Archived Strange Tach Problem

[JoshPeters]

1996 12v Auto



Ok, it's to the point I need to make progress on this thing to be able to plow the driveway where I rent. Here in MN we are actually having a winter.



Last summer I borrowed the truck to a friend. I had to go pick it up with a trailer. It had blown the front trans cooler line, oring. I got it home and that fixed then the snow started to fall.



I have no tach, trans in limp, and of course no charging. Tach sensor. Easy fix... WRONG!



I unplugged the wiring, 3 wires. One should(?) be 5v reference voltage, one ground and the other signal to the PCM to get all the above features. I have 5v, 5v and ground. So, I Ohmed the wiring to the PCM. All ohm OK.



Here is where it gets more confusing... . I unplug the PCM and I still have 5v, 5v, and the ground goes open!



Where does the 5v reference voltage come from? Is it the cluster or PCM? The wiring diagram I had moment to see showed it coming from the PCM.



The wiring diagram also showed a solder joint where the signal wire was for the Cluster, PCM and Sensor.



I have limited time to get this thing in a heated shop to get this figured out. There are other projects waiting to get in the shop space.



Thanks guys!

Josh

 

[JoshPeters]

Ok... more confusion.



Unplugged the PCM and the Instrument cluster and somehow still have 5v on both wires with the key on.



Help guys!

 

[Sticks]

PM Missouri Mule, he seems to have a knowledge on these.

 

[LVJUNIOR]

There is a wire (purple and black if I remember right) that goes to a buss block inside the dash forward of the obdII port. It sends the low voltage power out to the dash cluster and controls the guages. I don't know if it will help you but it is worth a check for a broken wire. Sounds like you ruled out the tack sensor the rear anti-lock sensor on top of the rear diff has also been known to cause a little havock. Like I said it might not be any of these things but at least it will give you something to try while you are waiting for a better answer and you still have the heated garage

 

[Missouri Mule]

5-volts wire violet/white provided by PCM=PCM A17

signal wire grey/ black=PCM A8

ground wire black/light blue provided by PCM=A4



The instrument clustter should not have anything to do with Engine Speed Sensor(crank position sensor,tach sensor). It is powered thru Ignition Switch.



If you are getting 5 volts to ESS from PCM that is good.

If you have ground that is good.

If you get any voltage on on signal wire while engine is not running that is not good (ESS BAD).

If you get any voltage in signal wire with key off that is not good ( ESS Bad).



Don't know why your are getting voltage when PCM unplugged

(Looking thru the wiring diagram the TPS is provided power thru A17 and the ground is also provided thru A4 on the PCM. (looking for more sensors that are provided power/ground thru A17/A4. )(A4/K4 ground=Intake Air Temp,Water in fuel,battery temp,)

 

[JoshPeters]

Thanks Missouri Mule!



I checked another 12v tonite that I came across. Nice guy let me probe his truck with my DVOM for 5 min in a parking lot (shows ya how much I need to get this on the road! LOL). His reads the same thing I read on my truck. So at this point I have to assume that my ESS is bad. Thanks for the description and pin outs!

 

[Missouri Mule]

Josh i'm going to check out my truck Friday with a multi-meter to see what the ESS is reading since my tach is working. I'll check with key off then on and if I can will check with it running to see what the signal volts should be. Unfortunatley the FSM doesn't give that info you need diagnostics manual for the PCM.

But it seems to me if the engine isn't running there should be no voltage on the signal wire.

 

[Missouri Mule]

I couldn't get the connector disconnected unless I took offf the top radiator hose (Swore I didsonnected it when I did the KDP but then there was nothing in the way at that time).

I 'll try the PCM side of it tommorow or later today.

 

[S.Morris]

On my sons 97 auto. Bad ess = no tach,no charge alt , and drive only in trans. No 1st,2nd or od

Steve