420 sec cutting out

[samurai]

Was out today in my 420 sec 1988 all of a sudden all the lights on the dash came on a it came to a stop tried to start it turned over ok but wouldn,t start left it a while then started ok was doing 30mph the power steering went off as well was almost impossible to steer hate to think what could have happened on the motorway you can hear the the fuel pump working when you turn the key ,then same thing happend at a junction ,not confident to go out in it now only had car a couple of weeks
any advice please

[bolide]

The usual MB suspects are crank position sensor, fuel pump relay, OVP relay, ignition module, distributor cap and rotor arm. Of these I would say the CPS is the most likely

If the car has a distributor cap, and it looks suspect, replace it and the rotor arm with MB parts only

I'd take the car to your nearest MB independent specialist

Nick Froome

[jith]

bolide said:

The usual MB suspects are crank position sensor, fuel pump relay, OVP relay, ignition module, distributor cap and rotor arm. Of these I would say the CPS is the most likely

If the car has a distributor cap, and it looks suspect, replace it and the rotor arm with MB parts only

I'd take the car to your nearest MB independent specialist

Nick Froome

Sorry Nick,

There is no CPS on this old Merc; this is early KE Jet which is basically mechanical with Hall sensor type ignition. As the fuelling system relies on fluctuating vacuum to operate the metering unit, this kind of fault is usually an air leak; or, as I have found with increasing frequency, individuals turning the air/fuel ration so lean that the car stalls very easily, particularly if it is an automatic as this S class is.

If you can get the car up to Glasgow Samurai, I can have a look at it and go over the whole system for you. If you look on my website you can see the 300 SE I did for a PHer just a couple of weeks ago.

Regards,

James.

[bolide]

I was wondering how it knew where the cam / crank was - but it has a distributor so it doesn't need to! Do the old K-Jet systems have an injection pump timed to the cam? I suppose they do, so no electronics needed

Do they still have an ignition module like the later cars?

Nick Froome

[jith]

bolide said:

I was wondering how it knew where the cam / crank was - but it has a distributor so it doesn't need to! Do the old K-Jet systems have an injection pump timed to the cam? I suppose they do, so no electronics needed

Do they still have an ignition module like the later cars?

Nick Froome

Hi Nick,

The 420 is actually KE Jet, which simply means it does have some electronic control on top of the mechanical system. This gives a more finite control of fuel flow under different conditions, and has features such as idle stabilisation and fuel cut-off on the overrun. Some of the later KE systems also ran closed loop with a cat.

There is no injection pump, only a high pressure pump either under the tank or immersed. The system is CIS Injection, or constant flow. The injector pressure constantly changes to deliver less or more fuel according to demand through a mechanical flow meter.

In terms of reliability it is second to none. If you have a 12 volt supply to the pump it will run, even if all the other electronics have totally failed!

As far as I know, Mercedes were the first company to use electronic ignition as far back as the 60s. It was called an Ignistor unit and utilised the points, but only as a switch working at very low current rates. This meant that they lasted a long time and didn't burn out. The unit then controlled the dwell angle and ignition firing to the coil. Eventually they made the system contactless. So the S class of this era still has an ignition module; but they are extremely reliable. I could count on one hand the number I have replaced in many years.

I love working on these systems because the engineering integrity is tremendous.