Disassembly of the TC cluster. This is where I modified my original 170mph face plate to except the digital clock at the bottom of my original odometer. I also used my cluster housing during assembly since it was still in very good shape.

 

TC cluster assembly completed and installed for testing of the instrument gauges. All of the original factory connectors will go back into their prior locations on the back of the TC cluster with the exception of the speed sensor wire(more on that later).

NOTE: Disconnect the battery before you attempt to remove/install the instrument cluster.

In this set of pictures I have the TC cluster installed and you can see that all of the instruments/lights are working as they should with the exception of the low fuel warning lamp(more on that later).

(These pics were taken before I installed the digital clock)

 

I'm diggin' it! Looking forward to the finished set up.

 

Here is a view of an original TC wiring harness. This particular harness was removed from an early Gen2 500SEL and is the one I am going to copy off of since I am with out one. Though this harness has a few cut wires that before I had no clue where they terminated are now mostly resolved. I found the trip computer wiring diagram(Schematics)on a Russian web site and it was confirmed to be what we needed not only to assist in the repair of the original TC harness but the actual installation of a Gen2 Trip computer too and that was a huge step forward.

I have the TC mounted in the 560 right now and I have run into another problem, my TC display starts to fade out and is completely out after about 3 or 4 minutes but if I gently push in on the TC connector it will come back on and I believe this may be caused by a few cold solder joints on the circuit board.

 

When I first tried out the TC, I had it setting in the engine bay next to the battery for ease of getting power/ground connections because that was all I needed at the time because I was trying to figure out why there were no illumination lights in the display nor the control switch. So before I found the schematics there was no choice but to take all of the protective loom from the wiring and trace out the illumination circuit and I started off by opening up the control switch(S67)where I found the illumination circuit.

The grey w/pink stripe turned out to be the light circuit, pin 7 on the control switch, I found pins 7&8 to be reversed @ the connector so I simply put them back the way they needed to be and now I have illumination on the display and control switch.

But as mentioned before, the display still fades out after 3 or 4 minutes and is totally gone.

 

Excellent...I will be following your progress with great interest, as I plan on overhauling my factory installed RR unit shortly. Keep up the great work!

 

added to DIY.

 

Absolutely amazing...! I am going to read it in further detail when I have a bit more time.

 

An excellent contribution. Thank you for spending the time to document your work.

 

Grumpy560 and I have been working on this project now for a while and spent many hours on the phone back and forth figuring out the wiring harness and differences in Gen I and Gen II units.

I must say that Steve is a brave guy in tackling this head on. We currently have no forum member who has retrofitted one of these units active and to my knowledge no US-spec car was ever retrofitted with the trip computer. He is missing the wiring harness to plug the "brain box" to the cluster and to the other components the brain box gets its information. I sent him my wiring harness that I pulled from an early 1986 model 500SEL Euro. The production date was 09.1985 making it one of the very first Gen II cars produced.

When I was pulling the wiring harness, I did so with the dashboard and center console in place. Knowing what I learned from the experience, I would have removed the dash completely to make this removal easier. As a result to tight spacing and no way of removing some of the harness intact, I had to cut some wires off.

Now the fun part begins, a fellow forum member also has a trip computer and is planning the retrofit. His unit is a Gen I and it turns out to have a different wiring harness where part of the harness pierces the firewall and connects directly to (I believe) the potentiometer under the fuel distributor. On the Gen II unit, there is no wire that goes to the engine bay but apparently connects to the ECU in the area of the passenger foot well.

So Steve and I had a large group of wires, some cut some not and with no leads on whats what. Steve was able to figure a few of the wires out and found an error in my re-installation of the controller socket where I switched pins 7 and 8(illumination and ground).

Eventually we've found a wiring schematic for a trip computer from what I believe is a W124. Knowing that the W124 is a post 1985 chassis, I assumed the wiring harness will be identical or close enough to the Gen II W126 from the same time period. I was able to blow up the wiring schematics clearly on my screen and translated each cable from the brain box to the cluster and the corresponding color of each individual cable. In addition to the brain box, we were able to translate and figure out cables that connect to the fuse box and into which # fuse.

Just to reiterate what Steve mentioned in one of his initial posts, the trip computer brain box/system is specific to either V8 380,420,500,560 or I6 260,300 M103's. I have not seen or heard of a 280 M110 with trip computer so I don't think there was a Gen I 6-cyl version. I have seen a 380 Gen I with a TC, Kalifornia has a 500 with TC, my TC came out of a 500 and Mike in NY has a 560 with a TC. So pretty much every V8 Gen I or Gen II had a TC option. Regarding the TC on the 6-cyl models, I've seen 300's in both W124 and W126 with the option, and probably(haven't confirmed) the 260 may have had the option, too. I have not compared the TC part numbers to see if there were any differences between the 6-cyl M103 and M104 motors but I believe they would be the same.

