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  #21  
Old 10-28-2012, 09:18 AM
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Originally Posted by 73and88 View Post
... After 10 minutes elapsed the temperature reading was 173 degrees F.
...

I'll post more about the use of additional diodes after I install them and then try the complete assembly in my 1986 911.
F? That equates to 78C in proper engineering units Magnet wire and solder aren't even *remotely* phased at 80C. Most common magnet wire is good to at least 150C. Lower is avail but so is insulation up past 250C. Old lead based solder is good to ~180C (the newer RoHS stuff even higher.) You've got a long way to go before you're at those temps. The pcbs and plastics are usually the limiting factors.

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:
Originally Posted by Dr K
did the car run with the new (Chinese) relays???
x2. Just because something is made in China doesn't automatically mean its crap (though I do share your general sentiment.) Stuff under designed and built to be as cheap as possible is complete shit no matter where its made. But you can build very good parts anywhere, China included, if thats what you spec (and pay just a little more for.) Many very high quality relays are made in china now. As an example I have work experience w/ Zettler (good stuff) and all made in China afaik.
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  #22  
Old 10-28-2012, 04:04 PM
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Default DME Relays...Probable Cause of Failures

Quote:
Originally Posted by Dr K View Post
73and88,
That's an awful lot of posting, detail, testing, and labor on your relays, but I really want to know - did the car run with the new (Chinese) relays???
Dr K...yes it ran with the Chinese relays and it ran with the original DME relay once I rebuilt it. I would not use the Chinese made relays as their construction is not as good as the original Porsche part.

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.
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  #23  
Old 10-28-2012, 10:38 PM
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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.
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  #24  
Old 10-28-2012, 10:59 PM
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Why would the interior of the relay reach 120C?
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  #25  
Old 10-31-2012, 10:16 PM
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Quote:
Originally Posted by }{arlequin View Post
hopefully he's running out of 'reasonable' improvements
now thats a stretch..

Great write up!
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  #26  
Old 11-12-2012, 09:21 PM
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Default 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!!
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  #27  
Old 11-15-2012, 07:13 AM
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Originally Posted by Jazzbass View Post
Actually, it's mostly just out of my interest in this part of the car. This was the very stuff I loved doing before I realized there were better career opportunities in software. That said, I did just add 10 hp to my car for free. I'm working on a new post about how.
Did you post this yet?
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  #28  
Old 11-15-2012, 10:19 AM
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Originally Posted by Elfer View Post
Did you post this yet?
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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  #29  
Old 11-15-2012, 02:23 PM
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Quote:
Originally Posted by Jazzbass View Post
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.
roger that.. thanks...
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  #30  
Old 03-07-2013, 09:46 PM
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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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