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Hello
The relays have allready silver coatet contact pins witch is superior gold in some aspects. The DME isnīt working like a computer on pure digital signals. The inputs are still analog and needs toi be filteret and convertet to digital signals. the digital output needs to be trsnfered to analog signals again. If you have a dirty power source your signilas are dirty as well needs more filtering and more calucaltion time while the digital output also gets some dirt when convertet again. The DME Relays have a diode and a small condensor to filter but they are not bullet proof ( thats why they can be changed simple ). Not only Porsche have the problem, those problems are very comon on electric devices runing in such extreem conditions. So mostly replaceable ( cheap other companys integrate it and you have to dump the complete brain ) units are developed for a typical lifecycle from a car, and thats typical some 6-7 years while more engenieering is spend into the real importent devices to make them as longlasting as possible end also forgivable for overgarging or people welding on the cars, and,,,, you know al those things happen under circumstances. So maybe the DME relay can be replaced by a new SMD based total integratet device and will not fail again as it is a moe sophisticatet design as ( maybe? ) used in modern cars and Porsche made allreday one upgrade to the 993 device. The point is if there is a merked willing to pay for gold contacts ( Well the Stereo boom boom marked does as those guys are consumer beleiving into comercial ) If you look around at other car brands you will find they all have the same problems yet not everyone will spend money into fixing a 1985 domestic or japanese car as this will exceed there value and the dealers like to sale new or newer used cars. Grüsse |
Roland,
Have you ever tried replacing the DME relay with two heavier-duty relays, such as a Bosch 0.332.002.182 -- rated at 40 Amps, and a 3 Amp, 1000 Volt PIV rectifier? |
Roland,
I have to wonder if those silver plated pins are a factor in the problem? Those DME relays don't seem to be sealed as well as the 'round' relays and the 'through-the-body' wiring harness connections! Has anyone closely inspected the socket and relay pins to see if there was any sign of the old 'silverplate' tarnish on the surfaces after a failure? Of course, just removing and replacing the relay tends to break through any tarnished surface corrosion, but I wonder if a thorough cleaning of the socket and relay pins , followed by a coating of Dow Corning 4, 11, or 112 Silicone Grease wouldn't help prevent further problems. I hope those relay switching contacts are NOT silver-plated as well! Silver was found to be an extremely poor choice for relay contacts in the 1960s by GTE Autmatic Electric Laboratories. There was a very bad migration problem, and microscopic 'dendritic' crystaline 'hairs' formed on the contact surfaces, and those would certainly accentuate any noise problem in the DME system! <b>Anybody have a known bad DME relay they want to send me for dissection and analysis?</b> Please email me, if you do! |
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and if the "intermittent running" happens in say .001 second every .050 second would that be perceived as "serious" or subtle ? I suppose if it had to restart its' operating system. . . :rolleyes: edit; Still not used to page two at 20 post. . .Roland with the insight again; on page two. |
Hello
Have you ever tried replacing the DME relay with two heavier-duty relays, such as a Bosch 0.332.002.182 -- rated at 40 Amps, and a 3 Amp, 1000 Volt PIV rectifier? In my young days we convertet one car to the Bosch/ Mercedes Benz setup, but those relais are 150$ each. Those have Filters and a overcharge fuse. The main reason was that the car was built on a 77 Chassis and we had to made the DME Unit from scratch and the car was used to drive some rallye events so fail proof and easy maintaince on the fly was a major task. All relais where on the dasboard for easy acces. Wireloom in aircraft quality duel fuel suppley runing redunant. In germany certain Rallye classes have to run with catalyc converters ( and if they donīt work you get disqulified ) I have to wonder if those silver plated pins are a factor in the problem? Those DME relays don't seem to be sealed as well as the 'round' relays and the 'through-the-body' wiring harness connections! Those silver coatet contacts where standard on most Mercedes and Porsche cars ( Just look close on the "simple" 356 bulb connector and check the pricelist, they are now 4$ each in germany while a simple 0,02 unit from british sportscar fits too ). The background is that silver contacts have the best long term stability and will not corrode like others yet it isnīt the best aviable, but Porsche didnīt built the cars for eternity. The two relais used inside the DME power suppley units are standard Siemens ( print ) relais like used on the other Porsche units. Good enough to run 30 amp load at 100% duty and the contacts are equiped with a diode to kill sparking. The device also has a condensator to cut peak voltage and noise geting into the Cumputerside. But it isnīt splitet over a transformator or other similar working unit. Also all devices use a similar ground including the alternator. The DME Brain has also a noise filter inside but enough wires to have some restnoise geting in there. The DME Wireloom is shielded to avoid EMV but mainly on the signal wires. The injector wireloom and some other open ends ( throttle body, idle valve, air meter device ) are still listenening with open ears and can catch EMV from a bad ignition. Only the 911 DME relais have the round connectors in 6pin dice form. The DME brain and the Relais are raised obouve the floor siting nearly 1" abouve "waterline". It also sits under the passenger seats nearly in the middle between the axles witch is the spot with the lowest frequency and amptilude on vibration while driving. ( Well Porsches are smart engenieered ) I use "Kontaktchemie" ( now CRC ) 60 for hard oxidation and clean all with Isoprophylalcahol yet on silver or other contacts I use "Kontaktchemie" Tuner 600 as this needs no washing after use. To prevent oxidation I use "Kontaktchemie" Gold 2000. I also have some EMV 35 on hand ;) @island Well just wantet to "help! Warren a bit and maybe get some usefull results from the thread. Right now this board has to much traffic and i canīt follow up all the stuff. Grüsse |
Where can I buy this stuff to prevent oxydation[sic]
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Thanks Roland!
The info about the filter was exactly what I was looking for. According to my wiring diagram in the Bentley manual, the input sensors aren't running through the relay - but it sounds like some of the filtering is happening there. This would make sense. BTW, can you find part numbers to convert to the 993 relay? That would seem to be the ultimate solution. -Eric |
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