How to test a relay?

Pull and guess as a way to find electrical problems is a mug’s game. A Google search suggests that the DME relay originated as a Porsche term, it may or may not be correct in this case and if it is faulty it would seriously affect a car’s ability to start. In the Porsche context it appears it starts both the engine management system and the car’s fuel pump(s), it is a safety part that cuts fuel pressure in the event of a shunt to avoid fire risks. I fear that you might need a little more in the way of electrical checking to confirm where the get up and go has got up and gone. Electrical systems on more modern cars are fearsome, unless you have both good tools and the skills to read a car’s fault you may fail. I have had garages fail to find electrical problems – the symptoms pointed at this device or that device according to the computer codes, both were replaced and the problem in the wiring remained. A single dodgy break in a wire was the cause of the (expensive) trouble. A clued up auto electrician found, and fixed the issue, (others had failed) it only needed wire and cable terminations, plus the time to trace and fix the issue. You have not defined the symptoms as you know them, in this case is there fuel pressure, where is power reaching or not reaching, is the fuel injection system working, etc. Note, I am no auto electrician, but it does sound as though you do need a leg up in that league to point you in the right direction.

One thing you could try is the substitution test. Check in your manual, and on the car, and look for another relay on the car – horn, wipers, heated rear window, main beam, – for one with exactly the same terminals and pin layout.

The theory is you don’t need any of the above to start a car – only when it’s on the road. You can do it one of two ways, put the proven working relay  in the DME slot and give it a try, or the old DME relay in the other accessory slot and see if that works.

Please bear in mind I’m not an auto electrician, I just messed around with them in my youth, which was a long while back now!!! I did work for BT, pre-electronic exchanges, on equipment that was nothing but relays – hence the substitution protocol !!

I feel your pain looking for a cheap ( realistically priced ) way round – mine’s a Merc!!

Don’t bother with breakers – try EuroCarParts, search for DME Relay, put your reg no in and see what price new ones are – I checked for mine and the dearest was £25. If it doesn’t work, you might get your money back or at the very least, a credit note!!

The fact that the OBD port has stopped working is real issue in my book. Since it appeared to work when the problems started, but stopped as the problems built up suggests the me a major issue with power circulation. ECUs can and do fail, I would check to see if there is someone in your area who could check it over for you if all else fails. While I used the term ‘major issue‘, because the effect is deadly, it could be as simple as an earth, a bad connection or a damaged wire, all of which will cause major grief until they are found. As you have done things, could it be that for example something needs to be reset?

Frankly, we are all guessing wildly and I strongly feel the this one for a good auto-sparks, who knows the ways of the model

I am sorry, I misunderstood the postings trail, if the OBD is now back on line you have sorted something out. As there are so many reports is there any commonality to them that point to a specific area? For example, fuel pressure, no fuel flow, fuel injection systems alarms could all come from e.g. a pump not getting fuel or no fuel, or even no power. A dumb example I know, but it does serve to point to a possible logical way through what otherwise feels like a maze.

The clock fail is no doubt due to the loss of battery at some point and easy to overcome.

On some cars the loss of battery power can upset the anti theft devices and some form of rest can be needed.

Now you have OBD working have you done a rest of the system to clear the alarms, which may have come from a series of failures prior and during the current situation?

Checking the web for similar faults on merc’s, highlights one of the ABS wheel sensors failing or, believe it or not, the brake light switch. It seems the brake light switch has more than one set of contacts and if one set fails ( even though the brake lights still work! ) can bring about similar fault codes.

These failures can put the ECU into “limp” mode – but that is at least supposed to get you home, albeit slowly!!

This is probably not relevant but maybe worth a mention. A couple of years ago my VW suddenly decided not to start and the AA bloke (who had fortunately previously worked for VW) fiddled around for a couple of minutes and then asked if I had a spare key. Spare key worked fine! Apparently there is a transponder (whatever that is) built into the key which had failed. Worth a try if your car uses a key rather than the later extremely insecure systems which seem to be prevalent now.