Got the fabrication done and ready to be installed in the car. In this picture I'm getting the center-line for the teeth of the "trigger wheel".



Stripped off the paint, marked the centerline and the position for the teeth and prepared the surface for welding.



And here's the finished "trigger wheel".



The final test of this strategy will unfortunately need to wait for a bit until I get the engine back in the car and get all the other components hooked up.

Attached Thumbnails

     

__________________
Current Stable

• 380SL (diesel)
• Corvette C5
• Manx
• Baja Bug
• F350 Powerstroke
• Auburn Boattail Speedster replica

I never finished off this thread, so I thought I'd update it with some pertinent information.

First, the solution has been working really well. Absolutely spot on speedometer and rock solid. No balance issues with welding the "teeth" directly to the drive shaft and the pickup has been working great. With one annoying exception...

When stopped, I would periodically get a "bouncing" speedometer and the odometer would slowly increment as well. Moving the car just a couple of inches would cause it to stop. I suspected that the sensor was mounted too close to the "trigger wheel" teeth such that when the sensor and trigger wheel were aligned, the slightest vibration would induce enough movement to trigger a false signal voltage change that would cause it to register with the computer as movement.

While I was under the car recently to just do a visual check and to do a tightness check on all the bolts since the swap was completed and I've got several thousand miles on it, I added some clearance to the sensor/trigger wheel.

That adjustment fixed the problem and now that "bouncing" needle syndrome has been completely eliminated.

So I can say with a great degree of certainty that if you need to add an electronic speedometer to a car without it, and you can't find a transmission or tailcone with the pickup internal, this external solution works. The bracket can be fabricated without welding or any special tools.

__________________
Current Stable

• 380SL (diesel)
• Corvette C5
• Manx
• Baja Bug
• F350 Powerstroke
• Auburn Boattail Speedster replica

nice work paul and thanks for finishing the report!

Question...

What parts are required to make an electronic spedo work in a car that previously had a mechanical spedo? I grabbed a 400E spedo to use in my 606 swap (into my '87 300D). I might have to go back to the junk yard and pull the dash and trace out wires next weekend.

That is a really, really good question. One that I tried to figure out when I was setting up my "driveshaft trigger wheel". Fortunately my car already had the electronic speedo, so all I had to do originally was hook up a trigger wheel and bracket. (Subsequently, I've swapped the 722.4 transmission for a 722.3 that I was able to put the 380SL tailcone onto and get a completely stock electronic speedometer)

Here's the section of the wiring diagram on the 380SL speedometer. It's obviously a "solid state black box" but this diagram only shows power and ground and an output to the cruise control. I have no idea where the VR pulses get to the speedo.



The wires on the VR sensor are solid brown and solid white (at least on the 380) so you might be able to trace them. It definitely appears that the cruise control is not a source, since the speedo provides only output to the CC.

Please post anything you find, because I, for one, am definitely interested in knowing how this works.

Attached Thumbnails

 

__________________
Current Stable

• 380SL (diesel)
• Corvette C5
• Manx
• Baja Bug
• F350 Powerstroke
• Auburn Boattail Speedster replica

Looks like I will go back to the junk yard next Saturday and spend some time pulling the dash and steering wheel to trace out the wiring.

I'm doing a 722.6 swap and will need something other than the small display that shows speed. I did grab the 400E spedo so I will get back asap to figure out the w124 system so I can retrofit it into my car. I do not have any welding capability so I might have to farm that out.

You might want to start at the other end and work back towards the speedometer. It might be as simple as determining that the VR sensor hooks directly to the speedometer black box. On the otherhand you may find that there is some kind of a "speedometer amp" or some other electrical device sitting between the sensor and the actual speedometer module, perhaps analogous to the tach amp (or EGR controller) for the 123s tachometer. Those devices also use a VR sensor as the input for the gauge.

__________________
Current Stable

• 380SL (diesel)
• Corvette C5
• Manx
• Baja Bug
• F350 Powerstroke
• Auburn Boattail Speedster replica

Interesting. I will have to do that. I clipped three plugs to keep for the gauge. I'm going to make a new thread and reference this one just to keep this one organized

A lot of work has been done to adapt. Has anyone ever considered a bicycle speedometer. A magnet could be applied to the rear axle or wheel assembly and the sensor put in the dash.

There are units made for motorcycles like the trail tech vapor that includes a tach and temperature gauge that is configurable.

Just saying.

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85 Merc 300D - Unwinding 31 years of wear
86 VW TD
Mahindra Diesel
Iseki Diesel

In 2007 I didn't own a diesel.

Some people want the quick and easy solution and others want it to be done correctly. I prefer it to be correct.

Hi Don,
If we were showing these cars, the only "correct" way would be completely stock. How boring would that be.

I pointed out a solution that will give correct speed an mileage. It is easier. It uses a reed switch that is tripped by a magnet. Appropriate technology for the rpm involved (600).

__________________
85 Merc 300D - Unwinding 31 years of wear
86 VW TD
Mahindra Diesel
Iseki Diesel

In 2007 I didn't own a diesel.

Really nice.

Wondering what speedometer, sensor and any other electronics/wiring you did as i'd like to replicate this on my 5 speed without mechanical speedo.

If you have a mechanical speedometer in place you might want to look into a GPS speedometer cable drive box. Once calibrated to how many cable revs/mile, you can change tire sizes, transmissions, rear end ratios, etc and it stays accurate because it's getting your speed from GPS.

I've never used one but this seems to be the "in thing" for transmission swaps and rear end ratio changes.

That's true, however, i would really rather one driven from the wheels turning rather than GPS.


GPS just tells me what i was doing a few seconds earlier where as a speedometer driven from the wheels is " now"


I have a gps on my 5 speed 717.422 transmission swap :/

I'm driving a Mercedes diesel, so the speed I was going a few seconds ago never differs a whole lot from what I am going now, even with the pedal on the floor.

Thanks for the info though, that was an potential issue I had never considered with those.