Guide: How to get the tach working in a w126 gas to diesel conversion - PeachParts Mercedes-Benz Forum

dude99 · #1 06-09-2012, 01:22 AM

I bought my w126 already having been converted from a 380se to a 300sd and there have been a few things that I have noticed that wen't quite right. One thing that had bugged me from day one was that the fellow I bought it from that converted it hadn't gotten the tach working. He had plugged a tach amp into the socket under the hood, put in the crank sensor, and put a diesel tach in the cluster (I didn't know he had put in a new tach) but the needle still didn't budge when I drove it.
I didn't bother to look into it until I found a Tach, pickup, and wiring harness from the wreckers in case I needed something. I finally found what was left of a 300sd in a wrecking yard that had what I needed (diesel w126's seem to be very rare in yards by me), so today had at it and got it working. If you have a gasser that you have dropped a diesel into, this is what you need to do to get the tach working.
A couple notes before we begin, I read in several places while researching how to do this that you could simply plug a tach amp into the gassers diagnostic port and plug in a diesel tach and have it work. Well let me set the record straight. That absolutely doesn't work... I tried it first.... also, you need a diesel tach, a gas one will not work, and you can't convert a gas one to a diesel one.

What you will need:

A diesel tach

A tach amp socket (you could make the one already in your gas w126 work by figuring out the pin outs and what not but getting the diesel one is just easier)

A tach amp

A crank sensor if your diesel doen't have one already, some wire, a spade connector, and a soldering iron and solder (butt connectors would work to)

Ok, Step 1
Remove the cluster and take it inside. Place it on a nice soft towel an swap out the gas tach for the diesel one. This is the easy part. It should only be held in with a few screws.

Step 2
Remove the gasser tack plug (actually a diagnostic port) leaving the wires behind.

Step 3
The only wire we can use that originally went to the diagnostic port is the solid brown ground wire. We will have to run a new signal wire to the tach as well as a switched power source to the tach amp. First we'll connect the brown wire from the new tach amp socket to the ground from the old diagnostic port and solder them together. This is also a good time to tape up all the remaining wires from the old diagnostic port and push them away. Then solder the green signal wire from the tach amp socket to your new piese of wire that we'll be running to the tach. I used white wire for this. Why you ask? Because it's what I had laying around at the time and I'm cheap and didn't feel like going out to buy some nice mathcing green wire... anyways.... it should look like this

and then this

Step 4
Run the new signal wire (white) into the cabin. I routed mine through the rubber grommet where the vacuum lines enter the cabin behind the fusebox. It doesn't matter really how you do it. Then fish them up in behind the cluster opening.

Step 5
Connecting up the tack. The Tach runs off of two wires, a power wire (black with red stripe) and a signal wire (green with black stripe). You'll need to strip back a few inches of the protective sheeth to get to them.

What we need to do is cut the signal wire so that it has a few inches leading to the connector and then solder our new signal wire that we ran into the cabin to it, like this

Step 6
Reinstall the cluster and reconnect all of the various wires going to it including the tach.

Step 7
Now we need to run switched power to the tack amp sockets black wire. I couldn't find an easily accessible wire that was switched in the area so I took my power off of fuse 14 in the fuse box using a spade connector. I had to make a small notch in the back of the fuse box to allow the wire to pass out of it.
Note: There is a red 12v wire that ran to the original diagnostic port. I didn't use it because it wasn't switched and I didn't want to drain my battery.

Step 8
Solder the new power wire to the black wire coming out of the tack amp socket.

Step 9
Install the Crank sensor if you need to. Odds are your diesel already has one attached but if it doesn't the sensor mounts to the bracket above the crank pulley. The connector end of the sensor wire plugs into the bottom of the tach amp socket. Here is a picture of the crank sensor plug

Step 10
Bolt the tach amp socket with the crank sensor plugged into it to where the original diagnostic port was mounted. Then attach the tack amp to the socket. I should note that I disassembled and re-soldered all the connections inside my tach amp before using it as they are notorious for having bad connections inside.