TC were found on very few cars, I would estimate around 1-2% of all W126's produced as having the option. Besides the W126, they were also found on W124's and on the R107/C107 cars in even smaller numbers.

After studying the schematics, both Steve and I have stumbled on one last little hurdle. The schematics show that a cable or 2 connects to the X-61 junction box. This junction box is supposedly located in the passenger area near the floor/front door area. Steve found that our US-spec cars do not have this junction box. I decided to go to a yard to find a Euro and see if perhaps this was a Euro-only item or not. The Euro I found in the yard was a 1983 500SEC (imported by Don Minkoff, who I believe is related to the Minkoff forum member we have on the board). I looked for the junction box and found no such thing. Either this was a Gen II Euro item or it came out later from 1984 on as TC's were available starting I believe 1984.

I'm hoping that either of our forum members with TC on their W126's (Gen II) would be kind enough to snap some pictures of the area of where the X-61 junction box is located.

One last thing that I'm working on figuring out is whether or not the digital clock is necessary for the trip computer to work and function correctly. I would like to keep my original speedometer or my non-TC AMG speedometer and still be able to use the TC. After hanging over on the German forums and talking in private with a member on this topic, he mentioned that one of their forum members installed a TC to his 300SE and kept his original speedometer without the digital clock. Steve believes that the DC is needed for the brain box to collect and calculate time to distance and other functions the TC does. So at this point, we'll have to investigate this bit further. If the DC is in fact needed, the DC could be jumpered and tucked in any other place.

I must say that going at this alone without any shared knowledge from either party is a scary thing. Especially if you have no one to turn to if you are at a roadblock. Now that both Steve and I have pooled in our knowledge we are very confident and believe the installation will be not as difficult as we initially thought. Just a few minor things left and we'll have a complete and detailed instructions on how to retrofit this rare and cool unit into our W126's and possibly W124's and R107's!:thumbsup:

 

i found some info on the trip computer retrofit on a german site, here's some info:
-the clock is not "especially" necessary for the function of the trip computer (it didn't go into any more detail than that)
-the x-61 connector box (input from the KE-control unit) was standard on GenII (i'm assuming they mean Euro) only
-brain box and display is identical for Gen I and II
-digital clock from W124 will not work in W126

 

You guys are welcome to snap pics of the TC unit(s) on my 500SE at some future GTG.

Deal?

 

Yeah, my junk is available ATM. Still don't have the interior back together so just let me know.

 

Thanks to all of those who have replied and are willing to help sort this out.

Since my last post I have been trying to sort out the problem with my TC display. The display would fade out shortly after it was turned on so I removed everything from the car again, looked over all the details of the wiring harness and found nothing wrong in that area. Made sure all connections were good to ground and power and that all pins were making contact. I then put everything back into the 560 and still had the TC display fade out after a few minutes but when light pressure was applied to the rear of the display it would come back on normally.

So I removed the cluster by itself and disassembled it completely. I took the TC display alone and put it back in the car, hooked it up with nothing else, low and behold I had a clear functioning display but I noticed that there were no illumination lamps on and the orange arrows were no longer lit.
I then installed the gauge cluster carefully next to the TC display, hooked up the battery and everything worked except for the back ground lighting in the TC display but the digital display remained lit and did not fade out. So I moved the gauge cluster over and put the odometer in place and hooked it up and and had the same result as above. WTF, everything works like it should with the separated units hooked up and out of the cluster housing but they won't work hooked up mounted in the housing.

So again I removed everything from the car and laid the parts out on my work bench. I started to reassemble each unit back into the cluster housing starting off with the TC display unit. I then put the gauge cluster unit in and when I was installing the odometer housing I noticed a thin metal strip at the bottom. This is a grounding strap that connects all 3 gauges together. I payed close attention while installing the odometer and noticed that the metal strip was not making a good grounding contact on the TC display unit, even though there is a screw to secure them together once the units are in place mine had some form of residue, sort of like what masking tape will leave behind if you let it get wet. I cleaned that metal strip and finished installing the units into the cluster. It was late when I had gone through all of this but I put the cluster back into the car and I now have a fully operational TC though as me and Dr.Grillz have discovered it will not function properly without the correct computer chip(more on that during the installation)@ the rear of the brain box.

 

Now that my TC display fault is out of the way and all of the wiring destinations have been sorted out I can move forward and start putting together a wiring harness for my TC.

I had a trip to the Nashville pick n pull planned this morning but the weather here was not good and I did not want to chance driving all the way there only to have to pull what I needed in the pouring down rain.

