• Welcome to The Truck Stop! We see you haven't REGISTERED yet.

    Your truck knowledge is missing!
    • Registration is FREE , all we need is your birthday and email. (We don't share ANY data with ANYONE)
    • We have tons of knowledge here for your diesel truck!
    • Post your own topics and reply to existing threads to help others out!
    • NO ADS! The site is fully functional and ad free!
    CLICK HERE TO REGISTER!

    Problems registering? Click here to contact us!

    Already registered, but need a PASSWORD RESET? CLICK HERE TO RESET YOUR PASSWORD!

98 Shifts erratically & No speedo

Dylly

Dylly

Member
Messages
183
Reaction score
3
Location
Saskatoon, Saskatchewan, Canada
(I'm from Saskatchewan, and we have plenty of snow right now, pulled the truck into a heated shop so moisture isnt out of the question)... Its a 1998 Chevrolet 1500 extended cab Z71.

This could be completely unrelated but... My thermostats were causing me trouble, so i pealed off the thermostat housing and accidentally dropped a bolt under the intake. After an hour of trying to get it out with a magnet i resorted to some air... i blew the bolt up to the front and fished it out with a magnet.. put it all back together and it was pumping heat perfect.. Here's where the problem started though..

I took it out on highway and noticed my speedo quit working, no cruise either (obviously) and the truck shifts whenever it feels like it and when it does shift it bangs the shifts... Sensor? Throttle position sensor? Vehicle speed sensor.. (two of them?)?

Thanks for the help
 
You may have disturbed a connector fishing for the bolt. Also have your grounds been cleaned lately?
 
no real way of checking them but you need to get it fixed asap or you WILL screw up the auto trans.
your vss sensors may be bad with the melting snow or too much snow under the truck may have broken a wire on the sensors they are on the trans usually only 2 wires that are twisted pairs. they must be twisted to work properly. they tell the computer how fast you are going which helps determine shift points.
wes
 
could be either or both buy one they are the same if i remember correctly.
unless you have the special test equipt no way to test. except if truck is on jack stands rear wheels turning preferably over 15 mph. using a voltmeter read a/c volts on the sensors. damm dangerous to do unless you make test cable sets.
wes
 
Do you still have the factory radio in it? If so you can turn the SCV all the way up and if it doesn't turn the volume up when the speed increases then most likely you have a bad VSS(the one in the rear housing of the transfer case). That sensor goes straight to the VSSB/DRAC module on the 6.5's, and then that module sends out a speed signal to the ABS on one circuit, the ECM on another, and the radio speed output on another. So you could have a bad VSSB/DRAC, bad VSS, or a wiring problem somewhere.
 
Unfortunately i dont have the original radio in it so i cant test it that way... I have both sensors ordered... I have ScanXL which monitors damn near everything... I went for a little rip only got it up to maybe 50km/hr or so and my Transmission Input Sensor showed an avg of 626rpm and the max was 2204rpm... output sensor shows an avg of 4rpm and max of 32rpm... it also gave me code "P0722" which is "Output Speed Sensor Circuit - Low Input".... Ideas?
 
check the tranny case connector and VSS wiring thouroughly for corrosion or broke wires. Snow and ice have a nasty way of goofing things up.

check the engine rear grounds,one is for the tranny,maybe you broke it loose with the blow gun pokin behind to blow that bolt back.
 
dylly that tells you right there that the rear sensor or its wiring is bad.
wish i had that info first.

not critizing (sp) that is why it is so important to give all the test info when looking for help on a problem.

the front sensor most likely is good and does not have to be replaced.

wes
 
Last edited:
I appologize for not getting the codes earlier. So i went and did some research online and found all the output speed sensor codes :

P0720 Output Speed Sensor Circuit Malfunction
P0721 Output Speed Sensor Range/Performance
P0722 Output Speed Sensor No Signal
P0723 Output Speed Sensor Intermittent

Would P0720 mean i have a cracked wire?... I know one code could mean multiple things though... i'm just trying to narrow it down because the wiring loom coming from the tranny to the engine compartment eventually turns into a rats nest which im scared to start pealing into... esspecially when it gets very hard to get at as it goes up the transmission toward the top of the engine.
 
