Davebb Posted April 12, 2012 Share Posted April 12, 2012 Hi. I hope someone can help me. I'm helping a friend with his 88 Mack. The speedometer quit along with the odometer. A shop replaced the vehicle speed sensor and said that fixed it. It quit again. I checked both the grounds and power at the speedo head and got an ohms reading of about 2500 ohms both at the vss (unplugged) and the vss wires (2) at the head (vss connector plugged in) . I think the ohm reading was too high. Anyone know what it should be? I've searched for hours on the web and can't find what it should be for his sender. Most opinions I can find say 800 to 1400 ohms.The vss is adjusted all the way in and backed out 3/4 of a turn. My friend took the head to a speedo shop and they said it works great and that they could set the dipswitches for him if he liked but they'd need the tire size and diff ratio to do it. I took the vss out and hooked it to an ac volt meter on the lowest scale and didn't get any reading until I actually touched the end of the sensor with metal and moved it back and fourth. The reading on the meter barely moved the needle. I thought I should get some voltage reading from up to an eighth inch away from the tip and increasing voltage the closer I moved the metal back and fourth nearer to the tip of the sensor.I really appreciate any replies, thank you.David. Quote Link to comment Share on other sites More sharing options...
Davebb Posted April 12, 2012 Author Share Posted April 12, 2012 I'm not sure what rw is. But it is a 1988 Mack superliner. It does have a Mack electronic speedometer.David. Quote Link to comment Share on other sites More sharing options...
ThaddeusW Posted April 12, 2012 Share Posted April 12, 2012 I'm not sure what rw is. But it is a 1988 Mack superliner. It does have a Mack electronic speedometer.David.RW is the model designation for the Superliner and the prefix of the VIN number (before they changed the VIN numbers). If it is a Superliner an example model number would be RW713.The electronic speedo has a few wires, power, ground and two wires to the sensor. If you move a piece of metal near the sender and the meter needle moves then it sounds like the sender might be working.Have you buzzed (continuity check) the wires between the sender and the gauge? Do that and also check the ohm reading of the wires. If you cant get the leads of the meter to reach, either extend them with a length of good wire OR just jump the wires at the sender side and measure across the wires in the cab. The reading should be near zero in terms of ohms.And as for the ohm reading of the sensor, its not an accurate check at all. If its a hall effect sensor or some other type of solid state sensor then the ohm value means nothing. I am surprised that you even got a reading at all.Try this: if the wires are good, remove the vss and use an electric drill with a bent rod in it or a gear like wheel and put the sensor next to it as you spin the drill, see if the gauge responds. Quote -Thad What America needs is less bull and more Bulldog! Link to comment Share on other sites More sharing options...
Davebb Posted April 12, 2012 Author Share Posted April 12, 2012 Thanks Thad. The drill sounds like a good idea. Quote Link to comment Share on other sites More sharing options...
theakerstwo Posted April 13, 2012 Share Posted April 13, 2012 My superliner has about 1300 ohms on the sendor.With the Vss out see if you can turn the tone wheel with a screw driver.If you can check it out.The sensor or vss with be same be on most all trucks and 900 up with be ok. I have never seen one with 2500 so that is almost open.ohm each signal wire from one end to other end and it should be less than 10 ohms. Quote glenn akers Link to comment Share on other sites More sharing options...
ThaddeusW Posted April 13, 2012 Share Posted April 13, 2012 My superliner has about 1300 ohms on the sendor.With the Vss out see if you can turn the tone wheel with a screw driver.If you can check it out.The sensor or vss with be same be on most all trucks and 900 up with be ok. I have never seen one with 2500 so that is almost open.ohm each signal wire from one end to other end and it should be less than 10 ohms.2500 ohms is far from open depending on the application. 1 million ohm or more resistors are available and used frequently. In fact your meter might have upward of a meg ohm of input impedance.I say this because I have worked with hall effect and inductive sensors that are two wire and have a very high input impedance when measured.Though, for the VSS on your truck it could mean the difference between a bad sensor and a good one, depending on its design. If its supposed to be 10 ohms then it could be a basic induction loop sensor. A simple coil of wire with an AC signal ran through it. when the magnetic field is disturbed by the tone ring the inductance drops slightly and can be detected with a tuned circuit. Pretty simple design really. Quote -Thad What America needs is less bull and more Bulldog! Link to comment Share on other sites More sharing options...
