Replaced DME Relay - Wow!

[DANNOV]

For the past few years I have carried a new DME relay as a spare in the emergency parts kit for my 1985 Carrera. Today, I decided to put the new DME relay in and carry the old one (the 16 year old original) as a spare. It was a very easy job to make the changeover. I always thought that the DME relay works or it doesn't, and as long as it works there should be no difference between an old relay and a new one. Was I wrong. This car is so much smoother and responsive. I noticed it immediately. My car seemed a bit sluggish over the past few months and I thought it might be a clogged injector; but, an urgent repair wasn't necessary so I planned to wait until the next regular servicing. The new DME quickly cured that problem. If any of you are carrying a new DME as a spare, I suggest do the changeover. You might be pleasantly surprised.

1985 911 Targa

[ebradway]

Ummmm... Dannov, doesn't the DME relay simply control power to the DME? And the DME manages digitial inputs and controls the EFI?

If the relay were filtering signals from the sensors or the signals to the EFI, I would see how cleaner signals would make a difference. But the relay just controls power to the DME. Maybe the old relaywasn't maintaining power to the DME, but then I would think the car would be stalling reularly. Another possibility (and somewhat likely) is the the DME gets it's ground from the relay. The DME reads signals as a difference in voltage from ground. If the old DME was giving a fluctuating ground then the signals from the sensors would be misinterpretted and you would get poor performance from the EFI.

This sounds like the recent postings about adjusting the cruise control cable to make the throttle more responsive... When I was my windshield the car seems faster too... (just kidding).

[Early_S_Man]

Eric.

Dirty POWER connection or source = unreliable computer!!!

DME = COMPUTER

Do you get the point?

[R22tech]

Hey Eric- just for the record: I was the one who posted about adjusting that cruise cable for throttle response and I still maintain, along with others who've done this, that it does make the throttle respond quicker. I'm sure it wouldn't work on a car with a properly adjusted throttle cable-but for those of us with "slacky" cables and no lift to get under there, it is a quick fix! And also: MY CAR DEFINETELY RUNS BETTER AND FASTER WHEN ITS DETAILED!!! THIS IS A LONG KNOWN FACT. HELL, IF I PUT RAIN-X ON THE WINDSHIELD I'M UNBEATABLE!!!! In fact, when I see you on Sunday, I'll let you try out my secret rubber conditioner and we'll see how improved your performance becomes.

[DANNOV]

I think Early-S-Man has it right. Perhaps I just should have cleaned the pins and the ground connection and it would have been as good as new. But, just to make sure I wasn't fooling myself - after all, perceptions of improved performance can be quite subjective - I put the old relay back in, as is, to see just what would happen. After about one hour of running, I have concluded that the car definitely does not run as well with the old relay, as with the new one. Could be something about 16 year old relays. A relay is a simple device but my experience with electrical relays in other electronic gadgets has not been good, these things can be flakey and when connected to a computer who knows what happens. In looking at the 911 electrical diagram the relay connects not just the DME but also the fuel pump and the oxygen sensor. It is all beyond me I just feel that the relay made a noticeable difference, not a mega-HP gain but nevertheless positive and noticeable.

[ebradway]

I'm looking in the Bentley book now to see just what the relay controls...

Ah hah! The DME Relay actually provides power to the fuel pump, as controlled by the DME. When the relay fails, the DME still has power but the fuel pump does not. It is possible that the relay was creating a load on the circuit, not allowing enough power to the fuel pump, resulting in symptoms similar to a weak or failing fuel pump.

BTW, dirty power to a computer does not create a lower performance situation - it would either function normal or fail altogether. That's the nature of computers.

And R22tech - I know it was you with the cruise control cable adjustment - I was just ragging on you because you won't let me crash on your couch!

[Jim T]

Yeah, I just cleaned the plug on my old Pentium, wow, that processor really flies now!

A new relay ain't gonna make a car that is functioning normally faster or more responsive. If your DME is not getting enough power or intermittent power, it not going to cause minor acceleration losses, but some serious intermittent running.

Its a relay, and its the digital age, baby!

Your phenomenon is the same experience by those who put in a K&N and "notice" how much "faster" their car suddenly is.

[Early_S_Man]

It is quite obvious that none of you wise-ass individuals citing platitudes about computers and what noise or EMI WILL or WON"T DO ... have NEVER worked in the real world troubleshooting NOISE, GROUNDING, or RFI/EMI problems in computer rooms, telephone digital switching offices (like at the D/FW Airport ...) with $100K+ worth of test equipment at hand!!! If you HAD, you wouldn't be talking in ABSOLUTES!

