Diagnosing speedo...

[CGauthier]

Ok guys, I'm trying to diagnose the electrical part of the speedo and the first thing i want to check is the vss at the transfer case. Now, I'm wondering if there's a way to do it with an ohmeter? Anyone know if there should be continuity with the thing just sitting there? If you jack up the back tires and let the wheels spin should you get some kind of voltage reading? I wanted to try this before draining all the oil out to take the thing outta the trans . If anyone knows anything in this area please let me know...



TIA,



Carl G

 

[CGauthier]

Ok, I just noticed something today that I think must be a clue. As I went to partsource today to grap some snap ring pliers I pull up to the window at the store and I noticed that the lights weren't on. I instantly got out of the truck and gave it one of these <a href='http://www.smileycentral.com/?partner=ZSzeb008' target='_blank'>#ad
</a> It may be coincidence but then again, it may not; but both the speedo and the motion sensor not working is a little spooky. Anyone know how these things tie together if they do? If anything, to me it's starting to look like the vss, IF the motion sensor works through it. Any thoughts?



Thanks,



Carl G

 

[dpuckett]

I'm a little about the motion sensor myself. You shouldnt have to drain anything to remove the VSS and adaptor. At least I've never had to, and I've had my share of fits with the darn things. Unscrew the VSS from the adaptor, and put a drill (sans bit, of course) onto the end of it and spin it. There is no backwards- it will register speed/ distance either direction. If the speedo moves when you hook up the drill, then you have a mechanical problem. If it doesnt move, then you will want to check the connector itself for any loose wires or bad connections. Best way to do this is to disassemble the harness and get down to the wires themselves. If the female ends arent perfectly round, it wont make good contact, and you will get either erratic or NO speedo operation.



Beyond that, I dont have any experience.



Daniel

 

[CGauthier]

Thanks Daniel... looks like i got an electrical problem then. I didn't have a heck of alot of time but the wires look like they've been played around with or something... not tucked neatly against the frame or nuthin, just kinda dangling there; with some tape slowly falling off. One thing that I would have liked to explore further is that i had to feel over top the xfer case to tell where the wire coming off the vss was goin, and it splits off in two directions-I'm gonna find out where. The female connectors looked good to me. It seems like the condition is getting worse all the time; I mean the speedo is not working alot more than it is working lately I'll get to the bottom of this <a href='http://www.smileycentral.com/?partner=ZSzeb008' target='_blank'>#ad
</a> If anyone knows a way to test the vss please let me know... .



Thanks,



Carl G

 

[Philip]

Carl



The wireing splits off to the switch that turns on your 4X4 indicator light on the dash.



Here is some troublshooting to keep you busy for a while. Unscrew the little step box where it goes into the side of the transfer case. Jack the truck up and support it properly. Start the truck and let it idle in gear. If the adapter comeing out of the case is turning. It is good.



If it was turning. Unscrew the VSS from that box and screw it straight onto the transfer case adapter. Then start the truck back up. Run it in gear. If the speedo works. Then most likely that little gear reducer box is bad. Open it up and look at the gearing in it. See if it has ground itself apart inside.



Let me know what you find. The wireing for the trans just kind of flops like you think. I had the wires to my VSS rub threw a while back.

 

[CGauthier]

Philip-I think I've got it down to being an electrical problem just because i did what Daniel said and spun the shaft of the vss around on a cordless drill and the speedo didn't budge... I may be wrong thou.

The dealer-what a freakin joke. I went there to ask about the vss, how much $$ it was ($100 Cad which was a little more than i hoped), if i could return it if it wasn't the problem (answer was no)-and how to test it,he started talking about an ohmeter then stopped... i dont no if he stopped because he didn't know what he was talking about or if he wanted to get my truck into the shop but I couldnt get him to finish off what he was saying about the ohmeter. He just kept saying the truck had to be hooked onto the shop computer--yeah right. Those homers took three hours to find out that the switch on the xfer case was the reason for the light not coming on on the dash... there's gona be alot of snowballs in Hell before they see my truck again <a href='http://www.smileycentral.com/?partner=ZSzeb008' target='_blank'>#ad
</a>



Carl G



If anyone has a little spare time, and ohmeter and feel like taking a reading on the two leads of a known working vss? Mine read like i touched the two ohmeter leads together, no resistance at all; I'm not sure how it's supposed to be thou. Thanx

 

[Paychk]

Carl, at the sensor disconnect, you should see less than 10 ohms resistance to ground on the black/light blue tracer wire. The white/orange tracer wire should have 4 or more volts with the key on. The power train diagnostics manual doesn't list any values for the VSS or distance sensor as it is sometimes called, if the above checks out, that only leaves the sensor and the PCM as possible faults.



PS I just checked my sensor and it shows no resistance value at all. I even spun it to see if I could get a voltage reading from it, again nothing

 

[CGauthier]

Carl, at the sensor disconnect, you should see less than 10 ohms resistance to ground on the black/light blue tracer wire. The white/orange tracer wire should have 4 or more volts with the key on. The power train diagnostics manual doesn't list any values for the VSS or distance sensor as it is sometimes called, if the above checks out, that only leaves the sensor and the PCM as possible faults.