Step 11
Start it up and hopefully watch the needle sweep. I can't say how nice it was to finally see mine move...

degier · #2 12-14-2012, 09:39 AM

Nice write up. Thanks!

vstech · #3 12-14-2012, 10:06 AM

I agree, nice write up.
but you need to get all those aluminum fuses out of your box!

Quote:

Originally Posted by dude99

I bought my w126 already having been converted from a 380se to a 300sd and there have been a few things that I have noticed that wen't quite right. One thing that had bugged me from day one was that the fellow I bought it from that converted it hadn't gotten the tach working. He had plugged a tach amp into the socket under the hood, put in the crank sensor, and put a diesel tach in the cluster (I didn't know he had put in a new tach) but the needle still didn't budge when I drove it.
I didn't bother to look into it until I found a Tach, pickup, and wiring harness from the wreckers in case I needed something. I finally found what was left of a 300sd in a wrecking yard that had what I needed (diesel w126's seem to be very rare in yards by me), so today had at it and got it working. If you have a gasser that you have dropped a diesel into, this is what you need to do to get the tach working.
A couple notes before we begin, I read in several places while researching how to do this that you could simply plug a tach amp into the gassers diagnostic port and plug in a diesel tach and have it work. Well let me set the record straight. That absolutely doesn't work... I tried it first.... also, you need a diesel tach, a gas one will not work, and you can't convert a gas one to a diesel one.

What you will need:

A diesel tach

A tach amp socket (you could make the one already in your gas w126 work by figuring out the pin outs and what not but getting the diesel one is just easier)

A tach amp

A crank sensor if your diesel doen't have one already, some wire, a spade connector, and a soldering iron and solder (butt connectors would work to)

Ok, Step 1
Remove the cluster and take it inside. Place it on a nice soft towel an swap out the gas tach for the diesel one. This is the easy part. It should only be held in with a few screws.

Step 2
Remove the gasser tack plug (actually a diagnostic port) leaving the wires behind.

Step 3
The only wire we can use that originally went to the diagnostic port is the solid brown ground wire. We will have to run a new signal wire to the tach as well as a switched power source to the tach amp. First we'll connect the brown wire from the new tach amp socket to the ground from the old diagnostic port and solder them together. This is also a good time to tape up all the remaining wires from the old diagnostic port and push them away. Then solder the green signal wire from the tach amp socket to your new piese of wire that we'll be running to the tach. I used white wire for this. Why you ask? Because it's what I had laying around at the time and I'm cheap and didn't feel like going out to buy some nice mathcing green wire... anyways.... it should look like this

and then this

Step 4
Run the new signal wire (white) into the cabin. I routed mine through the rubber grommet where the vacuum lines enter the cabin behind the fusebox. It doesn't matter really how you do it. Then fish them up in behind the cluster opening.

Step 5
Connecting up the tack. The Tach runs off of two wires, a power wire (black with red stripe) and a signal wire (green with black stripe). You'll need to strip back a few inches of the protective sheeth to get to them.

What we need to do is cut the signal wire so that it has a few inches leading to the connector and then solder our new signal wire that we ran into the cabin to it, like this

Step 6
Reinstall the cluster and reconnect all of the various wires going to it including the tach.

Step 7
Now we need to run switched power to the tack amp sockets black wire. I couldn't find an easily accessible wire that was switched in the area so I took my power off of fuse 14 in the fuse box using a spade connector. I had to make a small notch in the back of the fuse box to allow the wire to pass out of it.
Note: There is a red 12v wire that ran to the original diagnostic port. I didn't use it because it wasn't switched and I didn't want to drain my battery.

Step 8
Solder the new power wire to the black wire coming out of the tack amp socket.

Step 9
Install the Crank sensor if you need to. Odds are your diesel already has one attached but if it doesn't the sensor mounts to the bracket above the crank pulley. The connector end of the sensor wire plugs into the bottom of the tach amp socket. Here is a picture of the crank sensor plug

Step 10
Bolt the tach amp socket with the crank sensor plugged into it to where the original diagnostic port was mounted. Then attach the tack amp to the socket. I should note that I disassembled and re-soldered all the connections inside my tach amp before using it as they are notorious for having bad connections inside.

Step 11
Start it up and hopefully watch the needle sweep. I can't say how nice it was to finally see mine move...