[RoninLB]

I respect the honesty without the political BS. Tell it like you see it.

[DANNOV]

I don't know why the car runs better with the new relay...it didn't seem logical to me...I only report what I experience. I have since removed the old relay - after switch test - and the car is no doubt smoother and more responsive with the new one. There is no way I will put the old one in again, I don't even want it for emergency purposes. I suspect it is in the early stages of decline. Perhaps I should send my old relay to the biggest of you doubters and you can screw up your car.

[Kurt B]

What Dannov noticed is real.

Your computer also runs in cycles, sometimes running faster, better and more reliably than others. However, since it isn't controlling a combustion process, dependent on air density, heat, etc, and lots of other factors, which is it both responding to and affecting (not affecting the weather, but affecting what the car wil do the next instant) you will not notice slight changes in your Pentium system as you will on the macro engine system which is dependent on the computer system.


In fact, your computer is also much newer than the car's computer system. I'm sure if you monitored the small systems of a 'puter as old as our cars, you'd notice a difference--but the monitoring is the problem since these will be very small variations on a system that only computes and displays but does not affect a macro system, like an engine.


The main reason it makes a difference the age of his car and of ours. Everything drops a bit in efficiency over time. Minute degradation of solders, metal fatigue, degradation of wiring occur over this time. Each one may have a fraction of a percent change in efficiency and reliability, but over time, it adds up.

The degrading DME may affect other systems that have degraded. Not by cutting the systems off, but by affecting how quickly they respond to data received, and also dependent on how quickly other systems respond back to the DME.

Replacing one part may stem the symptoms for a time...especially if the part is brand new.

That's my experience so far. Small things Have changed the car's performance. All of my injectors were working. Every one, but I replaced them with new ones. It made a difference in the idle--steadied it out almost completely, but not quite.

None of the old ones had failed or was hampering the system (since I changed them one at a time to see if one was bad as I thought).

[brayne]

My .02:
My stock 86 cab developed a stalling problem over the last two weeks. It began dying under various conditions such as coasting to a stop, accelerating up an onramp, etc. It continued to start when cold without any problems but it would not start right away after stalling. Initial checking shows I have a 1 wire head temp sensor and 1/16" end float in the distributor. I plan to replace the sensor with the 2 wire version but I'm not sure about the distributor.

However, based on the advice I got on this board, I started by replacing the DME relay. Wow, what a difference! The engine idles and revs smoothly. The stalling problem completely disappeared. I still plan to fix the temp sensor but it will be for preventive maintenance now instead of troubleshooting. At any rate, based on this experience I would agree with Dannov that the new DME relay caused my engine to run noticeably better.

[carrera]

i just washed my car
wow! what a difference, it runs much better
i wonder ,if i wash it a second time, will it be even faster?

[Bobboloo]

All electronic equipment has to have a stable power source to operate correctly. The relay is a switch through which current flows. As switches get old corrosion develops on the contact surfaces. This corrosion affects the flow of electrons.(i.e. power source for the computer.)
Nine times out of ten electrical problems are a result of switches or connectors.
Just wanted to restate Warrens point.

Bobby

[Wayne 962]

Gosh, these relays must be genuine pieces of crap to fail all the time...

-Wayne

[bjola]

Yes Wayne these relays can be a real pain in the ...... From my 944 time (it is sold now) I have had a couple of problems with these relays but those have been either intermittent or totally failed.
I seem odd that replacing a DME relay that is still working will make your car run better considering what the relay is doing but you never know.
Anyway, I enjoyed the "quarrel" this subject started.
greetings from Sweden, Bjorn

[jlex]

What Porsche (or a smart after-market company) needs to do is to develop a DME relay with gold plated contacts. Give it a lifetime warranty. I think they'd make some money here.
regards,
jlex.

[Rot 911]

Just out of curiosity last night I pulled my DME relay. It turned out to be the original and was manufactured on 10-1-85! I went ahead and swapped it with my spare new relay and will now use it as my spare after 16 years of faithful service!

[DANNOV]

Originally posted by carrera
i just washed my car
wow! what a difference, it runs much better
i wonder ,if i wash it a second time, will it be even faster?

OK you won...you are the biggest disbeliever....I will send you my 16 year old DME relay....hope your car runs better with this ancient piece of electrical junk!

[MBAtarga]

Something tells me there is going to be a shortage of DME relays real soon...