Davebb

Thank you very much for the answers. I appreciate your time.

Hi all. Can anyone explain a 12 24v alternator? It's on an 88 superliner. It takes lots of rpm (2100) to start it charging. Is that right? The guy that I'm helping work on it says the truck has 4 12v batteries and that it's supposed to charge two of them at a slow charge rate and the other two at a much higher rate. That doesn't sound right to me. I've looked at the wiring diagram found on this forum under wiki and it seems that the alt has a separate 12v terminal. I wonder if somehow that has a trickle charge designed into it. The Alt. has a box mounted on the back of it wired to it. It would be nice to know what the maker of the alt. had in mind. Can anybody straighten me out on this?

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.

Oh. That's what I thought you meant. Thank you.

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?

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” - 16 Wiring. 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 2vac ebay link http://www.ebay.com/...19#ht_730wt_952

Thanks Thad. The drill sounds like a good idea.

I'm not sure what rw is. But it is a 1988 Mack superliner. It does have a Mack electronic speedometer. David.

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.