How to Repair Relay Control Module (RCM) - (Mystery no Start)
Joined: May 2007
Posts: 7
Likes: 0
From: NOLA
Re: How to Repair Relay Control Module (RCM) - (Mystery no Start)
My car sat for a while when I was deployed. Drove it a couple times, then it died and would'nt restart. Not intermittent, just dead. I matched the symptoms to this and had a friend re-solder the connections. Now it's the typical intermittent problem. Sometimes it won't start right away, other times it starts fine then dies randomly.
But, I discovered that if I hold the center relay on the board closed, the car will start and run 100% of the time. I already ordered a new relay from a Mercedes dealer, but the post office apparently lost it...
My question is: can I safely drive the car with the relay manually held closed until I get the new part? The plastic ring from a Gatorade bottle cap folded in half works perfectly to hold the relay closed and keep the car running. I used this to get the car home, but I don't know if it was wise or safe.?
Thanks.
But, I discovered that if I hold the center relay on the board closed, the car will start and run 100% of the time. I already ordered a new relay from a Mercedes dealer, but the post office apparently lost it...
My question is: can I safely drive the car with the relay manually held closed until I get the new part? The plastic ring from a Gatorade bottle cap folded in half works perfectly to hold the relay closed and keep the car running. I used this to get the car home, but I don't know if it was wise or safe.?
Thanks.
Thread Starter
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Senior Member
Joined: Aug 2008
Posts: 5,741
Likes: 96
From: Sierra Vista, Arizona
Re: How to Repair Relay Control Module (RCM) - (Mystery no Start)
Looks like the coil in the fuel pump relay is bad. This is starting to be a common problem.
Replace the relay and yer good to go. Contact me if you want me to replace it for ya.
Senior Member
Joined: Apr 2006
Posts: 25,432
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From: Ontario
Re: How to Repair Relay Control Module (RCM) - (Mystery no Start)
Joined: Aug 2014
Posts: 42
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From: Missouri
Re: How to Repair Relay Control Module (RCM) - (Mystery no Start)
I did the solder fix over the weekend and so far all is well. Still driving on pins and needles though, really paranoid that it will shut down again on me. Keeping my fingers crossed. Thanks for the repair instructions and all the advise, it is appreciated.
Joined: Aug 2014
Posts: 42
Likes: 0
From: Missouri
Re: How to Repair Relay Control Module (RCM) - (Mystery no Start)
It has been a week and and about 600 miles since I did the fix, so far so good. I am just amazed that someone actually figured this problem out. My RCM looked pretty good on first glance, but a closer inspection showed that the solder on the troubled areas was much less than the others. However, mine did not show any sign of cracking, so I was less than excited that this would fix the issue, but like I said So Far So Good.
Joined: May 2010
Posts: 68
Likes: 0
Re: How to Repair Relay Control Module (RCM) - (Mystery no Start)
I did my fix thousands of miles ago and it's still running great - like you, I didn't have any obvious issue, but it did look slightly different than the other solder joints. I touched it up and it's been running great. Major props to the OP for figuring it all out for us.
-Kevin
-Kevin
Joined: Apr 2014
Posts: 3
Likes: 0
From: Ingleside texas
Re: How to Repair Relay Control Module (RCM) - (Mystery no Start)
Help me ive been trying everything ... I love my xfire... please text me if you are or can help... no start... no gear indicator just a box ...2nd and reverse only after jumping starter and have to use special tool to shift... help.. (361) 523-8169..... my name is tony...
Senior Member
Joined: Nov 2005
Posts: 2,349
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From: Schertz, TX.
Re: How to Repair Relay Control Module (RCM) - (Mystery no Start)
Help me ive been trying everything ... I love my xfire... please text me if you are or can help... no start... no gear indicator just a box ...2nd and reverse only after jumping starter and have to use special tool to shift... help.. (361) 523-8169..... my name is tony...
Joined: Apr 2014
Posts: 3
Likes: 0
From: Ingleside texas
Re: How to Repair Relay Control Module (RCM) - (Mystery no Start)
Yes tried rest a couple of times.....
I thought I read on this forum someone saying they found a mercedes recall on the slk engine..ive been looking for text cause it had a link to where it was found... cant find it again....
