After Replacing DME Relay Troubles...
 

I just replaced my DME relay(with a solid state)after the 30 year one failed, and now my 86 Carerra feels like it is running on a few cylinders?

The car starts right up stays on, but shakes. No smell of strong fuel. Checked fuses and all appear intact.

**Just to clarify,

I was driving to work earlier last week when all of a sudden the engine died and had to pull off the side of the road. After arriving home, I dug for the URO
replacement relay which I had purchased a few years earlier.

Arriving at the scene where the car was at, I installed the relay and the car sounded like it was running on a few cylinder. At any rate, I drove it like that home
which was only a few miles.

I then placed an order for the SS relay and come to find out the car is acting the same with this relay.

Was just about to buy a solid state DME, will be watching this.

My car as acting funny 2 years ago (i assumed it was the URO parts relay).

I got a wittrin (solid state was out of stock at the time)...first one was dead on arrival...2nd one worked for 9 months and occasionally wouldn't start.

Got the solid state one in August and it's been going well so far.

Throwing this out there.. maybe it wasn't the DME relay that caused it to die?

Or something else happened during the stall?

I posted a pic of the relay on a different thread, but here it is!http://forums.pelicanparts.com/uploa...1583213534.jpg

What do the pins of the burned out relay look like -- overheated/dirty? Did you clean & inspect the mating connector sockets (say, use deoxit)? Also, you can find threads on PP on how to make temporary jumpers to test start and test run the motor -- to see if you got really unlucky with a DOA replacement rly. Guessing (but no information on this) some reportedly DOA relays may just be misdiagnosis of DOA when all that happens is multiple insertions in the sockets eventually clears contamination.

So you are saying that the new solid state relay makes the car run like it's on "a few cylinders", same as the one which burned out shown above (and in the other thread)? FWIW, I have been using the solid state relay (with pump prime) for at least 4 years, and my cars runs perfectly on it. As mentioned by High Life, perhaps there is something else going on. You should try the test Grant mentions above to be sure (jumper).

I don't think a DME relay has an effect on how the car runs so much as does it run or no run. Passes a go signal to fuel and ignition. Other issues it sounds like and would look first at the ignition. Bob

I have used the solid state relay with fuel prime for many years, and zero issues. It is basically your fuel pump relay. When a regular DME relay starts to fail you can get intermittent running.

I suspect that with the burned circuits on your old relay it means something is wrong with the DME itself. Find a friend with a similar model car, and put your DME in his car. If the problem follows, that will tell you it is your DME.

Don't put a good DME in your car because if you have a bad power or ground it can hurt his good DME.

I really doubt you have any issue with the solid state relay. It is likely something else.

I opened up the DME last night and unfolded the circuit-board to look at any suspicious burn marks, but noticed nothing that stood out.

If anyone within my area is willing to help out with the DME swap for testing, I would greatly appreciate it.

Thanks to all that chimed in...

Have you checked the Cylinder Head Temp sensor?

I replaced it about 6 months ago. The car had been running fine until this happened.

I don't think I have ever seen a circuit burn like that. Some thing is shorting. I would maybe start with the coil, given their failure rate anyway. posible the DME, or fuel pump. clean and check connections at fuse board and engine panel wires. Distributor caps and rotors go bad could be the running issue but wouldn't take out the relay. Good luck.

For troubleshooting the DME relay just use a jumper. It takes any variability out of testing.

https://forums.pelicanparts.com/porsche-911-technical-forum/665035-dme-relay-carry-spare-2.html#post6619164

Basically suggesting you chase down potential collateral damage/failure from the original failure; work your way outward from the known failure. I'm thinking you now have an intermittent connection which opens only when the firing engine provides just enough vibration to move poor connections. Only going after the rly socket 1st because the original relay fried (probably a bad/fractured internal solder joint) and the connector is the next thing in line to be damaged by excessive heat before the hot relay failed open. Suggest you want to eliminate a fouled connector. Also, check connector sockets front and back to make sure sockets are still seated properly and leads connected after your removal of the original relay. Jumper diagnostics may help before you search for (and even potentially introduce) unrelated problems.

Maybe something as simple as another component that overloaded the first DME relay (killing it in the process) and is still faulty. The fuel pump drawing too much current comes to mind:

FYI, a burned trace is not a normal failure mode for these DME relays. They have gotten a bad reputation due to cold solder joints. In your case the burned trace is more indicative of excessive current draw that heated up the trace and eventually causes it to fail as can be seen in your image.

If the fuel pump is marginal because one of its bearing is on its way out maybe it draws excessive amounts of current that are still under the rating of the fuel pump fuse but enough to cause this type of damage. The next actions:

- test fuel pump current draw
- check static fuel pressure and fuel delivery rate (if that is off it explains your poor running)

Good luck,
Ingo

------------

Would the (cracked) solder joint moving side to side be able to cause that kind of heat?

