DME Relay Repairing

[Jazzbass]

The 944 rewire and Andy's comments got me researching DME relays. Since I also have one of those damn things in my car, I did some research into the problems.

The DME relay is really 2 relays in one. The problem is not really the relays themselves, since relays tend to be pretty trouble-free - but the printed circuit board they're attached to. All of the heat, vibration, etc, of the car takes its toll on this board and the solder joints on it start to fail. The solder job on these is pretty crappy to begin with, so it doesn't seem to take much to make them fail.

Here's a pic of the DME relay that was in my car when I bought it:

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Inside - showing the relay side. You can see the two relays - the one on the left is the fuel pump relay and the one on the right is the main DME/fuel injector relay.

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The other side showing the PCB that the relays are soldered to. This is typically where the problems occur.

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A quick inspection shows that sure enough there is one solder joint that has failed (red arrows) and several I'm not too happy about (blue arrows). Note the solder joint that has failed has a ring around the pin - this is a crack in the solder joint. Cracks are caused by vibration of the component (the relays are heavy and then to crack their solder joints quicker than smaller components) and heat/cool cycles expanding and contracting the solder.

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Here's a close up of the bad solder joint, with the red arrow pointing to the crack.

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Fortunately, its an easy problem to fix with a soldering iron. Here I'm reflowing the solder on all the joints. When I do this I add a little bit of new solder, but not too much. The main idea is to completely melt the existing joint. When it cools, all the cracks will be gone. NOTE: Don't blow on the solder to cool it! This can cause the solder to cool too fast and you get what's called a "cold solder" - a joint where the solder doesn;t adhere properly to either the component, the board or both.

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All the joints reflowed. Note the excess flux on the board. I could wash this off, but I'm lazy and don't feel like it. Not a big deal anyway.

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There you go - DME relay repair made easy!

[boardman]

jeez chris most of those looked like cold joints before ya fixed em!! im glad to see ya popped it open so everyone could see how easy it is to fix someone elses crap solder work! i see this junk every day in the audio business

[Jazzbass]

jeez chris most of those looked like cold joints before ya fixed em!! im glad to see ya popped it open so everyone could see how easy it is to fix someone elses crap solder work! i see this junk every day in the audio business
My first real job in high school was repairing TVs (16 years ago). 50% of the work I did was reflowing cold/broken solder joints, the other 50% was replacing rectifier bridge diodes in the main power supply after people left them plugged in during a thunderstorm. I made $5/hr, they made $50/hr in labor :roll:.

[Gator_86_951]

I dig the soldering skills. Can you do surface mount parts? Guess it depends on the packages sizes. I was never very good at soldering, actually I am horrible with a capital H. I had to bribe the soldering lady at work with star bucks when I let the magic smoke out. :(

[Andy Clements]

Nice photo tutorial Jazz! I've fixed a few of those over the years myself. I always tell guys to save the old ones 'cause they can usually be repaired as a spare. Why is it that the 911 part has to be all fancy and use the round pins instead of the conventional blade prongs us lowly FEWC's get!

I knew the DME relay was basically just a dual relay in one can....but I was not aware of the explanation you found from Wong regarding engine RPM and the safety shut-off for the fuel pump. You can see why I thought a combo of toggle switches could replace the relay, but it's a bit more complex than that. I guess it would be good practice to open and inspect the relay annually for pending joint cracks that might ruin a race.

So any further verification on the kill switch function via disabling the 12V+ (pin 86) on the 944 relay......Mr. EE???

Nice research Chris!

[Lupin..the..3rd]

Thanks for the writeup and nice photos Chris! :)

[William Miller]

Nice!
Andy, BTW, the DME relay for the 964 and 993 also have the blade style connectors as do most of the other relays in the car.
The tiney terminals are for the coils and the larger (normal size) terminals are for the load that is switched.

Chris, I tried, but failed to re-melt the solder on a curcuit board that held a connector on the toy train just before x-mass.
To say the least the board was bad or I screwed it up. I may need your services when the parts come in!

[}{arlequin]

crap.... looks like a need to own a soldering iron... nice post jazz

[Jazzbass]

crap.... looks like a need to own a soldering iron... nice post jazz
I use a Weller WTCPT (http://www.wassco.com/wtconoutstat.html). Its one of those things that's on the pricy side, but worth every penny.

