speed sensor not working
 

i have tested the speed sensor on my 1998 mustangs reference wire and theres no voltage. I checked at the sensor connecter and at the pcm, so i beleive the pcm is at fault. I would like to send the pcm to get it fixed, but were are the best places that can be trusted to fix it. I live in Ottawa Il and do not know of any place around me that does this:confused:

Before you send the PCM out for repair, could you tell us how many wires go to your speed sensor. Two or three.
If its three, then you should have voltage going to the hall effect sensor as you stated. Were you able to verify that there is no voltage coming out of the PCM to the sensor by actually checking at the PCM ? Just asking in case there is a break or poor connection in that wire. Most of the time the input voltage to the sensor is 12 volts.

I checked at the speed sensor referance wire ( pnk/org ) and there is suppose to be .5 volts. It is a two wire and the other wire is ( gry/blk ) which the sensor sends information to the pcm which is a pulsing voltage of .5 volts. the wire at the pcm ( pnk/org) has no voltage at the run position or engine running. This is why i beleive its in the pcm. I was hoping that it was some where in the wiring but not so

If it is a two wire sensor it is a variable reluctance sensor which means you measure the AC voltage at the two wires as you pass the teeth in the output shaft past the sensor. You can do the same thing if you remove the sensor and pass a piece of steel back and forth across the magnet of the sensor. If you test it this way and get no kind of pulsating voltage from the sensor it is defective. Use the lowest voltage scale of your AC meter. If you do get the reading and you still have problems, now it's a good idea to confirm that there are no breaks in the wires

If the referance wire does not have any volts then how can the sensor produce a signal to send back to the pcm. I checked with a manual and it says first to check the referance wire and if there is no volts ( 5volts) the pcm is at fault. I crawled back under the car and checked it again and yes with the ac side and no reading with any scale seting. I feel that taking out the sensor to do a reading will be of no use if there is no volts going to it, if i am wrong please explain what i am saying. This car has only 18000 miles on it, and garage kept, mostly shows and cruise nights.

The sensors that have two wires have an internal magnet and as the field is varied it produces its own voltage signal that is measured as an Alternating voltage signal that increases in frequency as speed is increased.
A three wire hall effect sensor requires a reference voltage in order to work. Where is this sensor located that you are checking?
If the manual you are using is specifically for a 1998 Mustang (OEM) not just a general manual, than by all means go by the manual.

Its in the tail shaft of the trans ( manual 5 speed ) and the spedometer gear is on the end of the sensor. So you are saying that the two wires on the sensor has no volts unless it is spining

Yes, that is correct. You can check that sensor by connecting your meter to the two wires and have it set on the lowest AC voltage setting and spinning the sensor in your hand. You should a small voltage reading if the sensor is good.
If I may, why are you suspecting a problem with the sensor or PCM?

I was confused because the manual which was Haynes led me to run the first test which now does not make any sense. how can you test a wire when it is unpluged from the speed sensor, why they led you that way i do not know, but since the sensor produces its own ac voltage. If they would of said to back probe the connecter that would make more sense. So tomrrow i will take out the sensor, so thanks for your help and i will let you know the results

I don't know at this time I can help determine the cause of the problem, but it sure would help if I knew just what the problem was.

It all started when the motor would run at a higher rpm while backing off the throttle and when coming to a stop the motor would not drop to idle until the car completly stoped. It has a rich condition, but runs ok. When i stick my foot into it runs very good. I have a o2 gauge and it does not sweep very far into the lean. I have modify the engine and put a tune into it and it help the rich conditon to some extent but because of the response of the higher idle and dropping to the base idle when completly stoped, i am leaning on the speed sensor as the culprit

Did any of your modifications include changing the cam? If not, you might want to look into the Idle Air Control Valve on the throttle body. They can cause a problem like this.

Today i did check the speed sensor and it is good, so i checked the iac valve. The voltage going to it was fine and the carbon deposits were very thin, thats because of the milage. So my next thought would be that the solenoid may not be controling the pintle. The excessive amount of air if this is true would be telling the o2 sensors that there is abnormal amount of oxygen in the exhaust stream and it would be giving the pcm to enrichen the fuel. That may be why i am running rich, and at idle it smells and i did not as of yet change cams. I changed the heads,exhaust, plenum, throttle body, msd dis4. I checked the termanals on the iac and there was readings but i have no way to know what the readings should be if there is any. I do not like to throw money at replacing this with out more knowledge, but this makes sense

Is the check engine light on at any time?
There are many reasons for an engine to run rich. Among them is an inoperative/stuck O2 sensor, intake air restriction,incorrect inputs to the PCM, incorrect outputs from the PCM, misfires,bad injector(s) to name a few. It's time to get a scan tool on that engine and see what is happening with the engine data like fuel trim , O2 activity and IAC counts% . Can you get one and obtain some of that info?

The check engine light is not on and i do have a scan tool, since i changed the heads and so on that a tune was needed. Like i sayed the tune did help, so i thought it would be easy to diagnois it with out hooking it up but i guess i will hook it up and check it out once again

I found the culprit with the multimeter. The signal wire from the maf sensor had no voltage at idle or off idle, replaced the sensor and the fuel trim is responding like it should be

Good job ! Stay in touch, there are lots of folks on this site that could benefit from your knowledge.