I am hoping the community may be able to provide some insight into this mess of issues.

I own a 2004 w/ 25K miles. A couple months ago while driving the transmission downshifted into limp mode. I drove the remaining 2 miles home. Would not restart right away. Later it did start and seemed to run fine. The next time I drove it a good distance, it stalled and would not restart/keep running after numerous tries. Would either get a crank with no start, a crank with start/sputter/stall, or no crank at all. I had it towed home. At first I assumed it was a crank position sensor but that was only until I started reading codes and got a P0600 (Serial Data Link Communications Malfunction). At that point it would typically not crank at all unless I disconnected the negative battery cable one or more times. I did get it to start on occasion, but then it I could not move the transmission out of park without using a ball point pen.

Here is a full list of the symptoms:

P0600
MILs - ABS, BAS/ESP, Brake, Brake wear indicator
Spoiler deploys and won't retract
Speedometer does not function
Transmission stuck in 'Park'
When able to start and override selector into "Reverse" or "Drive", stuck in limp mode

Here's what I've tried so far:

Removed and examined RCM. I attempted re-solder of three known troublesome spots. Didn't correct the problem. I'm not very good at soldering, and I'm not sure whether there might be some other RCM issue. I wouldn't mind trying to switch out a known good RCM with mine, but haven't done so yet. I'm not sure whether my long list of symptoms is even consistent with the RCM issue.

I replaced my battery less than two years ago with one that was manufactured in 1/14. It is putting out over 12 volts. The battery terminals and ground(s) were not corroded, and I cleaned them just to make sure.

Today I was not able to get it to crank at all. Since I disconnected the battery numerous times as well, there are no codes. I'm sure I would still get the P0600 again if I were able to start the engine, but right now with just the key on, my reader says "pass" - not even any pending codes.

With all the symptoms and especially with things like the speedometer not working, I am worried that it could be a bad PCM.

Any opinions welcomed.

 

I hate to say it but get that battery checked before you spend any money. I had an eight month old battery fail just sitting in the garage over the winter for three months. It was good when I put it away but then it just died. The battery had been in use eight months then it was kaput - poof. The battery was a good one with a total of ten years decreasing warranty.
By the time I got it out and to the place I purchased it from, it was 4 days over the one year free replacement.

 

Thanks for the input. However, I don't think I have a battery issue. But I did investigate it further. It did turn out that from mostly sitting the past couple months (only running every couple weeks) that the battery voltage was low (11.7V). So I had the battery tested (331 CCA) and then recharged and it was 12.5V and 336 CCA. That should be enough to crank the engine in warm weather. That said, I questioned whether the battery was really performing the way it should based on the fact that the battery tester was reading "good". I ended up getting a new (Jan 2016) Autocraft Gold 730CCA under warranty. Voltage was 12.6V on my voltmeter (not sure whether the store tested for CCA). Same no crank result, so I am pretty convinced that it is not the battery - though I may have the store test the CCAs just to make sure.

Here is something else that I may investigate... I just noticed my ignition switch assembly may be a little loose. I've never done the sticky key mod though I plan to eventually - that or just replace the offending part with the MB part #A2024600904. What I am thinking is that maybe I have a bad connection between the ignition switch, the park/brake (autostick) interlock, and PCM. Since the ignition switch output must go through the PCM, is it possible that something related to the ignition switch is the source of problem(s), i.e., would a bad park/brake interlock data going to the PCM result in the PCM disallowing starter cranking, disabling shifting out of park, triggering all the brake and stability control MILs, and triggering a P0600?

 

I bought a refurbished RCM and all the issues went away. I will take some time to more closely examine my bad RCM and attempt to properly repair it and keep it as a spare. I'm surely thankful for all the knowledgeable folks in the forum, and I'm glad I was patient and persistent and didn't have to take it in for what might have been a far more costly service (diagnosis, misdiagnosis, more expensive part, unnecessary parts, etc.)

 

When you do get a chance to look at the RCM pay particular attention to the fuel pump relay and circuitry in the RCM, let us know what you find and I'm glad you got it fixed.

 

Send it to DJ ( tighed1 ), he can tell what is wrong with it and fix it for you as a spare.
( IF he has time, after getting his turbo NA back after 4+ years and 2 weeks )

 

Thanks! I've seen his numerous posts here and his generous offer to repair/exchange RCMs. I think I might try to play with it a bit first. Then if I give up I'll try contacting him. One thing that I can't do is easily compare it to the replacement I just received - the seller placed a bead of silicone sealant around the edge of the cover after doing the refurb and even though it would be trivial to cut through the sealant, I think I'm just going to leave it closed up.

