hohardcastle

I have a 97 that has charging issues. The alternator is working fine, I bypassed the computer with an external voltage regulator. So the charging system somewhat works. But the tach and speedo are not working. I tested to see if the speed sensor/crank sensor was getting the correct voltage to it and it wasn't. It calls for 5 volts and is not getting it.

So my question is, are the charging and tach and speedo all related? Could it be the PCM or something else?

Thanks for any help!!
Heath

Dave D(SK)

Charging and tach are related, I'm not sure about the speedometer. the crank position sensor output is one of the signals the PCM uses to determine the excitation voltage to send to the alternator and the output to the tachometer. If it receives no signal it doesn't tell alternator to charge.

I am having similar problems as well (see my other posts) except mine are intermettent. By search through this list, these electrical problems seem to be fairly common.

Dave D(SK)

TxDiesel007

Sounds to me like the computer is shot

too much money for a new one.. ask me how i know..

Rick

clutch1

Crank positon sensor..

SoTexRattler

Heath, do you have a DC factory manual? Well worth the price for troubleshooting.
If you aren't getting any +5vdc out of your PCM going to the crank sensor, you may indeed have a dead PCM. See if you can at least communicate with it and read any codes from the unit. If no luck check the PCM fuse. If still no luck, then verify that you actually have +12volts going INTO the compooter at the big connector on the side before prematurely replacing the PCM. They are way too expensive to "black box swap"...

Like others said, tachometer, charging ckt, A/C and cruise control functions are all controlled by the pcm which looks for the crank sensor signal to determine if the engine is "running" before it enables those functions. Speedometer and other PCM controlled engine gauges aren't affected by engine running status if I recall. That one symptom sounds ominous.
I'll look up the pinout diagram for DCvolts in/out of the PCM tonight when I get home.

K.

hohardcastle

I don't have a dc factory manuel where do I get one and is it one of the best manuels out there?

Thanks Heath

12valve

hmmmmm your truck sounds like a 98 12valve they are selling here (4x4 manual, QC, SB for just over $11K)

not tachometer, No AC, no Spedometer, and bypassed the computer for charging etc....

looked under the hood, and saw a cut wire going to the crank sensor! check all wires going to the sensor first!

On my 98 12valve I had same problem you have i did buy a new PCM ... $550 bucks + core. , AFTER that they found out it was tha darn crank sensor! Scanner got no comunication with the PCM, while the crank sensor's wire was burnt!

another thing: the sensor needs to be very very close to the wheel, to get a reading! a few papers thicknes away.

Dave D(SK)

Just replaced mine on Sunday. Service manual says a non-magnetic feeler gauge must be used and gap set to 0.049 to 0.051".

I have now replaced PCM and Crank Position Sensor. Still have problem, going through connections and grounds again.

SoTexRattler

OK Heath, I looked it up last night in the factory manual and there are TWO +12vdc inputs to the PCM. On the "C1" multi-pin connector going into the PCM, pin A2 and A22 should show +12vdc on them when the ignition switch is in the start/run position. Gotta have both for the PCM to do it's thing...

A2 is the "fused, ignition switch" +12v input when ign sw is in start/run position.
A4 is "Sensor Gnd."
A8 is "engine speed sensor input"
A17 is "+5VDC output"
A22 is the "fused B+" battery input. (should show +12 v at all times and has the heavier #16 wire going to this terminal)
A31 Gnd
A32 Gnd

Hopefully you'll find the problem external to the PCM.

K.

Oh yeah,... You can obtain the DC factory manual from the "TechAuthority" website listed in the back pages of your owners manual. (or Geno's Garage has them too).

K.

Lil Dog

Gonna ask a similar question, try not to hijack..

Tonight my rig lost charge. Tach, cruise all the others stuff runs just fine. Its just now I only have 12V on the batteries when running. I replaced the brushes in the alt about 6 months ago when I did the starter contacts. What voltages should one expect for excitation? I want to see if the alternator is actually getting a signal.

Racing_Joe66

figure out the problem?? i am having similar.

San Felipe Bob

I ended up with a new aternator.
Problems fixed.
Our trucks are very sensitive to charging voltages.
When the alt. goes south it causes "Noise" in the electrical system.
You would think new brushes would fix the problem...
No luck here.
Oh well.

I'm happy mine is working good again.
It was causing the trans to shift funky too.

SFB