[}{arlequin]

LMAO.... i knew i could count on you on saving me from 'wasting' my $11 at radio snack :lol:

[Jazzbass]

LMAO.... i knew i could count on you on saving me from 'wasting' my $11 at radio snack :lol:
That's what I'm here for. You know I couldn't let you go ghetto on this.

(I fixed the link, BTW)

[Jazzbass]

So any further verification on the kill switch function via disabling the 12V+ (pin 86) on the 944 relay......Mr. EE???

Note: I use the "new" pin out numbers of the DME relay (1-9) instead of the older 85. 86, etc, since that's what in all the factory diagrams and printed on the actual relays we're using.

Regarding Pin 3 (86) - killing power to this will kill your DME and the fuel injectors - this seems like a great way to kill the car, esp if you kill the ignition, too.

The +12V supply to the coil (pin 15) is supplied from the same power circuit - switched ignition (pin 15 of the ignition switch) - as the +12V to pin 3 (86) of the DME relay. Here's my design:

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There's an interior kill switch that will kill power to both the coil and to pin 3 of the DME relay. When pin 3 is killed, the DME, the fuel pump and the fuel injectors all loose power. The connector B(P1) is going to the main battery cut off, then to the coil.

As for the main battery kill switch in the ignition, I considered the idea of running the ignition kill part of that switch in series with the interior kill switch - thus opening either one would kill the DME relay and coil. But that would mean running from the fuse box, to the main kill switch in the engine compartment, back to the interior for the interior kill switch and DME relay, then back out for the coil. Kind of a pain, and unnecessary IMO since if someone kills the main switch the battery and the coil are instantly, effectively killing the entire system.

Here's the circuit for the main kill switch:

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[Andy Clements]

Chris...thanks for sharing the circuit design....as I'm getting ready to do the same on my car soon also.....but I'm still a bit confused....and I'm also no expert at reading circuit diagrams (but I'm not a complete moron either). I'm not sure I'm clear on your intensions/goals behind the design concept. Do I understand that you will have a kill switch mounted inside (within driver's reach) that will only effectively disable the ignition and fuel pump.....thereby killing the engine and fuel delivery (but not battery)? And you intend to have a second "main" kill switch accessible from outside (corner worker pull) that will kill the battery power as well as the ignition/fuel at the same time?

My understanding is that a kill switch in racing should serve several purposes....primarily to disengage all electrical power feeding from the battery. Likewise, you need the resistor to prevent a voltage spike from the alternator when the battery is killed, and for added safety we typically kill ignition and/or fuel pump at the same time with a multi-pole switch to avoid engine run-on and/or feeding fuel to a fire.

I'm trying to understand your purpose for an interior kill switch that only disables the DME relay which effectively is sorta like turning the key off? BTW, will TD's car retain a keyed start, or are you going to a push-button start configuration that might thereby require an ignition kill to shut it down?

Maybe I'm reading into this too much, but I've kinda always worked toward the simplified approach and/or the least number of items to fail. By that I'm referring to a single kill switch setup that is typically mounted in a multi-access configuration. I've seen kills mounted on the interior near the driver's-side-view mirror that can allow access by the driver OR a corner worked from one location (these are hard to fit). I've also seen setups where a switch mounted under the hood near the battery is cable operated from a pull located near the base of the windshield but is still reachable by driver or worker. Lastly, I've seen setups that use a single switch mounted under hood near the battery, but have dual cables with one routed for outside worker pull, and the other routed to the interior for driver pull.

The obvious disadvantage of an under-hood switch is that you must get out and pop the hood to reset it, but theoretically this should only be tripped in a real incident anyway. I'm not sure I see the benefit to having dual switches with one mounted inside that only kills ignition without killing battery power also.....assuming you are using it for safety in a real track incident. Soooo....can you explain your thinking behind the circuit setup so I can figure out what I'm doing wrong? If you truly have a better idea I'm curious to understand it so I can copy your design. :)

[Jazzbass]

Andy - you're right about both switches. The main kill switch on the second page is the primary "safety" switch for racing, and the one that would be flipped by a corner worker in the event of an emergency. It kills all power to the car and the ignition, and has the resistor connected to the alternator. Because of the current (lots) running through this switch, it physically needs to be close to the battery, with a pull cable routed somewhere accessible. This is exactly how the car is set up right now.

