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Discussion Starter #1
First off, hello everyone on allpar! I just joined because every time I do a google search for something on my truck (a 1990 d150) I wind up here reading something interesting, so this seems to be THE place to find information on mopar! By joining I hope to be able to get more specific assistance with my vehicle, and maybe I can share some of the things I have learned owning a dodge as well.

I have a 1990 Dodge D150 S base model - regular cab, long bed, 3.9L with a nv535/np2500 trans - two wheel drive.

The problem: My speed/distance sensor has gone goofy, especially first thing in the morning; I'm in second gear and my speedometer says I'm going 90. I know it is the sensor and it needs to be replaced, but I also know that the replacement will be the same poor design - ie reed switches and a magnet. I went to the parts store and looked at the replacement sensor, then I asked to see the sensor from a 1994 ram. The later sensor has three pins, meaning it is almost certainly Hall effect, and far more reliable. Switching my truck over to a Hall effect is no big deal for me - one or two transistors and I can easily convert the signal. The problem is in the mounting; my sensor mounts the same as a conventional speedo cable would - a threaded fitting with a square cable end. The newer sensor appears to snap into the transmission and use a bolt for attachment.

If anyone knows of a compatible Hall effect sensor for my vehicle (even if it doesn't send the same number of pulses per revolution) or some off-the-shelf adapter, I will post schematics and instructions so anyone else can make the same upgrade.
 

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Reed switches are reliable. Seems to have lasted you 22 years in a harsh environment til now. I'd get an OEM type replacement. Hall effect is also a magnetic switch.
 

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Welcome to Allpar! I would personally recommend just replacing the sensor with the OEM style. If you think about it, the sensor has lasted 22 years before wearing out, so if you replace it, you should get at least another 10 years out of the OEM style sensor, which puts the truck at 32 years old. This is not a common failure part, and I think you're going to be looking at more trouble trying to go with an updated sensor than simply putting in the old style. Just because it can be upgraded doesn't necessarily mean it's a good idea to do it. Of course, all of my electronics projects seem to end up letting that grey smoke out, so I'm naturally kind of wary of anything in that vein... If you do try to do it, you will be making a pioneering effort, this is the first I've heard of anyone trying to attempt this. Best of luck if you do decide to do it, and keep us posted!
 

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I'm not sure about this particular vehicle, but the type of upgrade that you are considering has been done on the Chrysler FWD cars. Both styles were held in with a 10 mm bolt and the opening was identical in size. The 3 wire sensor required a source voltage which could be obtained by tapping into the TPS source voltage line. The other two wires were mated to the original 2 wires and that is all it took. I made this change on my 91 Sundance just to see if it really worked. There was no problems.

I tend to agree with the others here in that it is probably just extra work that is not needed. Be absolutely certain to clean the sensor and wiring harness contacts with a good contact cleaner. That is often where the problem is.
 

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Discussion Starter #5
Thanks everyone. You make a good argument for the old style sensor. Still haven't gotten around to changing it, but I've had other issues since then. Wanted to swap in a 318... ended up swapping in a 360. Turned into a two week nightmare, but it runs and drives. Just a matter of time before the tranny explodes. In the process tho I learned more than I ever wanted to know about the differences (and similarities) between the 3.9, 318, and 360 - including motor mounts, flywheels, harmonic balancers, pilot bushings, and etc. Cranks are different between automatics and standards. If anyone else is considering doing the swap just expect to spend about twice as much as you planned and message me if you have any questions.
 
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