Whenever I’ve seen burned traces they were caused by overload rather than cracked joints. So it’s highly unlikely.

The trace is slimmest in the center between the joints and that’s where the heat is the greatest. A cracked joint either welds shut due to heat or opens up completely to the point where it doesn’t conduct enough current to heat the gap.

In either case there it wouldn’t produce sufficient heat to burn the entire trace like in your image. This was prolonged exposure to over current.

Cheers,
Ingo

I fully opened up the DME to take a look at the other side, which I hadn't. I found these two pins that appear to be missing the solder on the tip. I used my phone to do a 360 around them and it does appear that they are missing solder and not a lighting illusion.

What do ya think?http://forums.pelicanparts.com/uploa...1583381960.jpg
http://forums.pelicanparts.com/uploa...1583382002.jpg

In the top pic, the pin in question is in the middle and the bottom pic is to the left mid way.

Looks fine to me.

Hmm..

Other than that, I don't see any other anomalies within the boards.

Not all faults are visible when it comes to electronics and there is no definitive reason to believe the fault lies with the DME. If you need certainty the DME needs to be diagnosed properly and undergo functional testing.

As I said earlier your situation points towards the fuel pump as a likely candidate drawing too much current. It’s far less likely that two items went bad at the same time (DME relay and DME). Your observations say that swapping the damaged DME relay hasn’t rectified the poor ruining. That makes me think it’s a secondary failure and you still haven’t found the primary point of failure.

I would have to agree, and he knows lots more about electronics than I do. Something out of the normal range caused the relay to fry. Likely the fuel pump drawing too much power or a short or other issue. Find and repair that, then test.

Well,

After waiting a week for a replacement fuel pump and then spending 3 hours installing it, the issue remains. Before starting up the engine, I checked to make sure that the cylinders were not flooded with fuel by using a stethoscope. Also cleaned up the spark-plugs and did notice that the drivers-side bank ones were a bit wet, in comparison to the passenger side, which leads me to believe that that side is NOT getting spark.

Check the injector harness plug on the shock crossmember for corrosion or poor connection. Reach in behind the heat blower motor to find it. Also check grounds on #1 intake. Carrera distributor caps are kind of funky with that black cover that tends to hide defects in the cap, like cracks and coil wire terminal burn throughs.

Thank you for the response John,

I will take a peek later on tonight and report my findings.

Let's try to isolate the cylinders that aren't "working".

Does it idle rough as well? If so, you can "short out" (disconnect) each cylinder in turn and determine which one(s) aren't contributing. If okay, the engine idle will drop and run rough. If not, there won't be any difference.

The old way of doing this was to disconnect the spark lead either from/to the distributor/spark plug. Do this with an insulated plier.

On fuel injected engines, disconnect the harness connector from each injector in turn and see if that cylinder is contributing, as above.

Could be a faulty spark plug, spark plug wire. Also, check the distributor rotor and the underside of the distributor cap for structural integrity. Rule out the more obvious, simpler stuff first before diving into more involved troubleshooting.

Sherwood

Update:

Took a stab at all of John's recommendations and all appeared in good standing. Started the car and no change, as it still shakes and feels like it's running on a few cylinders.

I purchased a spark tester and had my wife help out with starting the car. All spark-plug cables were producing a good spark on all six cylinders.

Sherwood,

I disconnected each fuel injector connection and didn't really notice any real change, due to the car running rough. I took a video of the engine running and will upload it.

I will check the fuel pressure and report my findings.

with the engine running spray small bursts of brake cleaner into the air intake. If it picks up and runs smothley it is a fuel problem if not it could be electrical.

Thanks, I'll try that.

Update:

After doing more digging, I again started the engine and this time I took off the the injector harness plugs from cylinder 4,5,6 and the only one that did not make a difference when unplugging was 4.

This time I took the time and checked the Ohms across all harness injector plugs and injectors, and this is what I found:

Harness plugs:

1. 11.56 ohms
2. 11.56 ohms
3. 11.57 ohms
4. 11.57 ohms
5 11.57 ohms
6. 11.57 ohms

Injectors:

1. 2.6 ohms
2. 2.5 ohms
3. 2.6 ohms
4. 61.1 ohms (that’s correct)
5. 2.6 ohms
6. 2.6 ohms

I guess a replacement injector is in order.

Serge

Good find and the replacement injector will certainly make a difference.

Interesting how the engine always had a slight shake when it was running fine, and nothing I adjusted ever fixed it. I have a feeling this bad injector was always the culprit.

Ok folks,

To close this thread and report a resolution, the culprit was indeed #4 injector. I purchased a remanufactured one from GB Remanufacturing, checked the Ohms (2.5), installed and VROOMMM!

I took it for a quick run and it all felt just dandy!!