Porsche DME relays explained
If you have one of the Porsches listed below then you need to know about DME relays. Not because there’s anything intrinsically interesting about them but for the simple reason that yours could one day fail, and then the engine won’t start.
Actually, all you really need to know is just a couple of very simple facts. One: that you should always carry a spare DME relay in the car, just in case the original unit (however new it might be) lets you down and leaves you stranded. And two: precisely where in your Porsche said relay is located.
However, if you want to know more, then here goes…
DME stands for Digital Motor Electronics, or what you’re more likely to know as Motronic. That’s Bosch’s name for its widely used engine management system, itself often known generically as the ECU, or electronic control unit. The ECU is in effect the car’s ‘brain’, and controls (among many other things) the engine’s ignition and fuel-injection systems.
A relay is essentially an electromagnetic switching device that permits a low-current circuit to control (to switch on and off, in other words) a relatively high-current circuit (in this case those feeding the fuel pump and the ECU). It consists of essentially four parts: an electromagnet (a coil of ultra-thin copper wire with a moveable iron core inside it), an armature, a spring, and a pair of electrical contacts.
When a current is passed through the coil of copper wire the electromagnet is activated. This attracts the armature towards it. The armature is attached to one of the contacts, which in turn moves and connects with its opposite number, thus completing the high-current circuit. When the low-current supply is switched off the spring pulls the armature away from the coil, the contacts separate, and the circuit is broken. Simple and very effective.
The single DME relay in your Porsche is actually two separate relays cunningly combined inside just one black-plastic housing. (Note, though, that all of the Porsches covered by this guide have a number of similar-looking units within their often combined fuse and relay boxes.) One of the two individual relays controls the fuel pump, the other the ECU. And if one or both relays fails then it doesn’t take too great a leap of imagination to appreciate that the engine won’t start.
The precise workings of the DME relay and its related circuits are beyond the scope of this article, but it’s worth noting that both the 964- and 993-model 911 Carreras work in a slightly different way to the four-cylinder, front-engined cars. (And the 928 has a completely different system again, with no DME relay, as such, to worry about in any case.)
In the 964 and 993 the ECU has a permanent 12-volt feed that maintains it in standby mode (essentially to preserve its internal memory, fault codes and so on). When you switch on the ignition the ECU sends a current to the DME relay which then switches on the ECU ready to control the engine. Then when the the starter motor is cranked, the ECU activates the second relay in the DME relay and power is supplied to the fuel pump for as long as the engine is running.
In the other cars power comes from the ignition switch direct to the DME relay. This then powers up the ECU which itself then controls the second part of the relay that relates to the fuel pump.
Both systems offer a clue to the primary function of the DME relay. Essentially it’s a safety device. If the vehicle is involved in an accident and the engine stops the ECU will sense this (via the flywheel sensors from which the ECU constantly works out the ignition and fuelling requirements) and immediately cut the power to the fuel pump by turning off the DME relay. This significantly reduces the risk of an explosion from leaking high-pressure fuel lines.
The DME relay in your Porsche has a pretty tough life. Unlike most of the electrical system’s other relays, it’s active literally all the time that the engine is running, and because of the constant current passing through the coils and contacts it becomes surprisingly warm. And over a period of time this heat has a marked effect on the soldered joints on the printed-circuit board inside the casing on which both the relays and a number of other small electronic components are attached.
Look through a powerful magnifying glass at the PCB of a DME relay that’s had a few years’ use and you’ll probably spot at least one dry joint. Caused primarily by the effects of heating and cooling, expansion and contraction, dry joints occur where the tiny blob of solder by which the wire projecting from each individual component is fused to the metal track on the board begins to fracture.
It’s a common problem in the electronics industry, and at best the result is intermittent contact (which is why conventional fault-finding can be so difficult); at worst complete failure. Put it this way: if light aircraft had DME relays (and the Carrera 3.2 engine has been used for just this purpose) the authorities would probably insist that it was routinely replaced after 50 hours’ use.
DME relays – or, more specifically, dry joints on their circuit boards – can be repaired, but whether you suspect or know the unit to be at fault it’s better simply to fit a new one. At around £20 they’re certainly cheap enough for the peace of mind they should bring.
And – provided you know where it’s located; see below – it’s not a difficult job. Sometimes the metal contact blades (or pins) are a tight fit in the socket on the relay board, requiring the unit to be tilted from side to side as you pull it out (and sometimes this can cause the plastic cover to separate from the base), but you certainly shouldn’t need anything in the way of tools.
That said, some DME relays are remarkably awkward to get at. In the 924S and earlier 944s, for instance, it’s not only right up under the passenger’s side of the dashboard, but also neatly obscured by an additional fuse holder which you have to unscrew that and move gently out of the way for better access.
So if you go to your Porsche one day and the engine turns over but won’t start (although it may well try to fire) then the first suspect should always be the DME relay. If you have a spare (and, considering the notoriety of the things, you should always keep one in the glovebox) fit that and see if it solves the problem.
That’s not to suggest that a failed DME relay and no spare should leave you stranded. In fact, you can bypass the relay with a couple of pieces of wire. In a real emergency you can get away with a couple of paperclips, but ideally you need a pair of jumper leads incorporating an eight-amp, in-line fuse. In the case of a flat-pin relay you need to place them across holes 3, 7 and 5 of the socket (corresponding to terminals 30, 87, and 87b of the relay itself), or from 86 to 87 and 87b if you have a round-pin relay. If the relay was faulty the engine should now start.
