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Old 09-03-2003, 12:08 PM
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Join Date: Sep 2003
Location: York UK
Posts: 7
Fuel Pump Relay - Dry Joint photo

After suffering the symptoms of random fuel starvation on my 1990 230CE, I found enormous help on this forum and fixed the problem in around five minutes. By way of a thank you, and for the benefit of those who have never seen a dry joint on a printed circuit board, here is a photo of the fuel pump relay board with a zoom in on a classic dry joint on one terminal of the relay. A bit of heat and a blob of fresh solder and the pump ran first time.

Although not actually measured, I suspect that this joint is subject to quite a high current load even with a good pump. If this is the case, the fault is almost bound to develop at some time or other on most cars and I would urge all readers to check this relay periodically. I consider a sudden failure of the fuel pump to be as dangerous as a blowout and worthy of a routine check. Luckily, the engine cut out late at night on a quiet road, not while I was overtaking a fuel tanker at 70mph!

By the way, the relay location on my W124 230CE is behind a pull-out plastic shroud behind the battery (not behind the fusebox as I first thought, duh).

Cheers to all

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Fuel Pump Relay - Dry Joint photo-w124_fpr_dryjoint.jpg  
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Old 09-03-2003, 01:26 PM
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Join Date: Aug 2001
Location: secret
Posts: 3,044
Good info John!!

Mine would heat up due to the poor connection and actually lose the connection and the engine would die. I had a bad pump drawing too much current and causing the problem. After everthing cooled down it would run again. I replaced the bad pump AND the relay but took the old one apart and did what you did. So I have a spare now!!
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Old 09-03-2003, 01:31 PM
Q Q is offline
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I have successfully repaired mine as well. It was harder to get the cover of the relay off and on than it was to fix the solder joints. There were multiple points that needed resoldering. My problem seemed to crop up on days of higher humidity.
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Old 09-03-2003, 09:18 PM
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Join Date: Mar 1999
Location: Suwanee, GA, USA
Posts: 4,712
Many times the fuel pump(s) are drawing too many amps. This will cause a burn in the trace.

The current should be less than 7amps.
Donnie Drummonds
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Old 09-04-2003, 04:46 AM
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Join Date: Sep 2003
Location: York UK
Posts: 7
Thanks for the information Donnie. This begs the question: is it a worn pump that makes the relay connection fail or is the pcb not up to the job in any case? One possible answer would be to know how many failed in the first year on a good pump.

The 230CE was made in 1990 and has done over 150k miles. Although the pump sounds fine at the moment, I will check the current (when hot) and also recheck the relay joint in a couple of weeks to see if it shows any signs of deteriorating.

My electronics experience tells me that 7A is unlikely to cause the solder joint to fail per se, however, it may possibly have 'crystallised' over time due to repeated heating and cooling. That being the case, it reinforces my original suggestion about routinely checking the condition of the relay pcb on older models.

By a strange coincidence, I once had this fault on a BMW E30 and it left me stranded in the snow at midnight; a bad experience I do not wish to repeat.

I'll drop by with any updates. Cheers.
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Old 09-04-2003, 08:51 AM
Q Q is offline
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The problem with the relay is cold solder joints. If they had been soldered properly in the first place, they would not have failed. What happens is the pin that fits through the hole in the PCB does not get hot enough to adhere the solder properly. You'll notice that the pins where the problem exist are the largest pins with the body of the relay acting as a big ole heat sink when the soldering takes place.

Corrosion of the copper on the un-tinned (non-soldered) pins and holes in the PCB is what causes the open circuit to occur.

I have repaired my Idle Speed Control Module failure by resoldering its joints as well. Next is the cruise control module, which is flakey.
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Old 09-04-2003, 09:38 AM
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Join Date: Jul 2001
Location: SoCal
Posts: 1,303
Technically, the problem is not a cold solder joint. The solder HAS adhered to both the post and the pad on the board. The problem is mechanical, both in design of the board and often due to assembly errors.

In order for a solder joint to be effective, the connection must be mechanically stable and rigid before soldering. The problem in the picture occured because the post moved relative to the PCB hole because the hole was sized too large for a snug fit. This broke the solder joint, which then developed the black circle from arcing due to poor contact.

'91 MB 190E 2.3
'08 RAV4 Ltd 3.5
'83 Lazy Daze m'home 5.7
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Old 09-04-2003, 11:01 AM
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Join Date: Sep 2003
Location: York UK
Posts: 7
Under normal circumstances, I would agree with Steve about why some joints fail. Solder is pretty soft and cannot be relied upon for mechanical strength. In this case, I can't think what may have caused the pin to move other than thermal effects. I've seen dry joints on connector pins due to rough handling or vibration but, not being a metallurgist, it's hard to comment further.

When I was working in electronics it often seemed that many of the so-called electronic or electrical 'failures' were actually mechanical in nature or due to some outside factor such as dirt, water or heat. I guess the bottom line is that not much can really go wrong at the electron level in a well-designed system.

All parts are designed to work within reasonably safe limits for a certain length time. In the case of this part, the time elapsed last Friday night. But wouldn't it be gratifying to know whether the person who assembled this circuit board ever gave a thought to how many people would mess their pants in the fast lane when the fuel pump stops doing what it does best. The mind boggles... Hi, is that the MB service desk? I'd like to arrange an appointment for my wife and kids to come and meet assembler XY89.9 who was kind enough to leave his or her employee ID in my fuel pump relay.

Good Karma?
1990 230CE 150k+ UK spec.
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Old 09-04-2003, 04:40 PM
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Join Date: Apr 2002
Location: Surrey, Uk
Posts: 254
Nice pics John. That is exactly what happened to the OVP relay in my '89 230TE and, I guess, all the other ones we hear about. Simple repair.
Mick J
'08 Chrysler 300CRD (MB OM 642 engine)
'95 E220 estate
'89 230TE (R.I.P.)
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