Davebb Posted April 13, 2012 Author Share Posted April 13, 2012 (edited) Thank you both very much. Theakerstwo I think the 1300 ohms is the number I'm looking for. The way I checked the wires to the head was to plug the sensor back in at the trans and read the resistance at the unplugged head. They both read (the sensor and the sensor with wires to head) about 2500 ohms. Yes I did try to turn the tone wheel in the trans with a screwdriver and since I was able to turn the driveshaft some with the tone wheel I think is not slipping.Thad. I'm pretty sure the sensor is the magnetic type with the coil around the magnet that when the wheel in the trans passes it it generates an ac current. I found a vss on ebay that looks somewhat like the one in the Mack and it gave these specs for it.Magnetic pickup sender for Speedometer or Tachometer.Sine wave output.Normally connect to gauge open collect input.Output impedance: 390 +/-15 ohms.Sealed wiring, 18" long.Working voltage: 3 - 30 VDC.Temperature range: -40C ~ +80C.Dimensions: 3/4” x 4” total, thread length 3",Thread size: 3/4” - 16Wiring. Blue: open collect input from Tach or Speedo. Brown: Ground.Output Vpp: 500Hz > 2V, 1000Hz > 3V, 1500 >4V.Mounting: normally by fly wheel.It seems that passing metal by the tip at what ever 500hz calculates to (less than 500 rpm) should read 2vacebay link http://www.ebay.com/...19#ht_730wt_952 Edited April 13, 2012 by Davebb Quote Link to comment Share on other sites More sharing options...
theakerstwo Posted April 13, 2012 Share Posted April 13, 2012 !0 ohms with the senor disconnected. Quote glenn akers Link to comment Share on other sites More sharing options...
Davebb Posted April 13, 2012 Author Share Posted April 13, 2012 !0 ohms with the senor disconnected.I'm sorry theakerstwo. What do you mean here by saying 10 ohms? Is that what you got when you tested your wires. If it is that seems like it should be zero. Wouldn't 10 ohms be too much resistance?Another question. In those specs I posted in post #8 it gives the resistance in impedance. Anybody know what that translates to in dc ohms when reading across the 2 terminals? Quote Link to comment Share on other sites More sharing options...
theakerstwo Posted April 13, 2012 Share Posted April 13, 2012 I'm sorry theakerstwo. What do you mean here by saying 10 ohms? Is that what you got when you tested your wires. If it is that seems like it should be zero. Wouldn't 10 ohms be too much resistance?Another question. In those specs I posted in post #8 it gives the resistance in impedance. Anybody know what that translates to in dc ohms when reading across the 2 terminals? What imean is ohm each wire from one end to the other disconnected at both ends. No more than 10 ohms in the wire or its too much.It should be or best to be 0 ohms. Quote glenn akers Link to comment Share on other sites More sharing options...
Davebb Posted April 13, 2012 Author Share Posted April 13, 2012 What imean is ohm each wire from one end to the other disconnected at both ends. No more than 10 ohms in the wire or its too much.It should be or best to be 0 ohms.Oh. That's what I thought you meant. Thank you. Quote Link to comment Share on other sites More sharing options...
Davebb Posted April 20, 2012 Author Share Posted April 20, 2012 GOT IT FIXED! After chucking up a motorcycle front chain sprocket into my drill and spinning it next to the vss I got about 15mph on the head. Then checking the tone ring again to make sure it wasn't spinning on the shaft of the trans. I stuck a screwdriver into the hole up against the tone ring then marked the screwdriver outside the extension housing, put the marked section of the screwdriver next to the vss and realized it's not going in all the way. I ran a tap into the hole and a die on the vss and now I can tell for sure it's going in all the way. I screwed it in until I could feel it when slightly turning the drive shaft then backed it out a little under 3/4 of a turn and it works great. All this after two professionals couldn't get it working. 3rd time's a charm they say. The first time I set the vss it fooled me too.Thanks to all for the help. Quote Link to comment Share on other sites More sharing options...
Coley Posted April 28, 2012 Share Posted April 28, 2012 I found this all very helpfull as well, thank you all. I bought a truck which had just had speedo replaced but the replacement didn't work. Trying to solve problem myself and found one suspicious issue. One lead to the vss reads 12v dc current. Is this a problem? Thinking the guy who replaced it may have made a mistake. Anyone know? Thank you. Quote Link to comment Share on other sites More sharing options...
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