Needless to say you have to be careful when you’re doing this not to short out any other terminals (I take no responsibility if you inadvertently trash your ECU or any other component in the process…), and you should make or break any electrical connection only with the ignition switched off.
You can use this trick as a get-you-home measure, but it’s not ideal because it means there’s a constant power supply to the fuel pump, which can be dangerous in the event of an accident. You should also remove the link after you have finished your journey, otherwise the battery will be flattened if you leave the car idle for any length of time.
Another short-term fix is to allow the relay to cool. Often it will fail when hot (and because of the way electricity works the more those solder joints deteriorate the warmer the relay will become). You can take the relay out of its socket and blow on it to accelerate cooling. We’ve even heard of owners removing the plastic cap to help keep the relay cool and thus enable them to finish their journey.
Despite all you hear about DME relays, they certainly don’t fail on a regular basis, so you don’t need to get too hung up on them. On average DME relays seem to have a life of about 80,000 miles or eight years. That’s longer than most of us own a car, so routinely fit a new one whenever you buy a Porsche and you shouldn’t have to worry about it again.
If, on the other hand, you find that your relay is failing more regularly it’s possible that it’s having to cope with too much current. The obvious culprit in this scenario would be a faulty fuel pump that’s drawing more current than the typical 10 amps it should. Also, some after-market relays can be less reliable than others, so it’s worth seeking out a genuine Porsche-supplied item.
That apart, once you’ve fitted a new relay it should give you many miles of trouble-free motoring, and if you have a spare in the glovebox you’ll have peace of mind.
And what of modern Porsches such as 996s, 997s Boxsters, Caymans and Cayennes? Well, Porsche realised that DME relays were potentially troublesome, and so these cars have much simpler relays controlling the power to the fuel pumps. These items contain none of the soldered joints typical of DME relays, and so are inherently more durable.
Testing and repairing
DME relays are so cheap that there’s probably little point in trying to salvage or troubleshoot an old one. If you are so inclined, however, then there are some useful tests you can make once you have removed the unit from the car.
First, and using an accurate multimeter, measure the resistance between terminals 30 and 87b and between terminals 30 and 87. In both cases the reading should be open circuit (no connection, in other words).
Next connect a 12-volt supply to terminals 85b and 87 (it’s a good idea to make up some trailing leads with female spade terminals to ensure you don’t short out the contacts). This should activate the relay and close the connection between terminals 30 and 87b. Check that there is no resistance between them (or certainly no more than 0.2 ohm).
Now connect your power supply to terminals 85 (negative) and 86 (positive) and check that there is no resistance (or again no more than 0.2 ohms) between terminals 30 and 87.
If you see a higher resistance between the terminals when you make these two powered tests then you can assume that the relay is faulty. The problem will either be poor contacts or a failing soldered joint.
In the first case there’s little you can do; cleaning the contacts is only a short-term fix. If it’s a poor joint, though, you could try soldering it. Once you’ve removed the top of the relay any dodgy soldering will be immediately obvious. Look for a gap between the blob of solder and the wire that may be poking through it, and for flux (a brown, waxy substance) around the joint. If you’re handy with a soldering iron you should be able to make the joint sound again.
But I can’t stress enough that if your relay is faulty you’re far better replacing it than repairing it. Still, you could keep the repaired one as a spare.
Does your Porsche have a DME relay?
And where is it?
Shown below are all the standard, road-going Porsches fitted with a DME relay, together with the relay’s location.
Note that the relay in 944s, 968s, 964s and 993s is essentially the same part (although it was updated for the 993) and has six flat terminals (three narrow, three wide). The Carrera 3.2 unit has six round pins, all of the same diameter, and one plastic locating pin.
Note, too, that the 928 – in all its forms – has Bosch Jetronic fuel injection, not Motronic. And although these cars certainly have fuel-pump relays, they’re rated at a hefty 30 amps and contain no soldered joints. As a result failures are rare.
Carrera 3.2 1983–89
Beneath the driver’s seat, next to the ECU
964-model 911 Carrera 2/4 (but not Turbo), 1989–93; 993-model 911 Carrera 2/4 and Turbo, 1993–96
Inside the main fuse box at the right-hand rear corner of the luggage compartment. The 964 Turbo (but not the 993) has a conventional fuel-pump relay located in the fuse box
944, 1982–85; 924S, 1985–88
The DME relay (or fuel-pump relay as it was then known) is tucked right up under the passenger’s side of the fascia, and partly obscured by an additional fuse board
944, 944S, 944S2 and Turbo, 1985–91; 968, 1991–95
Inside the main fuse box at the left-hand rear corner of the engine compartment
Name and number
The DME relay in 944s, 924Ss, 968s and 964s originally bore the part number 944.615.227.00. This was updated for the 993, and carried part number 993.615.227.00. The newer part is a direct replacement for the 944 unit, and although it looks much the same it’s claimed by Porsche to be more reliable. If today you buy a new replacement DME relay for a 944, 924S, 968 or 964 it should have the 993 part number. The Carrera 3.2 round-pin relay carries part number 911.618.154.00.
Copyright © 2018, Philip Raby Limited.