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Discussion Starter · #1 ·
I have searched and cant find the info on diagnosing the problem when the speedometer stops working on a 2001 Rodeo 3.2 with code P0502 speed sensor A circuit low input. It is electrical and I dont know where or what ohms or volts to check for. It hasnt worked for 20k miles. I already replaced the vss for $100+ and I just bought the haynes manual and the only thing in it is to replace the vss. I downloaded one 148mb manual from here viewtopic.php?f=1&t=39947&p=310876&hilit=manual+download#p310876 but is isnt listed.
 

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If you've replaced the one in your transmissions trail shaft & still get the code its likely the 2nd vss (the one your ECU actually sees) which is in the speedometer itself.

Tailshaft rotates VSS -> VSS sends signal to speedo -> speedo displays current speed of Tailshaft AND a 2nd VSS built into speedo forwards signal to ECU. I don't think its a replaceable part, might need full new speedo.

~psguardian
 

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Discussion Starter · #3 ·
thanks for the info.

does anyone know the specs on how to test the vss behind the cluster? i would hate to buy one and it still not work and me not know if mine was actually bad or the new/used one was bad upon arrival.

Also do I replace the whole cluster or can the sensor be swapped out? The mileage issue if any doesnt bother me really just prefer 150k or under cluster. What model clusters are a direct swap to a 2001 Rodeo 3.2? Ebay seems to be reasonable with several under $70 shipped with a return/warranty policy if broken.
 

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I would imagine that if the speedo is dead then the VSS in the trans or the wires are broken. Is the later VSS gear driven like the late 90's ones? are the gear in good shape and not stripped?
 

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According to my 01 Rodeo FSM schematic the ECM/PCM and the speedometer both receive a VSS input signal from the transmission VSS. There is a splice between connector H-16 Pin 12 and connector H-5 Pin 10. One wire goes to the speedometer and the other goes to the ECM/PCM, the wire color code is Black/Yellow. The speedometer does not send any signal to the ECM/PCM.
 

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Discussion Starter · #7 ·
Buster28 said:
According to my 01 Rodeo FSM schematic the ECM/PCM and the speedometer both receive a VSS input signal from the transmission VSS. There is a splice between connector H-16 Pin 12 and connector H-5 Pin 10. One wire goes to the speedometer and the other goes to the ECM/PCM, the wire color code is Black/Yellow. The speedometer does not send any signal to the ECM/PCM.
ok thanks a lot. I looked at the haynes and found the schematic and it says the vss goes straight to the pcm. Tracing the wires is not going to be fun but it could still be in the trans. The speedo went out and afterwards the trans was rebuilt/swapped by a shop cause the alt power wire corroded and voltage dropped frying the trans. Mom doesnt know what the shop did but trans has an asain(sp?) plaque on it but the trans was also warrantied at 50k by Isuzu.

While I am posting might as well ask what is the part called in the trans that the vss connects too.
 

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If the transmission shifts properly the VSS signal to the ECM/PCM is present, so the problem is the speedometer or the wiring from the H5 connector to the speedometer. If transmission is not shifting properly in addition to the speedometer problem, the problem could be the VSS or the wiring from the VSS connector to the H5 connector. H5 is a 16 Pin Green connector on the left fender forward of the ECM/PCM.

The VSS is a Hall effect device that is generating pulses based on transmission output RPM, since you have a 2WD the VSS is mounted to the transmission extension cover. On a 4WD it is mounted to the transfer case. The VSS has a 3 wire connector, Pin 1 is +12 VDC reference, Pin 2 is chassis ground and Pin 3 is the signal output.

Btw, on a 01 Rodeo the ECM and PCM modules are combined so I refer to it as the ECM/PCM
 

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Discussion Starter · #9 ·
The transmission shifts fine. I will dig into it further next week when mom is out of town. A quick glance at the connector and it looks fine and the wire loom is still intact. Plus the suv has no other electrical issues. It is nice of Isuzu to tightly wrap all the wires in a two inch bundle. At this point I am thinking/hoping that is is just the cluster.
 

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You can remove the cluster in about 20 minutes. Measure continuity between the VSS input on cluster connector I-2 Pin 7 (Black/Yellow) and Pin 3 (Black/Yellow) of the VSS.
 

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Discussion Starter · #11 ·
Buster28 said:
You can remove the cluster in about 20 minutes. Measure continuity between the VSS input on cluster connector I-2 Pin 7 (Black/Yellow) and Pin 3 (Black/Yellow) of the VSS.
please excuse my ignorance. do u mean to check for power at those to make sure they read the same? i dont have a mechanic/ electric background just what I have learned through family but my bro in law is a 30 yr mechanic. working on removal of cluster now to diagnose before ordering one. I am mechanically inclined though.

Edit. I located the black wire with yellow strip behind the cluster. What exactly am I testing for? A specific voltage? Do I test it with the ignition on or does it have to be running?
 

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I mean read the resistance between the VSS sensor connector and the gauge cluster connector with an ohm meter. It should be zero ohms (ie continuity) indicating that the wire is not broken (an open circuit). If wiring is OK it is highly probable the speedometer defective, because you already replaced the VSS and the transmission is shifting properly. The PCM uses the VSS signal to determine the transmission shift points.
 

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Discussion Starter · #13 ·
Buster28. Thank you for your help. The vss wire was .001 on the gauge so the wire is good. Gonna order a speedometer cluster.
 

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Discussion Starter · #14 ·
Well got the new/used cluster and put it in and the speedometers still doesn't work. Wire reads 001 on test and a random piece of wire reads the same. Is there a way to test gauge outside car? Gonna contact seller and request refund for a bad speedo. Wish I knew someone local to swap cluster as it only takes 5mins.
 

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The speedometer has 3 connections
+12 VDC
Ground
VSS input
The fuel gauge and the speedometer share the same +12 VDC and ground. Does your fuel gauge work?
The PCM and Cruise Control share the VSS signal. Does your Cruise Control work?
 

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There should be +12 VDC on Pin 1 of the VSS connector when the Ignition switch is in run. The return (ground) is Pin 2 of the VSS connector. Place your voltmeter across Pins 1 and 2 and make sure you have reference voltage for the VSS.
 

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Discussion Starter · #18 ·
The voltage checked out at 12.5 at the vss connection. So the voltage and vss wires are good. The car shifts fine but the cruise and speedo don't work even after gauge replacement. So is the new/used gauge bad or is the vss sensor or internal trans tail shaft bad?
 

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The VSS is Hall effect device, all it takes to make it work is simply moving a piece of metal close to sensor tip. It is probably using gear teeth to operated it, each tooth equal one pulse, X pulses = 1 mile. That is why putting on bigger tires results in speedometer error, the number pulses/rev don't change. If you remove the sensor you can probably feel the teeth. Btw, the correct Isuzu part # 8-96014-422-0 Sensor, Revolution.

I forgot to ask if you cleared DTC P0502 after you installed the new sensor and did it come back after you cleared it?
 

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The sensor is a Hall Effect but the output is not. The VSS unit contains a transistor circuit that grounds the signal wire. Traditional Hall Effect sensor have a square wave output.
 
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