Cut out relay. or something like that - Mercedes Benz SLK Forum

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#1 Old 3 Weeks Ago
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Cut out relay. or something like that

Looking for a sulotion for this relay. its a black box with 2 contacts with nuts and then a black regular contct.
Located next to the battery, cables comes from generator and goes to the relay and then goes to batteri. the smaller contact have 3 cables and nopt sure were they goes or comes from.
measuring around 20 volts on the cable that comes from the renerator up to the relay and nothing out to the battery.
if a shortcut the 2 bolts with nuts the generator starts to work and delivers powerr to the battery.

Is there a way to change this to a normal relay or does this even change the current?

I meen normal relay that handels 100-120A 12v?

picture shows the relay i have now.. not mine but it shows how it looks.

The one i have is picture of the old one.. and thinking if its possible to change to the picture of the new.

Otherwise i have found one on ebay but its for year 2003 and up, and i have a 1997, they looks the same but maybe not work.

Please help me with this.

Best regards Thomas Carlsson from Sweden.
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#2 Old 3 Weeks Ago
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#3 Old 3 Weeks Ago
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That part is the battery cutoff relay. probably to devote power for starting or maybe in the event of an accident.. not terribly sure. replacing the part with a new one is the wisest solution.. some people will open it up and re-solder all the joints with success.
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#4 Old 3 Weeks Ago
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Thx... well mine does not work... messure 19 volts on incoming to relay.. and nothing out.

Looking for a diagram for wireing soo i can find were the small cables goes.

Thex again for the quick answer.
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#5 Old 3 Weeks Ago
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Hi,

Not a lot of help, but an observation ................

I do not understand how you are getting readings of more than 14.8 Volts on a 12 Volt Vehicle system, unless you have an Alternator Fault ????

Cheers Dave
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#6 Old 3 Weeks Ago
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Originally Posted by Dave2302 View Post
Hi,

Not a lot of help, but an observation ................

I do not understand how you are getting readings of more than 14.8 Volts on a 12 Volt Vehicle system, unless you have an Alternator Fault ????

Cheers Dave

Well its just from the alternator... no load at all.

Its brand new bosch one.

Had a small company mounted it for me... and for some reason i dont trust them now...
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@Charmed I can't add much information here, I have no first hand experience. I tried searching both WIS and Starfinder for this component -it's not clearly documented, I only found it on one diagram in WIS (none in Starfinder). I believe it's called X4/10 in MB diagrams and I've heard it referred to as the "crash relay", but I cannot find any diagram that shows what the smaller 3-pin connector actually connects to. I guess I would expect "airbag" but the airbag diagrams do not mention this component. The larger terminals, as you already know, connect to the battery and the alternator. Perhaps also to the instrument cluster light?



+1 on addressing your regulator - you shouldn't see such high voltage coming from the alternator. Maybe this is what destroyed your relay?
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#8 Old 3 Weeks Ago
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Originally Posted by efair View Post
@Charmed I can't add much information here, I have no first hand experience. I tried searching both WIS and Starfinder for this component -it's not clearly documented, I only found it on one diagram in WIS (none in Starfinder). I believe it's called X4/10 in MB diagrams and I've heard it referred to as the "crash relay", but I cannot find any diagram that shows what the smaller 3-pin connector actually connects to. I guess I would expect "airbag" but the airbag diagrams do not mention this component. The larger terminals, as you already know, connect to the battery and the alternator. Perhaps also to the instrument cluster light?



+1 on addressing your regulator - you shouldn't see such high voltage coming from the alternator. Maybe this is what destroyed your relay?
This is strange... i meen how does it change from letssay 19 volts to around 14?? have tested 2 alternators and both shows the same voltage... around 19-20 volts.


I too off the small 3 cable connector and measure it... 2 bottom cables is ground.. and the top is zero when car is not running and about 12 volts when its running...

and if i measure on the big cable the comes from den alternator i get 19 volts... and zero after it.. no matter the car is running or not.
That meens the relay is not activateds at all and does not charge the battery.

Gonna try just to put a thick cable over the 2 bolts on relay to see if voltage drops when chargeing battery.

For me its sound mot that strange thaat a alternator maybe giving out 19 volts when its totaly idle and npt connected to a battery.
And as soon its charging it drops to like 14.3 or something around that..

Whould be happy if someone knows how a alternator works when talking about this things and posts here or in a new tread.

//Thomas
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#9 Old 3 Weeks Ago
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HERE is an interesting read.

It's my understanding that battery power alone starts the car, once started, the alternator comes onto play to power other electrical systems and to keep the battery charged up to its optimum. Output from alternator should be between 13.8v & 14.5v.


As you have a new alt and it's giving the same readings as the old one, I'd now be taking a wild guess that you could be seeing a 19v output from the alt due to it not seeing a what it expects to be seeing and therefore being unable to regulate it's output?? Does that sound even feasible??

You may well find that replacing the relay fixes the issue

Just my 2c
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#10 Old 3 Weeks Ago
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Originally Posted by M4rCu5 View Post
HERE is an interesting read.

It's my understanding that battery power alone starts the car, once started, the alternator comes onto play to power other electrical systems and to keep the battery charged up to its optimum. Output from alternator should be between 13.8v & 14.5v.


As you have a new alt and it's giving the same readings as the old one, I'd now be taking a wild guess that you could be seeing a 19v output from the alt due to it not seeing a what it expects to be seeing and therefore being unable to regulate it's output?? Does that sound even feasible??

You may well find that replacing the relay fixes the issue

Just my 2c

Just wonder...
most charge system works like you start the car... the alternator start tp pruduse power to the battery... the alternator is labeld 90A 19V.
But on the alternator it is a charge relay (or what it called in english... mounted on the back of the alternator.. the task for that is to reduce power from 19V too accepted around 14 volts.

on these cars we also have a crach cut-out relay to shut down current between alternator and battery if something happends.


