w201 190 carb shut off problem

[jjrodger]

Hello All,

I have recently acquired a nice 1990 190 carburetor. It has one problem. Occasionally it will cut out or stall when running (usually at speed) and then refuse to re-start for a minute or so.

I have traced the problem to the shut off system. If I disconnect the electrical feed to the shut off valve on the bottom of the carb, the problem never manifests itself. Having established this much, I suspected the shut off relay (labelled "4 Zyl. Kickdown") in the relay box, number A0015457205. So I replaced it with a new old stock one (which did not look very new when it arrived). Still the problem persists.

Maybe my replacement relay is bad, although that seems a bit of a coincidence.

I wonder whether you could tell me what the feeds into the relay are. The terminals are labelled 87, 15, 31, 30, 87k and TD. I have not been able to find a circuit diagram for this car or any meaningful information even on the usually excellent WIS system.

All and any help appreciated.

 

[stwat]

That's a fuel pump relay, normally only used on the fuel injected 190e not the carb engined 190.

 

[jjrodger]

Interesting. It is the one in the car and the one specified by the EPC.

 

[stwat]

The car doesn't have a 'E' on the back does it?

 

[jjrodger]

Definitely a carb model, 2010232f787909.

Do you know where the relay is triggered from? I'm guessing it's the ignition switch.

 

[jjrodger]

I think I've figured this out and I think it is probably the relay that is the problem.

When the ignition (terminal 15) is cut, the relay switches a live feed to the shutoff valve to on for about ten seconds. Given that everything else with a terminal 15 feed works (e.g. radio, dash lighting, indicators, instrument cluster and no doubt much else), it must be the relay or its plug socket connection.

Make sense?

 

[stwat]

Hi, I'm not sure to be honest. Have you checked the fuses? They are known to look okay but fall apart once removed.

 

[grober]

there are at least two generations of fuel cutoff relay with slightly different wiring patterns. [ they may be interchangeable depending on how they are wired underneath ??]
for both
pin 30 main supply always live
pin 87 output to shutoff valve
pin 31 earth

early unit
TD goes to ignition unit via the diagnostic socket pin 1 [ I assume this is a switch off control??]
15 seems to loop out to the heated washer nozzles?????

The later relay has no TD pin instead pin 15 is fed via the manifold heater relay K3 connections found in the little relay holder in front of the fuse box which is in turn triggered from 8a FUSE 2 in the fuse box.

First thing to check are all the fuses [ esp fuse 2!] ----- the bayonet type can get corroded in their fuse holder contacts and start making intermittent contact so give em all fuses and contacts a clean with a non conducting abrasive [ nylon pan scourer pads are ideal ]
next have a look at that manifold heater relay connections before looking at the fuel cut off relay.

edit I did not make it clear the relay K3 doesn't control the cut off relay its just wired via its connections from fuse 2

 

[jjrodger]

Latest thoughts.

I checked and replaced the fuses, cleaning up the contacts.

The relay is the fuel pump relay but, of course, my carb car has a mechanical fuel pump. All it does in my car is shut off the fuel supply to the carburettor by means of a valve. It does this in two circumstances: whenever the ignition is switched off (in order to prevent run on) and whenever the rev limit of 6,200rpm is exceeded.

It seems to be triggered from the EZL (the ignition control unit on the nearside wing) and also from the ignition circuit (terminal 15). The EZL unit obtains engine speed information from the crankshaft position sensor.

My thoughts are that either the crankshaft position sensor or the EZL are bad. I am leaning towards the EZL because when I disconnect the shutoff valve, everything works fine. Whereas if the crankshaft position sensor was bad, I don't think it would start or run. I have bought a used EZL for £20 from Ebay. I haven't quite figured out which is the correct crankshaft position sensor.

Thank you for your help, all, we will see next weekend!

 

[jjrodger]

In the end, the problem was caused by a bad connection in the ignition switch. £25 from Merc, 20 mins to fit.