 

Not surprised (just found this thread). A bad "engine control relay" output from the RCM makes the PCM/PTCM not fully power up. YOu get all sorts of odd stuff, but usually, you get a no crank at all. If that line is resistive (relay contacts bad) there is no telling what it will do.

 

Yup. No start, engine cooling fan runs.
Recently had to re-repair a RCM. Owner sent it to me for the Mystery No Start repair. Fixed it, tested it in my car and sent it back. Owner received it and their car would not start.
Tried it again in my car and no start for me either. Replaced engine control relay and all is good.
Relay coils are going bad. Sometimes intermittently.

 

The box the RCM/BCM/PTCM sit in is force-air-vented with cabin air. This keeps temperatures under control in the box - but it also helps keep air moving to move moisture.

IMO, (after 31 years in electronic troubleshooting) sealing the RCM is a bad idea. I suspect the seller (or someone) thought they were sealing out contaminations that might pit the relay contacts, but IMO this is not a good idea.

 

Good point. I think the potential of a condensation issue would be more problematic than that of heat transfer. If the sealant was applied in low humidity environment, condensation would be minimized. If done in a higher humidity environment there could be condensation whenever the RCM is cool. I don't think that sealing the RCM is necessarily bad from a heat transfer perspective since the design of the RCM's housing provides no intentional means of convection, other than any tiny gaps between the tabs and flanges of the housing and RCM assembly. Though plastic is a poor conductor, I'd imagine that the RCM is primarily cooled via conduction through the housing. I'm not sure how hot the relays get, but I'm guessing when functioning properly they shouldn't create all that much heat. I might just leave it as is as an experiment, since I'll be carrying a spare in the future. What do you think?

 

As I mentioned in my original post, I did re-flow solder on the three connections that are reported to be troublesome. More recently I removed the solder, cleaned the surfaces up a bit and re-soldered. I think the connectivity is OK, and I think it was before as well. At 5x magnification things look pretty good "across the board" so to speak.

So I decided to try testing the individual relays (1 through 5, left to right, when opened module is laying flat on circuit board with relays facing you and fuses facing away). I simply used a common 9V battery which I connected to the two small vertical pins on the edge of each relay. Relays 1, 3, 4, and 5 all closed when I applied current to them. However, relay #2 (traction control) would not close when tested. Instead, relay #3 (engine control) would close even though my leads were touching the pins on relay #2. I'm thinking that the traction control relay is bad. Would that make sense, based on all the symptoms I described in my original post? Also, I've noticed that the horn (#1) and air pump (#5) copper coils are a little lighter in color than the middle three. Perhaps a sign of wear on the darker ones?

 

Did you resolve the issue?

 

In the end the issues with my original RCM related to the transmission, speedometer, and various brake/traction control MILs were solved by replacing the traction control relay. Engine would still not crank and continue running until I cleaned the contacts on the engine and fuel pump relays. So, at some point I will likely need to replace those relays as well. For now the RCM works fine and I have also procured a spare.

 

If the contact faces on the relay are pitted and not polished they will soon get pitted again and you will have the same problems with them. Change them now before you get stuck miles from home.

 

I had the "Mystery No Start" condition a few months back. Sent the RCM to DJ (tighed1) for repair. Feedback from DJ was poor solder joints on RCM board along with a couple of relays gone bad. DJ added the "Me So Horny" fix to my RCM & now the horn works without the car running. Best money I ever spent on the XFire.


Since that event, I bought a spare RCM from RockAuto.com &, like you, have it in my spare parts for the XFire.

 

I think that I would put the new one in the car and use the old one as a spare. The newer ones probably are better made as the problems became known to the manufacturers.

 

After replacing 2 bad relays (air pump, fuel pump) in my RCM works great now, I bought a brand new Mercedes OEM RCM as a spare. The new RCM had a manufacture date of 4/2015 so I opened up the new one and it had the same bad 3 solder joints. Repaired the new RCM and use it as a spare. The new RCM that I bought did not have upgraded solder joints. I wonder if the bad solder joints cause relays to over heat and fail, going to see how long the Repaired RCM lasts with the 3 bad solder joints repaired.

 

Thanks for your reply and I'm glad you got it fixed.

 

Excellent point. I was thinking the same thing. If the bad solder joints caused the relays to open and close at a much higher than expected frequency and created more heat, then maybe that caused premature failure of individual relays. If the solder joints are robust at initial installation, then maybe the relays, and thus the RCM, would last indefinitely, which would be great news!


I am assuming your new RCM is manufactured by Stribel? Does anyone know if any other manufacturers actually produce a comparable part? I've never seen a stock photo or physical specimen that was not Stribel. You'd think that at some point they would correct their product defect. But then again, since it's no longer used by the OEM, they get to keep a steady revenue stream. Realistically, there should be/have been an outright safety recall on these RCMs.