The interior witch is simply an engine kill and not meant to duplicate the functionality of the main safety switch. You're 100% correct - it is just like turning the key off (with the exception that power is physically cut from the DME relay - see revised circuit). The thinking here is that it gives the driver the ability to kill the engine fast in the event of a car problem (say low oil pressure) and makes doing so very easy - the interior kill switch is a large toggle switch with a safety cover on it, all the driver has to do it smack it and the car's dead. This as opposed to reaching up an turning a key. I know it sounds stupid - how much longer could it take to turn a key than hit a switch? My thinking is that when engine damage may be occurring, every millisecond counts.

I put this in because I've seen it in many race cars, even ones that still had keyed ignition, and was told that was the purpose. I agree that in the case of an incident, the main kill switch cutting off the battery would be the one to hit. To make that accessible to the driver, we'd have to do just as you say and double up the pull cable running into the car.

Here's the revised interior kill I did last night. The main difference is I'm physically switching power off the DME and fuel pump, not just switching off the DME relay. If the DME relay were to fail for whatever reason in an "ON" fashion, this would still allow the driver to kill the fuel pump.

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BTW, I wanted very badly to make this switch the main system switch for the car and put a push button start on the left side of the steering wheel, where it belongs. Unfortunately, TD wants to keep the keyed ignition because he thinks that we'll f**k with his car at night in the paddock after he leaves. Honestly, he's probably right :lol:

[}{arlequin]

Unfortunately, TD wants to keep the keyed ignition because he thinks that we'll f**k with his car at night in the paddock after he leaves. Honestly, he's probably right :lol:

you mean you didn't mention to him the fact that we're filling his rollbar full of D-cell batteries JUST for this purpose in case he wanted to take the battery out? :twisted:

[CrewChief]

I'm with Andy. KISS. But then, I also like to remove the key and go with a toggle with saftey cover. Call it aviator 'bling' I guess.

Are you using the interior kill to the DME in place of the driver activating the main cutoff? If so I'd be afraid some tech inspector would catch the difference and disallow the usage.

FWIW, you do NOT want to be constantly turning the large power switches off and on. Once when the tech inspectors makes you demonstrate it does work and kills the motor. And once to turn it back on. Because, they are a metal pin riding in a plastic cam groove. You know all those times guys have been fiddling with them in the paddock to get the car started? Well, the cam action wears quickly and suddenly the contacts don't close. The contact are not wipers but pressure (vertical movement).

Largely because I don't plan on using the cutoff like a key switch I don't even care about the alternator resistor. The diodes should be fine with a hot switch once a year or so.

[TD in DC]

I'm with Andy. KISS. But then, I also like to remove the key and go with a toggle with saftey cover. Call it aviator 'bling' I guess.

Are you using the interior kill to the DME in place of the driver activating the main cutoff? If so I'd be afraid some tech inspector would catch the difference and disallow the usage.

FWIW, you do NOT want to be constantly turning the large power switches off and on. Once when the tech inspectors makes you demonstrate it does work and kills the motor. And once to turn it back on. Because, they are a metal pin riding in a plastic cam groove. You know all those times guys have been fiddling with them in the paddock to get the car started? Well, the cam action wears quickly and suddenly the contacts don't close. The contact are not wipers but pressure (vertical movement).

Largely because I don't plan on using the cutoff like a key switch I don't even care about the alternator resistor. The diodes should be fine with a hot switch once a year or so.

It is my understanding that we will retain both the standard multi-pole kill switch on the outside of the car (that uses a throttle cable to get the pull switch over to the lower driver's side corner of the windshield), which should satisfy tech, and the additional driver activated kill switch in the cockpit. Can't imagine anyone would have a problem with additional safety measures, no?

[}{arlequin]

don't think it's been viewed as a problem but rather an additional complication.

