Test the vehicle speed sensor with a multimeter.

 

check for codes

 

check for codes

he wrote "no codes"

 

The speed sensor is used for idle control when coasting to a stop, as an anti-stall.


Can also test the idle validation switch built into idle control motor along with full sweep tps test.


All sounds crazy but really isnt any thing other than a few resistance ohms and volt checks. If you ask how, then you will get answer. If not interested in testing, thats ok too.

 

Im sory for the stupid questions but can anyone describe how to test the speed sensor with the multi-meter? i had the sensoe tested in the past and for some strange reason after 3 of those sensors every one of them did not read the speed properly they were all off by like 10 mph or so compared to the spedometer. perhaps i have another issue somewere? i know i have replaced the IAC motor and the speed sensor before, i really dont think it was that long ago, ether of them i guess could have gone bad i would just like to isolate which one it is. thanks all

 

No one is saying they are bad. There are other possibilities like faulty connectors or wires or computer on other end.


Its like making a phone call and not getting through. Maybe your phone (sensor) is just fine and the ball was dropped further down the line or on the other end.

 

Im sory for the stupid questions but can anyone describe how to test the speed sensor with the multi-meter? i had the sensoe tested in the past and for some strange reason after 3 of those sensors every one of them did not read the speed properly they were all off by like 10 mph or so compared to the spedometer. perhaps i have another issue somewere? i know i have replaced the IAC motor and the speed sensor before, i really dont think it was that long ago, ether of them i guess could have gone bad i would just like to isolate which one it is. thanks all

In an 89 the speed to the speedometer is driven by a cable not electronics. If the speed is different between sensors it is some thing mechanical not electric causing the speedometer to read off. They also have an electric sensor that only the computer reads when driving.

 

i'd take a look into the throttle body (open the throttle) with a good flashlight and see what kind of carbon buildup you have in there.  if there is i'd remove the throttle body and clean it good.  carbon will buildup around the perimeter of the butterfly blades and in the body in that area and can mess with idle and transitional airflow.

i just recently went through this with my wife's car.  similar symptoms but eventually tripped a code that led me to the TB.

at least it's free.

 

Ok ill look into the  TB for carbon build up, and ill check out the connections on the IAC and the speed sensor.. as for the difference in speed. the sensor (electric) threads onto the transfercase, and the spedometer cable threads onto the sensor so in theory the sensor should read what the spedometer reads right? they both come from the same source... the spedometer DOES match the posted speed that im driving. the sensor that goes to the computer does not i guess ill start with the connections, and checking for carbon and go from there...

 

the spedometer DOES match the posted speed that im driving. the sensor that goes to the computer does not

How do you know whats going into the computer?

 

Maybe he has a drb2

 

I didn't think that showed an actual speed.

 

I hooked my Matco scanner up to it, it shows all kind of perameters one being speed readout.

 

Ok new update and more backround information.... today i was able to mess with it and first i adjusted the IAC according to the (photo parts) section on this site. next i opened the throttle (engine off) and cleaned out the inside of the throttle body with some carb clean ( it really wasnt that carbon'd up) still no improvement, next i eleminated the EGR valve and seemed to help a little bit. then i pulled off the throttle position sensor and tried another i had kicking around (didnt know if that one was any good or not) seemed the same tho the only thing that seemed to clear it up almost completely was that i hooked up the TPS without it being attached to the TB and it seemed really smooth. this just does not make sense to me any thoughts? Also on this truck I do not have the AIR pump on it. the only things i have hooked up are the EGR, charcoal can, and the three valves at the back of the intake.I also plugged the line that goes to the three valves that comes off the front of the TB. so right now the only lines that are hooked to the front of the TB are the line for the map sensor, line to the air cleaner diaphragm thing, their is one more line hooked to the front 3/8 in size that has a lillte almost filter on it that runs to the back somewhere, and the PCV is hooked to the back of the TB. the truck has headers and no cats, and passes emissions test in Connecticut just fine. Any thoughts on were my problem could be? i know their is a lot going on in this thing. ALSO:::: the truck does not have to be moving to get the problem, if the truck is idling and you very gently apply pressure to the throttle it bogs the rpm drops and the truck almost dies, it seems to do this from 500rpm to like 1000-1500. 
please help: thanks all!!