Joined: Apr 2014
Posts: 3
Likes: 0
From: Ingleside texas
Re: How to Repair Relay Control Module (RCM) - (Mystery no Start)
Its the special tool that comes with the car its a mutated flathead screwdriver wrench also used to pop convertable compartment
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Joined: Jun 2008
Posts: 1,920
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From: Spacecoast, Florida
Re: How to Repair Relay Control Module (RCM) - (Mystery no Start)
Help me ive been trying everything ... I love my xfire... please text me if you are or can help... no start... no gear indicator just a box ...2nd and reverse only after jumping starter and have to use special tool to shift... help.. (361) 523-8169..... my name is tony...
What Is Limp Home?
Well, that means, your car has lost a wheel so it is driving on three wheels to home.
Seriously, when the Electronic Transmission Control module (ETC) detects intermittent or active problems, it prohibits gear shifting, in order to avoid serious transmission damage.
There are two different types of Limp Home modes, depending how serious the problems are. The names of these two different Limp Home modes may sound counter-intuitive but that is what Mercedes used.
Mechanical-hydraulic emergency running mode (This is the light duty Limp Home)
In this Limp Home mode, the transmission locks onto the 3rd gear or the last known good gear.
This particular Limp Home mode can be reset by turning the ignition off and then on.
This mode could set generic OBD II code and also intermittent code in the ETC.
Electric emergency running mode (This is the bad boy
)
In this Limp Home mode, the gear engaged at that time is retained and the assigned fault code is stored. After a shutdown, wait for more than 10 seconds, and restart, the transmission is locked in the 2nd gear and reverse gear only. When the shifter is moved from P to R or N to D, there is a huge clunk.
The transmission will be in this mode until the codes are cleared from the ETC by Mercedes HHT (Hand Held Tester) or SDS (Star Diagnostic System) or some third party special scanners or even the reversely engineered ones. Generic OBD II scanners cannot clear these codes from the ETC (although they can clear the corresponding generic OBD II codes).
When the transmission is in this electric Limp Home mode, it feels like it is completely shot. Many times (actually I will go out on the limb and say most of the time) you do not need a rebuilt transmission or a new ETC.
What to do if the transmission is in limp home mode?
The most useful thing to do is to have the ETC trouble codes read. This requires a Mercedes HHT (Hand Held Tester) or SDS (Star Diagnostic System) or some third party special scanners or even the reversely engineered ones. These codes are pretty accurate at pointing to the fault that caused the limp mode. For example, it may have codes for speed sensors or the shift module.
If no Mercedes scanners are available, at least try to read the codes using a generic OBD II scanner. The generic OBD II codes are not as precise as the proprietary Mercedes codes but they might shed enough light to have a good enough diagnosis. For example, if a code P0715 is read, then it is fairly certain that it is caused by the speed sensors.
After fixing the underlying cause of the (electric) Limp Home, the transmission would still not shift. The ETC has to be reset (codes cleared) before it will act normal again.
Without an ETC reset, people often (unnecessarily) replace the ETC. Of course, a new or working ETC will make the car shift again but all is needed to be done is to clear the codes in the ("bad") ETC. When the ETC goes into (electric) Limp Home, it shuts off all the power to the solenoids so the ETC will test "bad" because of this.
There are different versions of the ETC and certain ETCs can only work with certain valve bodies so exercise caution when replace either the ETC or valve body. It is a good idea to use the existing part numbers to find replacements.
Well, that means, your car has lost a wheel so it is driving on three wheels to home.
Seriously, when the Electronic Transmission Control module (ETC) detects intermittent or active problems, it prohibits gear shifting, in order to avoid serious transmission damage.
There are two different types of Limp Home modes, depending how serious the problems are. The names of these two different Limp Home modes may sound counter-intuitive but that is what Mercedes used.
Mechanical-hydraulic emergency running mode (This is the light duty Limp Home)
In this Limp Home mode, the transmission locks onto the 3rd gear or the last known good gear.
This particular Limp Home mode can be reset by turning the ignition off and then on.
This mode could set generic OBD II code and also intermittent code in the ETC.
Electric emergency running mode (This is the bad boy
)In this Limp Home mode, the gear engaged at that time is retained and the assigned fault code is stored. After a shutdown, wait for more than 10 seconds, and restart, the transmission is locked in the 2nd gear and reverse gear only. When the shifter is moved from P to R or N to D, there is a huge clunk.
