W126 300se problems

[richardmorris]

Hi all. Just had a call from a friend. The problem I thought we'd cured on his big merc is back. Basically when the lights are on it would cut out. Thought a new distributor sorted this, but now it cuts out when the heater blower is on full.

 

Obviously something electrical, but what? I had a meter on it the other week and the alternator was giving out about 13.6v on idle, the battery is about two years old.

 

He is getting worried about taking it to Belgium on Wednesday--apparently the Morgan isn't big enough!

 

Any ideas?

[mercrocker]

W201s have an over voltage protection relay that gives trouble of this kind - is there something similar on the bigguns?

[richardmorris]

There is an OVP, but I know this was new when he got the car 18months ago. Could have gone I suppose.

[richardmorris]

doesn't help any, but here are some photos of it. 

[spike60]

13.6v sound a little low, I can check mine to compare. It had a battery and alternator not long before I go it. I'm not sure that would cause cutting out though. Dounfs more like OVP relay or earthing problem. 

[richardmorris]

It always starts well and idles fine, just turning the heater on full is like turning the ignition off. He says he's checked the ovp, but I've just suggested fuel pump relay, so he's off to find that!

[Barry Cade]

Yup, OVP causes mad symptoms on these, and is connected in some ways with the fuel pump relay. Check them both, carefully.

 

This was mine... Bought as a non runner- usual dizzy cap and lack of use issue, sorted it all up and made enough profit to buy a packet of fags. A 10 pack.

 

[spike60]

13.6v with everything switched on, 14.1v with nowt on. Mine idles at 700, when you switch the fan on it dips to about 500 then corrects back up.

[richardmorris]

14.1 seems familiar too.

 

He's just called back having found the relay (0035450005) and is going to check it. He also mentioned that the side lights are taking a few seconds to light up- dash lights come on straight away though. THAT sounds like the OVP to me, although he has the newer updated one from Mercedes- twin fuses and red cap.

[richardmorris]

Well he's cleaned the connectors up and put things back, but doesn't seemed to have cured anything. He's getting in contact with the mobile merc guy I know who will know what he's doing more than we do anyway.

[richardmorris]

Right, I've just been over on the way home. Battery reading 12.6 dropped to 10.6 on starting then 13.8 with engine running. Turned on lights, 13.7 and then heater blower on full and radio on still 13.6 revved it and up to 13.8.

 

Left running for five mins under load and it seemed fine. I hate intermittent problems!

 

Symptoms of it stalling seem to be Lack of electricity and not fuel ( doesn't chug to a halt like running out, it just dies cleanly). It will always restart instantly though without the lights on.

[chaseracer]

This all sounds very familiar.

[richardmorris]

What did you do (apart from flog it?). Harp by the way is planning on braving it to Brussels on Wednesday night with no eu breakdown cover in a 30 yr old Mercedes with running issues. He should be on this forum!

[chaseracer]

Proper HT leads, decent plugs, idle control valve, reseat relays, replace OVP relay, run on 98/99RON with timing suitably set.

[richardmorris]

Proper HT leads, decent plugs, idle control valve, reseat relays, replace OVP relay, run on 98/99RON with timing suitably set.

Done all of that. Apart from idle valve, but then this goes wobbly when driving not idling. Bloody car. He calls it the blue bit&h !

[Bear]

Are all the fuses nice & clean and ideally, not the shitty aluminium contact ones?

And... just a stab in the dark, but... this is of the era when dim-dip headlights were a thing, so is there a ballast resistor somewhere that is maybe shorting or unable to earth, so finding exciting new routes to do so? Could be combined with a failing earth elsewhere, hence stuff shutting down occasionally but working when you checked it.

(Also ignition switch contacts).

[richardmorris]

Are all the fuses nice & clean and ideally, not the shitty aluminium contact ones?

And... just a stab in the dark, but... this is of the era when dim-dip headlights were a thing, so is there a ballast resistor somewhere that is maybe shorting or unable to earth, so finding exciting new routes to do so? Could be combined with a failing earth elsewhere, hence stuff shutting down occasionally but working when you checked it.

(Also ignition switch contacts).

No dim dip, thankfully. I am thinking ignition switch. But it's not that easy to check. Have checked the earths and all ( I can find) seem fine. All fuses new and checked at the weekend. The whole car is rust free actually and all connections generally good having lived most of its life in an underground carpark at the French embassy.

[Barry Cade]

This is going to sound stupid, but check the dizzy cap carefully, and if it's anything other than O.E, replace it. They're not cheap but had stalling/cutting out issues with the straight 6 even with a new ECP Beru cap.

[richardmorris]

It was replaced the other weekend with new leads- helped make the problem less frequent, but still happening.

[trigger]

This doesn't help in anyway but I've had one of these parking outside my house for the last week. For a car thats done over 210k it still looks like new.