 

worked on it again today before it started pouring rain.. i would consider this a off idle stumble, if i had a carb then id say it was the accelerator pump, if im under the hood air cleaner off and gently push the throttle open it begins to run rough and makes a loud hissing/ sucking front the throttle body, again the egr is still disconnected and I plugged off the solenoids at the back of the right valve cover. ant tests that i can perform to lead me in the right direction? i have a multi-meter ready to go. it dose not seem to have to do with speed like I thought before but rather fuel/ throttle pressure

thanks all

 

No thoughts huh... someone lead me to the TBI GODS, as id rather not just start throwing parts at it. thanks

 

do you have a multi meter, fuel pressure gauge, and timing light?

  will need to test for

  injector operation, tps operation, ignition operation, iac operation, fuel pressure, timing, wiring harness, asd relay, might be worth it to also check the gap for the hall effect, test the coil, would also recommend compression testing,

  mark any differences between how the engine runs in neutral and or park, vs. how it runs in gear, to rule out any trans issues.

  when was the last time the injectors were serviced?
  what is the spray pattern? (should be visible with timing light)
  are there any vacuum leaks?
  are all the temp sensors working properly?

  some common areas are the iac motor, injectors, and injector harness.

  with hard numbers for these tests a proper direction can be determined,

  for proper testing procedures I would look at posts from captain obvious, (co as some threads reference him) you could contact him directly, but he has relayed much of this testing info already, so it should be available through the site search function.

  luck with this endeavor and keep us posted. 

 

I have a similar issue with my truck. People always say to test this or that but they never say HOW to test it.

For example, the speed sensor. Is it the one on the transfer case or in the rear end? IF i test it with a muiltimeter what terminals do I check and what am I looking for

 

speed sensor would be on transfer case, sensor on rear end would be for abs.

 

I have a similar issue with my truck. People always say to test this or that but they never say HOW to test it.

That is because most people wont go through the procedures anyways, so no sense in typing long drawn out descriptions only for them to respond back "i give up, im selling it" or "im converting to carb" etc.

Admittedly many may not have the correct tools or current know how, but if they arent asking detailed questions or willing to get ahold of multimeters testers or gauges, no need to confuse them with technical stuff.

I cant remember if this year of truck had a 2 wire or a 3 wire speed sensor. 2 wire speed sensors are self powered when in motion and do not have constant 5 8 or 12v on any line. As a generator, it makes ac voltage not dc. These types (just like old distributor pickups) can have a simple ohm check between terminals.

If it is 3 wire, these type have a power in, ground, and a pulsed power out which acts as a signal. 5 8 or 12v goes in, and the sensor spits out a rapid on-off pulse based on speed. An led may flicker rapidly, but a test light wont, plus may smoke the sensor. Chrysler claims you can ohm test these, but that is mostly bs. They pulse 8x per revolution. They are still mostly mechanically gear driven so if gears are stripped they wont work even if sensor is good. A basic understanding of wiring diagrams is needed, along with understanding which pin does what.

Dont have a diagram at this second, you will need to get that on your own.

 

That is because most people wont go through the procedures anyways, so no sense in typing long drawn out descriptions only for them to respond back "i give up, im selling it" or "im converting to carb" etc.

Heh, I have seen a lot of that on here. I might have even said it I was going to a carb once.

But thanks for the more detailed post.

 

Ok so i was able to get a few results on my problem here. I tested the injectors i OHM'd em rite a 2 oms so those check out good, checked vacuum and its smoth at idle right in the green range on the OTC gauge dosent bounce or anything. next i checked fuel pressure it has 20 lbs at idle and last i checked these were suposed to have 14-15 lbs at idle. SO i checked for vacuum at the fuel pressure regulator and NO VACUUM! ok so i took vacuum off the intake and hooked it to the pressure regulator and then it was right about 14-15 so thats GOOD.... but now why am i not getting vacuum at the pressure regulator i pulled the short line.. blew through it its clear i pulled the throttle body and blew through the port were the line hooks up and its clear? ok now im confused why is their no vacuum? please advise im thinking that i need to fix this first before i go any further. THANKS

 

Its not really a vacuum going to the fuel press reg. Its called a vacuum port, but all its for is if the diaphragm in the reg, leaks the fuel will go into the engine, instead of dumping all over the outside where it will catch on fire. There should be no reading at idle on that port, it may have a reading at higher throttle, but not designed to be a vacuum, like some other fuel inj systems.

 

Yep no vacuum.

20psi means return line pinched/plugged or regulator stuck.

 

no one mentioned TPS sweep test?

you will need to use either an oscilloscope or an analog voltage meter to perform a voltage sweep test on the TPS. The delay in a DVOM makes dang near impossible to catch any voltage issues. i never could diagnose one with a digital meter, unless it was a scope with a high enough refresh rate (Modus, Vantage)

The TPS, a potentiometer, usually fails in the part/throttle cruise position where it is constantly being used, wearing away at the contacts.  typical failure is a sudden loss of signal voltage which tricks the ecm to think the throttle is closed when it is not

hope this helps