In all there are 9 wires coming out of the TC harness that will need to be terminated. again, this is only for a Gen2 W126 with the M116/M117 V8 engine.

1. Grey/Pink..................Illumination circuit. This is terminal X6/1(1pole)and is located in the center console, mounted to the heater box @ the left side of the CCU.
(note) there are 2 grey/pink wires loomed together from the TC harness.

2. Brown.......................Ground circuit. There are 2 ground wires loomed together from the TC harness. They terminate @ the W1 Ground bundle behind the Instrument cluster and to the left.

3. Green/black stripe.......Speed sensor input for TC harness. Terminal block X21/2 located behind the instrument cluster. There are actually 2 parts to this. One from the TC harness and one teed from the chassis harness from the speedometer through a Y connector.

The next 4 wires from the TC harness go directly into the fuse box.

4. Black/pink..................TC display and brain box power supply from accessory switch. Terminal 15 fuse # 6 in the fuse box.
(note) there are 2 Black/pink wires loomed together from the TC harness.

5. White/Blue stripe.........TC Diagnostic port. Terminal X29 normally located in front of the fuse box @ the right side however U.S. marketed Gen2 W126's do not have such terminal and is a Euro only option. This single wire does not need to be connected and can be deleted from the install if you do not wish to have a diagnostic port.

6. Grey/white..................TC background display lighting. Terminal 15 fuse # 7 in the fuse box.
(note) the background lighting is full time even when the TC is not in use.

7. Grey/Red..................... TC Low fuel warning lamp system/engine run mode signal. Terminal 58R fuse 11 in the fuse box.

Note: The above mentioned wire designations are labeled per the original schematics and I will give further detail on the changes for #s 6&7 on U.S. W126's
........................................................................................................

8. Solid Blue wire..............TC brain box memory back up. Terminal X35 located in front of battery on outside of false firewall. Connect to terminal 30/61 Battery.

9. Yellow/Violet.................TC signal from ECU. Terminal X61 located behind the passenger side kick panel & below the ECU.
(NOTE) The X61 does not exist on Gen2 U.S. market W126's. This was a Euro option only. The Yellow/Violet must terminate to pin #4 on the ECU connector.

(note)
#'s 8&9 above are loomed together from the TC harness and go behind the heater box over to the passenger kick panel and terminate at the X61 block below the ECU on Euro models as mentioned above, thus we can tap into the existing engine harness or you can choose to open up the connector at the ECU and add the Yellow/Violet to pin #4.

I will give better detail during the making of my wiring harness.

 

Thank you for an excellent thread. The TC and tach on my 86 euro 420 SE appear to have a ground problem. Tach slowly died but occasionally when driving over rough terrain or wind gusts hit the car from the left it flickers but does not read. So I know tach is good. TC was also working sometimes does not. Tach problem arose after an MB indie replaced the entire ignition switch unit so I assume the ground problem was caused there. I'm not electrically inclined at all so perhaps a photo of the ground wire for the tach location at the cluster and location at the fuse box would help me. BTW I noticed my car has what might be the "junction box" under the floorboard at the passenger side; a small square plastic box with a bunch of wires coming in and out of it.

 

One last glance of the TC in operation before I removed everything to begin work on the new wiring harness. I made a trip out to the local pick a part and tore into a 1988 300SEL to retrieve some more of the parts that I needed.

I was after the main wiring harness but I did not go that far, instead I removed just a portion of a harness from the driver side lower dash and on to the back seat and that included all of the wiring harnesses from the doors. This would give me most of what I would need to do the job and would cut down on the pins that would need to be soldered on afterwards if I sorted out the harness in sections.
All of the wires going to the brain box will have connector pins already so it seemed easier to go that route instead of spending a few hundred on new wire and I see nothing wrong with reusing what I pulled from a parts car though I did check all of the wiring thoroughly before I started assembling the TC harness.

I am taking this one step at a time because I have to change most all of the color codes from the original harness so my new harness will not match that of the one I am making a copy of but it will function just the same. There are 23 wires in all that go directly to the TC brain box connector and I have managed to complete 15 of them as of tonight. The control switch and TC display harness is done but the most time consuming part will be soldering the new pins to the end of each wire once I have them all in place.

This would have been so much easier if I only had the original TC harness to start with but this makes the install much more interesting to say the least.

 

I`m sorry that I did not log in for a while and I did not see this topic. Maybe I could help you with some info regarding of the harness for the TC. My 500SE have this option and in this moment the car is taken apart for a rebuilt (after an accident) so I have acces to all the parts that you are interested... Please, if you still need some more info, ask me and I will help you as much as I can.

 

Dan500se, would you happen to know what your differential ratio is? Also, if you have the part number for the black "chip" that's located on the back of the silver brain box it would help in determining any differences between other models.