The transmission will be in this mode until the codes are cleared from the ETC by Mercedes HHT (Hand Held Tester) or SDS (Star Diagnostic System) or some third party special scanners or even the reversely engineered ones. Generic OBD II scanners cannot clear these codes from the ETC (although they can clear the corresponding generic OBD II codes).
When the transmission is in this electric Limp Home mode, it feels like it is completely shot. Many times (actually I will go out on the limb and say most of the time) you do not need a rebuilt transmission or a new ETC.
What to do if the transmission is in limp home mode?
The most useful thing to do is to have the ETC trouble codes read. This requires a Mercedes HHT (Hand Held Tester) or SDS (Star Diagnostic System) or some third party special scanners or even the reversely engineered ones. These codes are pretty accurate at pointing to the fault that caused the limp mode. For example, it may have codes for speed sensors or the shift module.
If no Mercedes scanners are available, at least try to read the codes using a generic OBD II scanner. The generic OBD II codes are not as precise as the proprietary Mercedes codes but they might shed enough light to have a good enough diagnosis. For example, if a code P0715 is read, then it is fairly certain that it is caused by the speed sensors.
After fixing the underlying cause of the (electric) Limp Home, the transmission would still not shift. The ETC has to be reset (codes cleared) before it will act normal again.
Without an ETC reset, people often (unnecessarily) replace the ETC. Of course, a new or working ETC will make the car shift again but all is needed to be done is to clear the codes in the ("bad") ETC. When the ETC goes into (electric) Limp Home, it shuts off all the power to the solenoids so the ETC will test "bad" because of this.
There are different versions of the ETC and certain ETCs can only work with certain valve bodies so exercise caution when replace either the ETC or valve body. It is a good idea to use the existing part numbers to find replacements.
Last edited by syfi; Oct 16, 2014 at 11:50 AM.
Joined: Aug 2013
Posts: 11
Likes: 2
Re: How to Repair Relay Control Module (RCM) - (Mystery no Start)
Would you please clarify whether the RCM repair or replacement works if there is no battery activity?
Here's my situation. Have had my '08 roadster for one year. No problems until a hellacious rain storm a month ago. Dropped over an inch of water in less than an hour. Lillian was in the parking lot and got drenched. Went to start her up, no lights on dash. Got a jump; she starts right up and I had no problems until this weekend. No less than three times after being parked that when I returned no power. Crazy thing is when I lift hood or release hood latch, power back on.
Mechanic has no idea what to do. Previous post mentioned cars where dash lights are on. Could this also apply to me?!
Here's my situation. Have had my '08 roadster for one year. No problems until a hellacious rain storm a month ago. Dropped over an inch of water in less than an hour. Lillian was in the parking lot and got drenched. Went to start her up, no lights on dash. Got a jump; she starts right up and I had no problems until this weekend. No less than three times after being parked that when I returned no power. Crazy thing is when I lift hood or release hood latch, power back on.
Mechanic has no idea what to do. Previous post mentioned cars where dash lights are on. Could this also apply to me?!
Last edited by tennisdiva; Oct 27, 2014 at 02:24 PM.
Joined: Aug 2013
Posts: 11
Likes: 2
Re: How to Repair Relay Control Module (RCM) - (Mystery no Start)
First thing we checked for was battery connection. Those look good. No corrosion or loose wires. Any other "connections" come to mind that might be causing this issue?
so thankful
Joined: Apr 2013
Posts: 32
Likes: 0
From: Huntsville, AL
Re: How to Repair Relay Control Module (RCM) - (Mystery no Start)
Hey everyone.
I'm having an issue where my car is beginning to not start up after sitting for a day or so and I was wondering if might be an RCM issue.
What's happening is that all the electronics (lights, radio, heater) will work but the car will not crank unless jumped. Just a clicking sound. Once jumped it will start right up and work for a couple of days. The last time I had this happen I went to an auto parts store and they tested the battery, starter, and alternator and said they were fine.
I love my car and I'm starting a new job on the 1st of December so its really important that I get this figured out and working.
I'm having an issue where my car is beginning to not start up after sitting for a day or so and I was wondering if might be an RCM issue.
What's happening is that all the electronics (lights, radio, heater) will work but the car will not crank unless jumped. Just a clicking sound. Once jumped it will start right up and work for a couple of days. The last time I had this happen I went to an auto parts store and they tested the battery, starter, and alternator and said they were fine.
I love my car and I'm starting a new job on the 1st of December so its really important that I get this figured out and working.