More Fuel Pump Relay Questions, sorry!

[isobars]

Hi Everyone,
Starting failure and cutting-out.
Well, woe again.
I thought this problem had been resolved last year with a new dizzy cap, plugs, etc, but apparently not.
I tried to start the car, 124/ 300CE 24v today, nothing, and no fuel pump buzz.
I unhooked the battery, 30 minutes later reconnected, the fuel pump buzzed, the car started immediately – as it used to do. Five seconds later it cut out instantly, no splutter, just stopped.
I unhooked the battery again but couldn’t recreate the buzz magic trick again. Not even remotely interested in starting.
Could someone advise, please, should the pump relay buzz every single time I turn the ignition?
What does the relay do, simply switch on the pump, which remains on until the ignition is turned off?
When people talk of bridging the relay is that simply a matter of removing the relay and putting a wire in two holes?
I’d like to have a look in the dizzy in case the seal – which wasn’t replaced – is shot. How do I get to the dizzy and HT leads?
All this said, why is the FPR my prime suspect?
Am I barking up the right tree here?
The car stood for a very long hard winter outside before I bought it last January. I took it out of a dry garage on Monday for a run in dry conditions, it had been in the garage since September. It started first crank, never missed a beat – it’s never stuttered on a run. When I came back after twenty minutes the windows were wet inside. Since Monday the car has been outside, but under a breathable cover. When I got in the car the rubber ring sealing the ignition barrel had droplets of water on it – maybe the cover isn’t as breathable as I thought?
Thank God for the sanity of a W123!
Thanks in Advance for any advice.
Cheers,

Paul

 

[grober]

I think I have got this correct if memory serves. When the ignition is first switched on the fuel pump relay switches on the fuel pump/pumps to pressurise the system. This when you hear the fuel pump buzz. Once the system is pressurised [ and there's a device called the accumulator which maintains system pressure for some time after the pump is switched off] you wont hear it again until the fuel line pressure has dropped over several hours deactivation. There is also a safety device which cuts off fuel unless the relay receives a signal that the engine is running. My guess is that following the initial pressurisation phase the relay either failed to get the "engine running signal" or failed to continue to provide power after starting. Trouble shooter | Motor | Find Articles at BNET Either way the first thing is to replace or repair the fuel pump relay. They often fail due to dry joints on the internal circuit board which develop due to vibration. Often removing the relay can and running over the soldered joints on the small internal PCB can repair them. Otherwise its replacement. [ the DIY repair doesn't apply to the large relay found on some later models]

Covering up a car is never a good idea. Far better to let them breath= dry out in the open air to avoid condensation problems. If you feel the car is very damp then remove the carpets open the windows and leave the car in a garage with a dehumidifier running for a day or two. Out in the hot sun will do equally as well.

 

[isobars]

Thanks Graeme!
Very helpful info – though I may have to read that article a second time!
The car had actually stood three days so if I assume the system wasn’t pressurised when I first tried, then the pump didn’t prime initially, but it did after I disconnected and reconnected.
So does that suggest there’s a problem with the relay in as much as it wouldn’t switch on the pump without being re-booted, and if that’s the case if may well be that if one element has failed, then other bits of information swapping within are equally liable to be dickey, that is, the bit that hears the signal that the engine has started?
FPR is strongly favoured here to be a duffer?
Thanks again for your help.
Cheers,
Paul

 

[Druk]

As a temporary measure and to test the operation of the FPR you can bridge terminals 30 and 87 which will make the fuel pump run continuous with the ignition on. A couple of turns of copper wire round the pins on the FPR and re-insert it into it's socket. If the car then runs satisfactorily then the FPR is suspect.

 

[isobars]

Hi Druk,

Thanks for the wiring tip.

I have a dilema though, there are three 87's marked on the relay, 87h, 87k and 87v.

**gger!

I think the 24v is different to the 12, perhaps?

Cheers,

Paul

 

[grober]

This link I posted might be useful. It refers to an N10 relay but the principle is the same.
http://www.mbclub.co.uk/forums/favourites/93685-how-repair-resolder-relay.html

 

[LastMinute]

Could someone advise, please, should the pump relay buzz every single time I turn the ignition?

The relay shouldn't buzz. (That's bad.) The pump should run for a few seconds each time you turn the ignition on.

I think the 24v is different to the 12, perhaps?

It is. the M103s have a standalone fuel pump relay ("KPR"), but on the M104s the fuel pump control is part of the engine subassembly control ("MAS") unit.

 

[grober]

The relay shouldn't buzz. (That's bad.) The pump should run for a few seconds each time you turn the ignition on.


It is. the M103s have a standalone fuel pump relay ("KPR"), but on the M104s the fuel pump control is part of the engine subassembly control ("MAS") unit.

For this type of relay replacement [ possible use of a temporary substitute for test is a possibility] is really the only answer as I don't think they are amenable to repair.

 

[Druk]

Hi Druk,

Thanks for the wiring tip.

I have a dilema though, there are three 87's marked on the relay, 87h, 87k and 87v.

**gger!

I think the 24v is different to the 12, perhaps?

Cheers,

Paul

All I've got is a 12v wiring diagram. It isn't 87v cos that goes to the cold start valve. My drawing shows two wires from 87, one to the fuel pump and one to the Lambda sensor but doesn't discriminate. Later cars must have provided separate teminals. You wouldn't do any harm to either by powering them up artificially cos the relay does that anyway once the engine is running. As Grober said the relay only cuts out after a second or two as a safety feature in case of an accident where the engine is stopped but the fuel pump would keep running. Try each in turn to see if one activates the pump.

 

[Druk]

The relay shouldn't buzz. (That's bad.) The pump should run for a few seconds each time you turn the ignition on.


It is. the M103s have a standalone fuel pump relay ("KPR"), but on the M104s the fuel pump control is part of the engine subassembly control ("MAS") unit.

LastMinute...does that apply to the 104 24v or just the 104 HFM engine? OP seems to have found a FPR with the requisite pin numbers.

 

[isobars]

Thanks for the relay repair link, Graeme. Superb piece of info. An incredibly high standard.
Druk, I'll try both, thanks.
LastMinute, does yours, (pump, not relay), buzz every time, is there no pressure built-up?
Cheers,

Paul

 

[LastMinute]

I don't think they are amenable to repair.

According to [POST=980395]this post[/POST] they're not easy to get in to, even by violence...

LastMinute...does that apply to the 104 24v or just the 104 HFM engine?

Ah, sorry – I wasn't making myself very clear. By M104 I meant the 104.980 in the 300[C/T]E-24s (and I'm pretty sure the 104.981 in the R129 300SL-24 is almost identical); I don't know enough about the later 104.942 or 104.992 with HFM. My 300E-24 definitely had a MAS in place of a standalone KPR.

LastMinute, does yours, (pump, not relay), buzz every time, is there no pressure built-up?

Yes, the pump always ran for a second or two when I turned the ignition on. Even if I turned the ignition off and immediately back on again (when there should already have been enough pressure). The only time the pump didn't run, I had a duff battery...

 

[isobars]

Many Thanks to you all for the advice.

Cheers,

Paul