[TD in DC]

Are you using the interior kill to the DME in place of the driver activating the main cutoff? If so I'd be afraid some tech inspector would catch the difference and disallow the usage.

don't think it's been viewed as a problem but rather an additional complication....

[Andy Clements]

It is my understanding that we will retain both the standard multi-pole kill switch on the outside of the car (that uses a throttle cable to get the pull switch over to the lower driver's side corner of the windshield), which should satisfy tech, and the additional driver activated kill switch in the cockpit. Can't imagine anyone would have a problem with additional safety measures, no?

Agreed, you should satisfy tech with the main kill switch. I think the real question was what was the purpose and need for a second interior DME kill switch. Jazz's explanation makes sense with respect to an engine problem (low oil pressure perhaps) that might benefit from an immediate engine cut action. The only benefit I can see of killing the engine without also killing the battery at the same time would be to maintain aux electrics like fans, Dash2, electric gauges, lights, etc........and OF COURSE TD's cool suit so he didn't melt on the tow back to the pits :D I can see how killing the engine quickly might be useful to prevent disaster, and theoretically if you didn't lose battery in the process you could restart the car and limp back after the session without having to pop the hood and reset the kill switch.....of course assuming you were willing to restart without knowing the source of the low oil pressure :) . I guess it all seems like some fancy EE :lol: thinking to me....but I can agree there are conceivable situations where it might be good.

I'm not sure the scruits would care if you could prove the main switch functioned properly, but I'm not sure if I would consider the second switch "driver safety" as much as protection for car/engine itself.

I swear I'm not trying to be difficult here....just trying to learn and figure out if Chris really has designed a better mousetrap....so I can swiftly copy it (and make it lighter so I can catch TD)!

[CrewChief]

Didn't mean to imply the internal kill was bad. Only exactly what was stated: the rules mandated cutoff is supposed to disconnect all voltage from the system. The internal kill does not. As you have clarified I don't see any reason anyone other than you would even know it was there.

Other than that, like Andy, I don't see a tremendous need for the switch. But as we used to say in OK, "Anything that pleases you just tickles me plum to death".

Oh yeah, I want to be there when all you guys with key operated race cars are frantically searching for same (left in the motel room) at 7am in the paddock!:twisted: :lol:

[TD in DC]

Oh yeah, I want to be there when all you guys with key operated race cars are frantically searching for same (left in the motel room) at 7am in the paddock!:twisted: :lol:

After all the abuse my car has taken from my "friends" :lol:, including moving it across the paddock, I have grown to believe that I need to retain the key.

The truth is that my car is frequently stored outside. Although a key is not foolproof by any means, I prefer to have more, rather than fewer, obstacles for thieves to overcome.

[Jazzbass]

I'm not sure the scruits would care if you could prove the main switch functioned properly, but I'm not sure if I would consider the second switch "driver safety" as much as protection for car/engine itself.
The interior switch was never meant to be a crash safety feature - hopefully I never implied that the main, external, battery kill would be removed in favor of this. It isn't, and it won't.

At the end of the day, all I'm doing is putting 1 switch in front of the DME so the driver can shut the engine off quickly if he wants to. Think of it as simply an easier way to turn the car off, rather than fumbling for the key. Personally, I don't see it as much of a failure point, either, as power from the ignition to the DME physically runs right behind the switch panel. All I'm doing is running that power through this switch.

Plus, if TD ever decides to get rid of the key, then this switch becomes the main ignition switch for the car.

[Dailydriver]

Hi all, great info...I've just inspected mine and nothing obvious there. I have a 964 with spade connectors and have read somewhere that i can bypass the relay by connecting the 3 load pins together and if the thing runs again problem found...anyone know about this?
Cheers
Gary.

[Dr K]

Holy thread resurrection, Batman! :D

[Dailydriver]

well why not...as long as the joker doesn't reply:) ... You know how it is when you first get a problem there's always an old thread to look up. I'm not convinced it is the relay yet though as it stopped mid drive, failed to start. left it at the side of the road and went back hours later and it fired 1st time so I drove it 6 miles and again it stopped. sadly it's been failing to start since.

[Trak Ratt]

Always nice to see someone who looks around or uses the search option first... even if it's a Brit!