 

Dan500se, would you happen to know what your differential ratio is?...

I do not know it, but I can check it out if you can tell me where I can find the ration on the diff (I know that is there somewhere...)

...Also, if you have the part number for the black "chip" that's located on the back of the silver brain box it would help in determining any differences between other models.

Tomorrow (now is 22.00 CET) I will be where I keep my cars and I can look at that "chip". I will take some photos if that will help...

 

Moving right along as the newly made harness is now ready to have the pins soldered on. Once I finished inserting all of the wires into the connector I went back and double checked each and every one to make sure the colors matched the numbered locations I had written down.

Once I have all of the soldering work completed I can put protective loom on the harness and tag each connector for it's terminated location. some of the wiring loom I removed from the donor harness will be used in places and I will use heat-shrink for other parts of it so it should look almost factory like when it's done.

 

Steve if you & Mclare ever got together on a wiring project what a team you would make.
Inspirational work- keep it coming

 

Awesome work, Steve! Bringing a rare and cool option to life in your SEL.

Al

 

The new TC wire harness is about 70% complete and will probably be ready for permanent installation by the end of the up coming weekend. I still have about 31 terminals to solder on to the wire ends and it is a slow process as I have to remove the old wire from the pins so I can use them again and I need to make sure I have a good solder joint.

Here is some of what I have done and as you can see there is still a slew of loose ends especially at the TC brain box connector and that one will take most of a full day for me to do since there are so many bundled up there.

 

The TC harness is now completed and ready for installation into the 560. This will take some time as I need to position the harness accordingly so that the TC brain box can be relocated to the passenger side compartment.

As you can see I have wrapped the main harness with yellow tape and the leads out from there are marked with white tape.

I still have one more short harness to make and that will be to add the X61 terminal block that U.S. market W126's did not have from the factory. I will need to run a single(fused)wire from the X35 terminal block located in front of the battery on the false firewall into the passenger foot well where it will be joined by another single wire that leads from the ECU @ pin #4 and these 2 wires will have a connector in place that will plug into the TC harness @ X61(red)connector below the ECU.

I will run the TC keypad controller harness down the passenger side of the trans tunnel to the switch.

The round plug at the end of the harness plugs into the TC display behind the cluster, the speed sensor input/output was originally in 2 sections but I changed that on my harness and deleted the need for terminal block X21/2 on my install.

NOTE: Pay close attention to the White/Blue wire in the original harness @ pin #32 on the TC brain box connector. This White/Blue wire, along with the power wires went into the fuse box. The White/Blue does NOT terminate in the fuse box but merely goes through and out the front along side other wires where it ends as a single pole connector known as the TC diagnostic test port @ X29/3 on Euro models. DO NOT terminate the White/Blue wire to any fuses in the fuse box. I was going to delete this single wire as I felt there would be no need for it but I went ahead and left it in there. This applies to both Gen1 and Gen2 versions of the TC brain box.

I have ordered new vac pods, lines and fittings along with a complete AC O-ring kit.

Here is how everything looks as the harness has been partially set in place.....................

 

NOTE: Pay close attention to the White/Blue wire in the original harness @ pin #32 on the TC brain box connector. This White/Blue wire, along with the power wires went into the fuse box. The White/Blue does NOT terminate in the fuse box but merely goes through and out the front along side other wires where it ends as a single pole connector known as the TC diagnostic test port @ X29/3 on Euro models. DO NOT terminate the White/Blue wire to any fuses in the fuse box. I was going to delete this single wire as I felt there would be no need for it but I went ahead and left it in there. This applies to both Gen1 and Gen2 versions of the TC brain box.

Wow, you really saved me there! Now I just need to look up what the test port looks like.

Great write up! Easy to understand, no little abbreviated code words that I need to know to understand whats what, great pictures and easy to follow. Thanks!

 

Over the weekend I completed the wiring harness for my TC and partially installed it into the 560.

After the harness was in place I started terminating the wires to the designated locations mentioned earlier in the thread. I saved the fuse box connections for last and went on installing the TC components.

I have just a few wires left to terminate and they are in the fuse box and of course @ the X61 terminal where I still need to make another short harness that will go from the ECU area and out to the X35 terminal block on the false firewall. I will add a 10amp inline fuse at this location. This is where I will tap into the ECU @ pin #4. As you can see in one of the pics posted earlier, the red connector below my ECU is now my X61 junction location.

Oh, the TC is operational in most modes now but I still have a few minor details to work out in the fuse box(more on that later)but we are almost done with the install of a Euro trip computer into a U.S. version W126. In my last post on this subject I will explain what changes are going to take place in the fuse box concerning the correct fuse #s that the TC will need to be terminated too.