In my case the crach cut-out relay is broken, soo i dont have any connection between alternator and battery.

Is it possible that if relay is broken and the alternator has no conntection to batter i can destroy the thing that is mounted on the back of the alternator soo instead of sending out 14 volt it sends out 19V.

if thats the case i have to first change the crash cut-out relay and then change the thing on the back of the alternator.. or maybe i will have the same problem again.


i dont know if a spinning alternator can take damage if its not connected to a battery.. (broken crash cut-out relay).

I meen in my ears a alternator should have something at the end of the cabels that force the alternator to work.. otherwise it just puching current strait out in the air without any resistance.
A alternator needs resistance or it can broke down..


Im from sweden and not that good at english, soo i hope you guys understand what i meen.

//THomas
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#11 Old 3 Weeks Ago
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I'm used to it, my other half is originally from Malmö

I know what you are saying and as I understand it, the volt regulator gives as much or as little as required, capped at 14.5v or so. I just don't get why two of them give 19v, maybe they need to see the battery or maybe some sort of feedback from the relay. It is a conundrum

Even better article on alternators HERE
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#12 Old 3 Weeks Ago
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Ahhh Malmö... then you can speak swedish now :P

Mmmm im gonna try to get a new relay or atleast do a bypass on it now to see how much voltage is going thru. hopefully i wont break anything else if its giving out 19 volts in system.
going to have a meter to check at the same time so i can just remove it if the current is to high.
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#13 Old 3 Weeks Ago
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I wouldn't bet on it, I can just about stumble through the days of the week at best .. I can absolutely 100% guarantee that your English is better than my Swedish


OK that sounds like a plan. The labelling would be a 90A 19v MAXIMUM output.

The 2nd article states:

"Amperage or current is regulated by the state of charge of the battery. When the battery is weak, the electromotive force (voltage) is not strong enough to hold back the current from the alternator trying to recharge the battery. "

So .. as it's not connecting to the battery (because of the faulty relay) .. that might explain the now-unregulated max 19v output reading ..
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I would suggest you remove your alternator and have it properly tested before you do anything else.
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Hi,

As above, I would not replace that relay and just start the Car, because this will allow the possibility that you could fry a lot of electronics, bulbs etc etc very quickly.

There are some Alternators that will just go pop as soon as there is a disconnection from the Battery / load, and some that don't, I am always cautious of running them with no battery connected these days.

It could be that the Alternator is just maxing out due to the dud relay, but yep, I concur with @efair, get that Alt professionally tested, if they say it's ok, then fine, replace the Relay and start the Car, if Alt is FUBAR, get it fixed, but do not start Car until the relay fault is corrected

Agree with @M4rCu5 , that all makes perfect sense.

Sorry if this sounds confusing, just don't want you to turn your car into a big paperweight

HTH, Cheers Dave
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Well. Now looked at the box that alternator came in. It say 90a 19volt.

Soo qick added a cable over the 2 bolts and forced the current to battery. Like bypass the relay.

And it charges 13.6-13.8v on battery.
IF I turn off the engine the battery is 12.6-12.8 volt.
Trying to start stereo and amp. Even blower and headlight. It wont go under 13.4v when engine running.

That meen that crash cut-out relay is broken.

Pretty hard to find on the net.
Only a few on ebay.
Wonder what Mercedes Orginal cost.....
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It's a simple 12V relay. The middle two terminals receive power to energize the magnet which then closes the circuit of the outer two contacts. The closed circuit is for carrying higher amperage like from the battery to the starter for example. It is very easy to check to see if it is OK. Send some power to it and check the continuity of the closed circuit.
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Quote:
Originally Posted by Charmed View Post
Well. Now looked at the box that alternator came in. It say 90a 19volt.

Soo qick added a cable over the 2 bolts and forced the current to battery. Like bypass the relay.

And it charges 13.6-13.8v on battery.
IF I turn off the engine the battery is 12.6-12.8 volt.
Trying to start stereo and amp. Even blower and headlight. It wont go under 13.4v when engine running.

That meen that crash cut-out relay is broken.

Pretty hard to find on the net.
Only a few on ebay.
Wonder what Mercedes Orginal cost.....
Aha ..

And I've found the answer to the other major question HERE read paragraph 4
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#19 Old 3 Weeks Ago
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Quote:
Originally Posted by Charmed View Post
Well. Now looked at the box that alternator came in. It say 90a 19volt.

Soo qick added a cable over the 2 bolts and forced the current to battery. Like bypass the relay.

And it charges 13.6-13.8v on battery.
IF I turn off the engine the battery is 12.6-12.8 volt.
Trying to start stereo and amp. Even blower and headlight. It wont go under 13.4v when engine running.

That meen that crash cut-out relay is broken.

Pretty hard to find on the net.
Only a few on ebay.
Wonder what Mercedes Orginal cost.....
Aha ..

And I've found the answer to the other major question HERE read paragraph 4 <img src="https://www.slkworld.com/images/smilies/thumbsup.gif" border="0" alt="" title="Thumbs Up" class="inlineimg" />
Dude. Thats just what i wanted to know.

Therefor i can measure 19volts when not attached to battery

&#x1f609;&#x1f609;&#x1f609;&#x1f609;&#x1f609;&#x1f 609;&#x1f609;&#x1f609;
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All the voltage regulator does is prevent the alternator from going berserk over charging the battery and destroying itself in the process. When the battery voltage reaches a certain value it breaks the alternator's circuit shutting it down.

Without seeing the wiring diagram, I have not idea what role the relay plays unless this is what disconnects the alternator from the battery. In modern cars, all the circuity necessary is contained within the voltage regulator.
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