might just see something that you can't...

Stretch · #15 11-06-2016, 11:28 AM

Should I measure the current draw from the pump then?

(Answer I think I will - can't hurt)