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DME Relay Troubleshooting
 
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Pelican Technical Article:

DME Relay Troubleshooting

Steve Vernon

Time:

1 hour1 hr

Tab:

$14 to $20

Talent:

**

Tools:

Multi-meter, wire, male spade connectors, 12-volt battery for testing

Applicable Models:

Porsche 944 Turbo (1986-89)

Parts Required:

New DME relay

Hot Tip:

Wipe down the inside of the airbox

Performance Gain:

Running motor

Complementary Modification:

Check all fuses

If your Porsche 944 will not start or suddenly dies on you, one of the first places you should check is the DME relay. On the older 944s, the relay is known as the fuel pump relay and is located on the fuse panel under the dash on the driver side. On later 944s and 951s, the relay is located in the fuse /relay panel in the rear left side of the engine compartment close to the windshield and A-pillar. These relays are prone to failure, a lot of failures. They usually fail from broken joints in the circuits, but they can also fail from the diode and or have issues with heat sink.

This article will show you how to check your relay for proper function and show two emergency stop gaps to get you home, but these stop gaps should only be used in the most extreme of emergency situations. If you are going to drive a 944 or 951, invest in a couple of extra DME relays and always carry a spare. There is even room in the fuse box for the spare. With a proper functioning relay, buy a spare and swap it out with the good relay. If the car starts and runs with no problems, then you know the new relay is good. Switch them back and place the spare in the fuse box. While all of these tests will help track down a problem, it is still easier just to install a known good relay.

The relay on the later 994s actually has two relays within it, one that powers the DME and KLR on the 951 and a separate relay that powers the fuel pump and heater for the O2 sensor.

Our project car would crank over but not start and while we knew that it had enough power to turn the starter, it is always a good idea when checking any electrical problems to make sure you have a fully charged battery.
Figure 1

Our project car would crank over but not start and while we knew that it had enough power to turn the starter, it is always a good idea when checking any electrical problems to make sure you have a fully charged battery. Place your probes for the multi meter on the positive post (red arrow) and negative post (yellow arrow) of you battery. You should have between 12.2 and 12.7 volts. This battery is a little on the low side but acceptable.

Next, release the two clips on the fuse/relay panel cover (red arrows) and remove it from the vehicle.
Figure 2

Next, release the two clips on the fuse/relay panel cover (red arrows) and remove it from the vehicle.

This is not an uncommon site for older Porsche fuse boxes.
Figure 3

This is not an uncommon site for older Porsche fuse boxes. There are two spare DME relays (red arrow) in the storage area of the panel.

Check with the fuse and relay list and diagram located under the lid and remove the DME relay (red arrow).
Figure 4

Check with the fuse and relay list and diagram located under the lid and remove the DME relay (red arrow). Gently wiggle and pull the relay out from its location in the panel. These DME relays fit snuggly. If you just grab the relay and pull on it, you may just end up pulling the cover off the relay.

On the side of the relay will be a wiring diagram showing the function of the relay; the terminals correspond with the numbers located on the bottom of the relay.
Figure 5

On the side of the relay will be a wiring diagram showing the function of the relay; the terminals correspond with the numbers located on the bottom of the relay. The schematics are as follows: 87b (4)- power to fuel pump, 85b(9)-ground from DME to pick up fuel pump, 87(8)-power to DME and fuel injectors, 85(1)- ground via DME, 86(3)-ignition switch and 30(2) is battery or power. The terminals are numbered on the bottom.

Place your multi-meter in the position to test ohms or resistance.
Figure 6

Place your multi-meter in the position to test ohms or resistance. You are going to be testing the resistance across terminals to determine if the relay is functioning correctly. Begin by placing the probes on terminal 30 (green arrow) and 87b (yellow arrow). The resistance should be infinite or a reading of 1 or OL depending on your meter. If your reading is different, the relay contact points are not open, which means that the relay is no good.

Next, place your probe on terminal 30 (green arrow) and terminal 87 (yellow arrow).
Figure 7

Next, place your probe on terminal 30 (green arrow) and terminal 87 (yellow arrow). The resistance should be infinite or a reading of 1 or OL depending on your meter. If your reading is different, the relay contact points are not open, which means that the relay is no good.

If the readings are correct so far, you are going to need to apply a power source to the relay.
Figure 8

If the readings are correct so far, you are going to need to apply a power source to the relay. The car battery will do. Connect the power source across terminals 85b and 87 (red arrows) and take a resistance reading across terminal 30 (yellow arrow) and 87b (red arrow). The reading should be almost zero or between 0.1 and 0.3 ohms

Connect the positive side of the power supply to terminal 86 (blue arrow) first and then the negative side to terminal 85 (green arrow).
Figure 9

Connect the positive side of the power supply to terminal 86 (blue arrow) first and then the negative side to terminal 85 (green arrow). Because of the diode in the circuit, if you install them the other way, the electricity will not flow and the relay will not pick up. Place the probes on terminal 30 (yellow arrow) and terminal 87 (red arrow). The resistance should be close to zero again. If any of these tests fail, the relay is bad and should be replaced. If the relay is good, then your problem lies somewhere else.

