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#21
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Normal running voltage should never hurt the relays. Vibration is the typ killer. Yours being discolored does indeed indicate it was overheated. But why one and not the other? It probably wasn't due to battery voltage. Its from the current load and resistive contacts. A dirty contact or circuit overload will send the temps skyrocketing. We have controls that come back from the field where the large 30A+ terminals have literally desoldered themselves if they weren't tightened properly. A couple watts in the coil is no match for the I^2*R heating of the load contacts. FWIW, If you add more diodes you may not be able to pull in the relay during cranking on a low battery. IMO, DME relays aren't severely under designed. Most make it 100k+++ miles and several decades. They are mechanical parts. Mechanical parts fail from time to time. Quote:
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Stephen www.salazar-racing.com 1970 914/6 - 3.0L GT 1983 911SC - 3.32L IROC 1984 930 2008 S2R1000, dirt bikes (some gas, some electric), Sherco trials bike Sold: 2001 Boxster (hers), 2003 996tt x50 , SpecE30, 1996 E36M3 GTS2 racecar, 2015 Mustang GT |
#22
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DME Relays...Probable Cause of Failures
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It appears there is a design fault in all these DME relays regardless of who makes them. That is why they often fail and why people suggest always having a spare in the glove box. I will not install any DME relay in my 911, Chinese, O.E.M. or otherwise unless I am completely satisfied with my new modification to eliminate the design fault. |
#23
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I agree that the high temps inside the DME relay are not enough to melt solder. However, according to one Relay manufacturer, anything over 120C can be detrimental to the insulation used on the coil wires with insulation failure and wire shorting a possibility. From what I can see with my tests, I would not be
surprised if the interior of the relay housing reached 120C and above. In my own case, the contacts of the failed relay looked rather clean. No signs of burns or heavy pitting. The coil is open and the coil wires certainly look discolored from heat. I'll try the extra diode and the cooling holes in the dust cover and see how the combination works out. |
#24
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Why would the interior of the relay reach 120C?
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Chris M 1985 911 Carrera with a couple cosmetic only mods 2006 E90 330i 1999 E46 328i |
#25
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now thats a stretch..
Great write up! |
#26
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DME Relay Explained
Further tests running the DME Relay with the plastic dust cover on for 10 minutes at 13.8VDC shows very high relay coil temps....as high as 120 degrees C.
The two internal relay coils dissipate around 4.5 watts of heat total. That heat is trapped in the closed dust cover. It just keeps getting hotter & hotter until it reaches thermal equilibrium. Take my word for it, with the dust cover off, touch the relay coil or the metal relay frame and you'll quickly burn your skin! The very high operating temps along with vibration are probably a major cause of fractures in the solder joints on the printed circuit boards. The high temps are probably also responsible for some degradation of the coil wire insulation. This will result in shorted wiring in the coils, even higher current draw and eventual failure. Long live the DME relay....and if your in car heater fails on the coldest day of winter, simply put your hands under your seat and grab that DME for a nice toasty feeling!! |
#27
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10HP
Quote:
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Everyday | Driven everydaydriven.com 02 Coupe 83 Esprit Turbo |
#28
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You know, I don't know if I ever did. I must have been referring to upgrading the early DME ROM to a later one. Since your car is a later Carrera it wouldn't be much help for you - you already have that ROM in your DME. Really applied to 84-86 guys.
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Chris M 1985 911 Carrera with a couple cosmetic only mods 2006 E90 330i 1999 E46 328i |
#29
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roger that.. thanks...
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Everyday | Driven everydaydriven.com 02 Coupe 83 Esprit Turbo |
#30
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Jassbass has skills. I hope he don't mind me printing a page as an addendum to my Bentley
manual. I have searched hard and long to find this. Great work.. |
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