w126 cluster compatibility

[nels2173]

Will a w126 gas version instrument cluster work in my 300SDL? The only thing I can see different is glow plug light and that fuel economy gauge. Will all the plugs work etc? Thanks

[nels2173]

I should add that both my speedometer and odometer do not work, the rest of the gauges light etc work fine. I dont care about the odometer but wouldnt mind having a working speedometer. thanks again

[JamesDean]

You likely are not getting any speed signal to the cluster that or power/ground. The speed signal drives the motors that run the speedometer and odometer.

Does your cruise control work? The speed signal is connected to it as well, without it (assuming the rest of the cruise system actually works) the cruise won't work.

The speed signal is generated by a sensor in the tail shaft of the transmission. The cable travels up into a connector to the right of the accelerator. Then from there I believe it goes up to the speedo and down to the cruise, maybe.

Swapping clusters, most likely, will have no effect.

[nels2173]

My cruise does not work either, I know the cable your talking about and it's plugged In for sure but suppose it could be grounding out somewhere? I have check fuses not sure where to look next, maybe run a new cable? Any ideas for me? Thanks

[JamesDean]

If your other gauges are working its unlikely that the problem is a fuse but its never a bad idea to check. Make sure you look for corrosion, etc.

For the speed signal: What I would do is check to see if speed signal is present first. Grab a multi-meter. If it has the ability to read out frequency put it on that mode. Connect your + probe up to pin 11 on the cruise control plug and - to a ground location. The speed sensor will output 40-50 Hz at 25 MPH. If you flip to voltage metering you should see AT LEAST 2.6 V while in motion.

If you Do see the appropriate voltage/frequency then you can likely conclude that the sensor and cable are good.

There is always the possibility that the capacitors in the speedometer pod have aged and failed.

Another thing you can do is pull the cluster, on the 6 or 8 pin connector that plugs into the speedometer pod. You can check for 12V and ground. I can't remember the pinout off the top of my head but I think (If yourre looking at the back of the cluster) the bottom right pin is 12V and two pins to the left of that is ground.

Hope this helps!

[nels2173]

Thanks for all the info, do you know what fuse # the speedometer is on? There seem to be several that say "instrument" on them. If not no big deal.

Also, is the cruise unit under the dash in the knee bolster area? thanks again

[JamesDean]

The speedometer is on the same one as the rest of the cluster I think. I would have to look at my cars to see. Not sure off the top of my head. I had a general policy of whenever I got a MB to just replace all of the fuses with nice new ones.

If your other instruments are functioning I doubt its the fuse

Yes the cruise is under the dash.

Check out the pics here from my 300SD. Its in the same location on your SDL, only there is some additional things in the way.

https://sites.google.com/site/mbcruisecontrolrepair/home/component-locations/amplifier-locations

[nels2173]

And so if the volts/hz are there then I assume its a blown capacitor or something in the cluster and possibly replace that and if not do I look for a ground speed sensor wire or?

[JamesDean]

Originally Posted by nels2173

And so if the volts/hz are there then I assume its a blown capacitor or something in the cluster and possibly replace that and if not do I look for a ground speed sensor wire or?

Sounds like a good plan to me.

1. Verify 12V/ground at speedometer pod 6/8 pin connector
2. Verify Volts/Hz for speed signal presence.

If 1 returns failure, then track down why you've got no power to the pod.

If 1 is ok and 2 fails, then track the wire down and see if you've got a disconnect or break.

If 1 and 2 are both good, then its probably in the cluster itself.

[nels2173]

does look like the pins/process for checking this? Is it easier to see in HZ?

[nels2173]

Remove the connector
test resistance between jack 3 (ground) and jack 5 (sensor wire) 2K ± 200 ohm
If you connect a mulitmeter (dc voltage) to those wires you should read a voltage when driving (30mls/h > 1 volt)

[JamesDean]

Those sound like the right pins. Sounds like a decent process as well.

The voltage should read at least 2.6 Volts RMS on the multimeter.

[nels2173]

The ohms tested ok from what I could tell but wasn't able to get any volt reading while driving down the road, does this mean bad ground, bad sensor? Why else wouldn't I be getting a reading?

[nels2173]

I also tried hz and wasn't getting a signal either

[nels2173]

I also tried hz and wasn't getting a signal either