You can bypass the relay for testing purposes to eliminate the relay completely from your troubleshooting list.
Figure 10

You can bypass the relay for testing purposes to eliminate the relay completely from your troubleshooting list. Use three pieces of 14-gauge wire along with three male spade connectors. Note: this should only be used to test or in an absolute emergency situation; do NOT regularly drive your car with this in place. Connect the spades to the end of each wire and then the other ends all-together. Make sure the area where they join is safely attached and protected. Insert one spade connector into terminal 87, one other into terminal 87b and then lastly the spade into terminal 30 (red arrow). You should hear the fuel pump running and be able to start the vehicle with the ignition key. Do NOT leave these in place, as the fuel pump will continue to run and the DME will continue to draw power. This is unsafe and will wear down your battery. If the car will not start with these jumped, your problem lies elsewhere.

In an absolute roadside emergency, you can wrap a paper clip around terminals 87, 87b and 30 and reinsert the relay long enough to get the car to a safe location.
Figure 11

In an absolute roadside emergency, you can wrap a paper clip around terminals 87, 87b and 30 and reinsert the relay long enough to get the car to a safe location. This is NOT a substitute for a proper relay. If you have a paper clip on you, then you should have had an extra relay or two in the fuse box, which you should use instead of this MacGyver approach.

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Comments and Suggestions:
Allan Comments: Hi Nick 1990 944 S2
All the gauges in the cluster shut down and don't come back even if the car dose start again. It takes a while for them to come back with random restarting and waiting.Is it possible that the faulty DME is affecting this Ive ordered a new one? Or is it a completely different problem.all the fuses are good.
October 20, 2016
  Followup from the Pelican Staff: Are they powered on but not working? You can start by checking power and ground to the cluster when the problem is present. If good, move to testing the signals to the gauges. - Nick at Pelican Parts  
Allan Comments: I have a 1990 944 s2 and am shore i have a DME problem but with the sudden shut down of the car when it worms up i also get complete lose of all dash gauges even if the car dose restart after it cools down.
October 18, 2016
  Followup from the Pelican Staff: What are you missing when trying to restart it? - Nick at Pelican Parts  
Aaron from Barrie Comments: My 85.5 944 was running perfect. Backed into the driveway and next day turns over but no ignition. Never had a no start from this car. I've been through all the obvious things listed, but no spark still. Does this system use a resistor like the the old days? Or is the transister in the DME what I should be testing? If so, how? I know how to get access, just not sure what I'm measuring.
June 22, 2016
  Followup from the Pelican Staff: Black to the coil is power from ignition switch. Green and black is the trigger from the DME. look for a primary signal on the wire from DME. - Nick at Pelican Parts  
riverhead Comments: HI
Is the relay the same to a pre 85.5 car?
Mines a 1985 wont start, older relay base under drivers side interior.
someone has been cutting the wires from the back of the DME relay seat. The relay is missing.
Is there a way to re-insert the wires into the back of the relay base?
Car sittign up about six years. Two outside. Turning over.
No fuel pump sound and no spark.

Any help appreciated
October 2, 2015
  Followup from the Pelican Staff: You can replace the relay connector, or repair it with new wires. Give our parts specialists a call at 1-888-280-7799. They can figure out what part or repair kit you need. - Nick at Pelican Parts  
MAK54 Comments: Thanks for the info. Power to the sensors come from the DME and then goes back to the DME correct. It has fuel pressure, no spark. The sensors were new last year and fuel pump this spring. I do have a feeling it may be the sensors, I was hoping for a way to test them. Thanks MAK54
May 18, 2015
  Followup from the Pelican Staff: The DME is only the receiver of the analog signal, those sensors should produce an AC voltage when cranking the engine, however I have not had much success testing them by voltage. They sometimes test OK, but when I swap them the car starts. If you find a spec for the AC voltage when comparing the two please let us know! The adjustment can be off from sensor to sensor so I suspect that is why testing for a/c voltage is not a perfect test. You may however test the resistance between the first two terminals on the sensor connector and the resistance should be between 800-1600 ohms, then between the first and third terminals should be no continuity. I have found however that these tests are not absolute. - Casey at Pelican Parts  
MAK54 Comments: I did check the fuses also, all ok.
May 16, 2015
  Followup from the Pelican Staff: See reply to other question... - Casey at Pelican Parts  
MAK54 Comments: I did the McGyver fix and still no start. I have battery voltage to both sides of the coil but no pulse. I disconnected the speed and timing sensors at the bell housing which are new two years ago, and haven't any voltage to them. I fear a DMC, but want to make sure. The car is a 1983 944. Any other ideas before I spend a bunch of money.
May 16, 2015
  Followup from the Pelican Staff: Ok, so you have no spark, now an important piece of the diagnostic equation is do you have injector pulse, and do you have fuel pressure? You will need a noid light set, to plug in to one injector. http://www.pelicanparts.com/catalog/ShopCart/TOOL/POR_TOOL_CAT447_pg28.htm

If you have no fuel injector pulse, its likely the speed/ref sensors, or a dirty ground connection on top of the bell housing. Less likely causes are DME and DME relay if there is no fuel injector pulse.
If there is fuel injector pulse I'd be thinking fuel pump or pressure regulator.
The speed and reference sensors are analog style sensors, and they produce AC voltage, you will not see voltage coming from the DME to that style of a sensor.
Let me know what you find.
Here is a recent tech article about 951 Speed and reference sensors. - Casey at Pelican Parts
 

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