Thanks Alan, that's exactly the kind of answer I was looking for-except we're testing the wiring instead of the vss, good enough for me I need to get my grubby paws on that manual-too bad I spent all my $$ on guages today

if the above checks out, that only leaves the sensor and the PCM as possible faults

Is that what the book said? So, if there's anything wrong in the circuit. . faulty speedo, whatever then that diagnostic won't check out? Man, I owe you one

 

[Bob4x4]

It has been a while since I have worked on a first gen,but the speed sensor produces a pulse. If I remember correctly it is 6 pulses per revolution. Use an ohmmeter while you you turn it by hand.



Bob

 

[Paychk]

Carl this page describes the VSS as having 15 or 16 reed switches (wired in parallel) located around a magnet that is rotated by the sensor shaft. So I put my meter to continuity, then rotated the sensor and the continuity was make/break. Also at some points there was no continuity and others it was constant (possibly magnet between or over individual switches). So try that test, also that white/orange wire feeds up to the 6 pin connector on the gauge cluster, so make sure you get the same voltage there as at the sensor. I surmise what happens is the VSS shorts the voltage to ground and this pulsing is what drives the speedo directly, cause that white/orange wire is the only thing that is common to the computer and the speedo. It appears the computer doesn't drive the speedo directly.



PS the page: http://www.allpar.com/eek/sensor.html

 

[BushWakr - RIP]

Hey Carl, Alan, CGauthier..>>>>>

Would one of you guys like to assemble the specific/factual info in your HOW TO post on this problem and put it in the "Best Tips" thread... it would be good to have the successful outcome in there as well.

The reason I ask is which ever one of you colates and posts this tips/solution can hope back in there and tweak and edit it as this issue unfolds..... I am a little backed up righ now and would do it myself but I could use a bit of help at the moment...



If no one wants to do it just let me know and I'll get to it when I can... no harm no foul...



pastor bob.....

 

[CGauthier]

Would one of you guys like to assemble the specific/factual info in your HOW TO post on this problem and put it in the "Best Tips" thread... it would be good to have the successful outcome in there as well.

The reason I ask is which ever one of you colates and posts this tips/solution can hope back in there and tweak and edit it as this issue unfolds..... I am a little backed up righ now and would do it myself but I could use a bit of help at the moment...



If no one wants to do it just let me know and I'll get to it when I can... no harm no foul...



pastor bob.....

Hey PB, I may not be the most qualified guy for the job, but if it's raining out here all day tomorrow and noone else has jumped on it I will do my best to do up the thread and to have the problem wrapped up too... after all it'd be nice to feel like I'm putting in a little bit of what I'm takin out - which is a hard thing to do in this forum with all these smart guys, errr inmates around





Carl Gauthier

 

[Paychk]

Carl



If for some reason you can't get to post this "how to" I'll back you up and do it.

 

[Paychk]

Here is a not so good schematic of the VSS circuit

 

[Paychk]

Here is the second sheet

 

[CGauthier]

Thanks Alan... If you wanna make the "how to" thread go ahead, I'm pretty swamped right now. I'm gonna try and get back on this hopefully this weekend... then we can put a finale to the speedo diagnosis "how to" and I'll be able to tell how fast I'm goin lol. Problem is the last couple times i've been driving it the speedos been working, go figure lol.



Thanks,



Carl G

 

[vssman]

I'm not sure what year dodge changed over to a three wire VSS unit. But here goes anyway... The three wire unit that I know is used on 2nd gen trucks (maybe on some 1st gens too) uses a hall effect switch and 8 pole rotating magnet. There is also two 4700pf caps for noise control. There is really no way of checking it without an oscilloscope and it runs on 5V. If you think that's the problem, then go to a junk yard and get one. They are all the same, except for: round pin to flat pin change and sealing o-ring diameter. The o-ring is slightly larger in truck applications to compensate for the out of roundness in the vss adapter used on trucks. They were originally the same size until DCX got compliants on oils spots in the driveway. I used to make these units in a previous life before out profitable shop went to Mexico.



Disregard if it's a two wire unit...

 

[Paychk]

The auto 2-wheel trucks use the 3-wire unit, manuals or 4X4 with the 205 use the two wire VSS.



PS What ever you do don't use 12 volts to try and test the VSS, it will result in instant fry. The VSS puts out 8 pulses per revolution of the sensor.

 

[CGauthier]

Ok, so tell me if im reading this right... the pcm sends 8 volts into a connector, which then goes to some splice or somthing (G7)?-which then goes to the daytime running light (cause im in Canada eh), the printed circuit board, and sends power to the vss. Other side of the vss (the LB wire which i'd assume is the - or ground) then goes off to another splice (k4) which then as far as the speedo is concerned goes into another connector at the left rear of the engine which then returns to the sensor return on the pcm. . am i right? If so I should be able to find the problem; an uneducated guess right now gives me a bad hunch the pcm may not be feeding the 8v, because the daytime running lights dont work either- which wouldn't be cool

For those of you that would like to see the schematic as a whole picture, you may not know but if you print the two pics up and put AD 26 on the right side of AD25 the lines join... Any help is appreciated



Thanks for the schematics Paycheck



Carl G

 

[Paychk]

Ignore the three wire VSS that is for auto equipped trucks only. The two wire is for manual or 4X4 equipped trucks.