Follow up on intermittent start problem and the fuel pump relay (W126 300SE 1989 UK)

Just to update those who assisted me with my intermittent start problem. I pulled the fuel relay and got the cover off in no time. I almost asked the group where I was to find this relay, but its difficult to
miss! It all looked great in there at first glance. A little closer looking did show up some minor cracking around one of the relay connection solder joints to the PCB. The clincher I think tho (hopefully) belonged to the rivetted connection of the conductor plate to the back of the pin labled terminal 15 (live at ign on). It showed some signs of duress and copper verdigree. I wondered what lurked under the rivet head and the blade of the connection, so I decided to clean up what I could see of the part and solder that too. All the other rivetted plates to a man looked perfect, so I left them alone. I WD40'd the relay base before re-inserting it. To date I have had no further re-ocurrence. Fingers crossed. BTW, there is a PIC microcontroller IC in there. Pretty advanced for 1989. Must have cost a fortune then.
Cheers... Rob.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Rob, be careful still. I went through pretty much the same and after not very long it acted up again..... wife was 'really' ticket..... :-)
cheers, guenter

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
A couple of comments here. These cars are full of fairly simple (by 21st century standards) circuit assemblies than control various things - the cuise control amp, heater gubbins, and in this case fuel pump relay.
A common failure is simply a mechanical one and it's usually at a solder joint, which is why the oft recommended technique of resoldering EVERYHING on the circuit board works so very very often.
Tiger is right in that a new relay will most probably fix it. By doing what you've done so far (fixing a couple of trouble spots) you have almost certainly identified the source of the problem, because by fiddling with this thing you've at least changed the nature of the problem and at best have fixed it.
Say you have it fail again and replace it with a new relay. Now, that old one sits there on your bench with one or maybe two mechanical problems preventing it from working good as new. We're not talking a mechanical failure here such as a worn cam but simply some place where the electrons get stuck - sometimes.
That it works at all means the PIC and firmware works, it's just not getting the data it needs as sostensibly some wire has a broken connection.
If indeed it does act up again then reflow ALL the solder joints. And it woudlnt hurt to go over it with an ohmeter first to see if you can identify any problematic areas of electrical discontinuety.
It does sound like you're on the right path though.
--
Need Mercedes parts? http://parts.mbz.org
Richard Sexton | Mercedes stuff: http://mbz.org
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
If you are handy with a solder... reflow it. But you are much better off buying a rebuilt fuel pump relay.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Motorsforum.com is a website by car enthusiasts for car enthusiasts. It is not affiliated with any of the car or spare part manufacturers or car dealers discussed here. All logos and trade names are the property of their respective owners.