If you were prying, pulling, pushig, etc---you more than likely damaged one or both of the wires. I find it hard, not impossible, for the sensor to go bad at the same time.

do you have a manual to find and test wires from ecm. If the wires are already a rats nest, I would think maybe a ground got pulled or the dreaded high resistance open?? the copper in the wire probably broke but the insulation around it did not.

find wires at ecm, short wires at sensor, test with ohm meter. It should be at least less than 4 ohms.
 
At the ECM ... like in the glovy? ... oh boy thats gonna be interesting. Thanks though!... I pulled the sensor last night and according to my haynes manual it should read between 1000-3000 ohms and it read 1300... so you could be right... do you know what kind of voltage it should read disconnected at the sensor? At least then i can check and see if theres current comming TO the sensor..
 
there is very little current the sensor is esecencally a coil that creates a electrical pulse when the magnitic reluctor passes by, creating an ac pulse. this pulse then is modified to the various signals needed by the system.
wes
 
Dylly said:
At the ECM ... like in the glovy? ... oh boy thats gonna be interesting. Thanks though!... I pulled the sensor last night and according to my haynes manual it should read between 1000-3000 ohms and it read 1300... so you could be right... do you know what kind of voltage it should read disconnected at the sensor? At least then i can check and see if theres current comming TO the sensor..
Click to expand...

There isn't current going to the sensor, the sensor generates current and sends that to the VSSB/DRAC on a 6.5 truck(gassers from 96+ go to PCM). The VSSB/DRAC will be the small white box mounted underneath the PCM behind the glovebox.
 
1300 ohms sounds about right,a tad low maybe.Both sensors should be within 10 ohms of each other.
The higher the speed,the higher the voltage.
 
Well thanks for the help guys... I went out to NAPA and bought the sensor (apparently theres 2 different possibilities)... one was $80 the other was $250. The $80 one was supposed to be the one that i needed (back of the transfer case) but it turns out the $250 one was infact the one that fit the wiring harness.. bad news BUT I checked the resistance accross the new one and it was the exact same thing as the old one so i just hand tightened it in, took it for a drive No difference, so i pealed it out and did what i should have done to start with... unplugged the VSSB then hooked up a nifty fault tester i have at the sensor, and checked it at the VSSB ... Current was running through the purple wire but the green one was dead... so i started tracing them back... the tool works awsome if the wire is easy to get at and as we all know, those old wiring looms arent fun to deal with so anyway here's what happened:

I had all my days mixed up
Day 1: changed starter - Everything worked fine
Day 2: went to muffler man, got my exhaust all put on and pyro welded into the down pipe (everything still working fine)
Day 3: Changed thermostats ... Everything buggered up
Day 4: put gauges in ... Still buggered up

Turns out on Day 3 after i put my thermostats in i ended up cutting the guy that owned it before me's Wiring for a subwoofer amp (the wiring was way to light for what i'd ever use.. 20 Gauge or something... i go 4 gauge or nothing)... So i used his already drilled hole to feed the pyro connector through... but the hole was to small, so i went and drilled it out a bit bigger and the bit-bit the firewall and shot its way through and knicked the big wiring harness that goes into that black cover before it comes into the cab (drivers side)... 4 wires total, 3 knicked, 1 torn (torn one being the green one).

The reason why i never fell upon this to start with is because i thought i did everything to do with the gauges on day 4 meaning the gauges had nothing to do with it... so after fighting with the harness and connecting all the wires back up... shrink tubing them etc everything works fine again.

Big lesson leart here on my behalf... Dont drill where someone else drilled unless you're absolutely sure its clear... if it was me i would have never drilled a hole there to begin with... but it was already there so i gave the previous owner way to much credit.

Thanks again for helpin out!
 
You must log in or register to reply here.
Back
Top