Ron E

I found many sharing the same symptoms. No tach, no speedo, not charging, no OD. Most were fixed with a new engine speed sensor. I got a new one. Didn't help. I borrowed a scanner. It would light up but gave a "will not communicate" message.
I purchased a SM CD from Geno's. Tons of info, diagrams, everything. I haven't found the issue yet. But, I'm pretty sure that's my fault and not the service manual's.
So, I started going through some checks. With 12.3 V remaining in the 2 month old batteries, pin 17 in the PCM is reading 7.3V instead of the correct 5V. The voltmeter is a good one (Fluke) so I'm not suspect of the reading. The battery check was done with the same voltmeter.
I'm going back to searching but thought I'd post this odd volt reading hoping that someone may know what it means, and what to do about it.
Also...
Checked ground at ESS connector (from PCM) ground is good, the earlier 7.32 V. Is there any way to verify the signal from the ESS to the PCM? I thought about skinning back a small spot with everything hooked back up and the engine running. Don't know what I'd be looking for from a voltmeter on volts or ohms. Possibly pulsing too fast for a basic voltmeter to keep up with?


Thanks in advance, Ron

pind

Ess is a hall effect sensor. If you unplug it and hook up with your meter set to ac. You will see a voltage when cranking the engine, won't be much. But it will be there.

7.32 is definitely not right on the sensor voltage line. Should be 5v. Possibly a slight variance., but not 2.3v.

Ill have to dwell on this a bit.

Ron E

One other thing that I found odd enough to go get another voltmeter to verify. With the switch on, engine not running with 12.15V at both batteries, the K6, cav A17 pin in the PCM shows 7.12V. With the PCM connector plugged back in (still ign on/engine off), I checked the voltage at the ESS connector. There it read .802V. I was thinking a break in the (violet/white) wire. That still wouldn't explain the extra 2 volts, but it would be a start. Then, I checked the ohms of that wire. The ohms show a perfect connection. "Assuming" that wire to be isolated, how can the ohm check show a good wire but still lose 6 volts between the 2 ends?
And, you're right. I don't really know what I'm doing when it comes to automotive computers, sensors, etc.

Ron E

After the wiggle and bending the harness test had no effect on the readings I had another idea.
To be sure about the wire, I skinned a small spot within 2" of each connector on each end of the violet/white wire and added a jumper-wire. The jumper-wire also drops to .802V when the connector is plugged in to the PCM. So, that suggests the original wire isn't the problem. Apparently the pin at A17 of the PCM goes from a too much 7.3V down to a not enough .8V when connector is plugged in to the PCM. That probably would be a clue if I had one. I still don't.
I'll keep posting SWAG's as they come to me. Maybe I'll learn something. I have the room and another vehicle.
So many shops throw stuff at these kinds of problems. Although their aim is probably much better than mine, there are tons of people who swear they've had to buy components they didn't need while paying some pretty steep labor rates as well.
If anybody has any ideas, share them. If it works, great. If not, one more thing eliminated. Thanks...

pind

What was the resistance reading using the ohm meter? Reason I ask,is a lot of people use the diode test function for an audible reference. While this works, you could have as little as one strand,and still get the continuity tone.

Now, pin a17 is the sensor positive, correct? In which case, you might have an internal pcm problem.

The pcm should send a stepped down (5vdc) voltage to the sensors. As the resistance of the sensor changes, it returns a variable (.5 to 4.5vdc) signal to the pcm,or ecm, depending which parameters are being monitored.

Next question, when cranking, did you see an a.c. voltage from the ess? Did you check the condition of the tone ring for the ess? Is the depth set correctly? Not saying any of these things are the problem, but worth checking out.

Also, for the will not communicate issue. Take a close look at the pins on the obd connector.

One last stupid suggestion. Check all of your fuses, very carefully. I have had fuses look fine,but be broken where the fuse meets the leg. That's another widowmaker.

Ron E

pind, I'll double check all of this to make sure I have it accurate. On the fuses, I did try a couple of fuses in position 9. Only other I checked was the 120A in the power box under the hood. I'll be back soon. Thanks again...
Ron

Ron E

pind....
With the meter set on the lowest range (200) I get .3 resistance across that wire. (.3 is also what the meter shows with the leads touched together)

On A17 pin, with switch on I got 7.38V today. (No negative symbol on display) black lead on neg battery post. same reading w/ neg lead on alternator housing

No separate tone ring that I can see. Just 2 slots in the balancer. Sensor set dead on .050". (Tried it at .035" once because I read it somewhere)
Rechecked the fuses using ohms on meter. using the 200 range. They all read .3 to .4 resistance

Darn it! I forgot to check the signal wire from the ESS while running. I'll fetch that info in a little while.

Due to the 7+volts on A17 pin, I've suspected the PCM. However, I don't understand how this thing works well enough to put much weight in my guesses.Not so bad sticking a un-needed ESS on there. It stings a little more if I buy a un-needed PCM. But, it keeps looking that way. What do you think?

It sure makes me appreciate the totally non-electronic operation of the old '89. Closest thing to a computer it has is a voltage regulator.

pind

Try finding the pcm ground, then grab a sensor voltage reading using Its own circuit.

Ron E

I'm not close to it right now. I'll check it against it's own ground ASAP. Thanks again.

Ron E

I was tied up in a non-truck adventure last night. I'll get the ground check done tonight.

Mike L

Was alcohol involved? Tawdry women, high speed police chases? C'mon man, enquiring minds need to know now that you put it out there!

Cheers!

Mike

Ron E

pind. checked the black/lt. blue strip wire for continuity. It checked at .4 using the same "200" range. The SM "the PCM provides a ground to the ESS" through this wire. Its position A4 on the PCM plug.
Then with everything plugged in, ground lead to skinned spot of the black/lt. blue wire near PCM connector, positive lead to the ESS signal wire (A8 at PCM connector). Started it up, and it read a .78V on the 20V range. I saw nothing suggesting a pulse. But, I doubt the sampling rate of the meter would have a prayer of being fast enough to display any pulse. Hope this helps. Thanks, Ron

Mike, my "adventure" revolved around helping a buddy who had some light flood damage. Did have a beer afterwards. Its at the very lowest end of the adventure spectrum.

texasprd

No, the typical multimeter would not have a hope of showing you the real-time voltage change from the ESS - if it could, you couldn't read it, considering how many times per second it is occurring. To really see the waveform from the sensor, you need an automotive lab-scope/"graphing-multimeter" for that - something like a Snap-on Vantage or Modis, or the PC-based systems like Pico. Here's a one-channel scope that's pretty inexpensive: uScope 1-channel automotive Master Kit or uScope 1-channel automotive Master Kit

Of course, you have to know what the waveform is supposed to look like. Snap-On's scopes give preset scope setups and waveform examples - others may not.

Ron E

Texasprd, Good info on those scopes. No doubt the tech side of this stuff is going through the roof. Mechanics talk scanners like they used to talk torque wrenches.

I can see one of those being a good investment even for the DYI'er. Especially the first, cheaper version. That's just how vehicles are "evolving". The newer ones start/drive/run great. But, when there's a problem, it can overwhelm a layman. And, I'm currently in that category and its a12V. God help me if I were ever to have to work on a 24V!!

Ron E

Shouldn't the new ESS have a detectable magnet? The old one does. No detectable magnetism on the new one. (It won't pick up a paper clip.)

Nope. it does have magnetism compared to the old one. I re-tried the old one anyway. Still walking.

EDIT:I meant to add it has some magnetism. You can just feel it with a pocket screw driver. The old one will pick up a 10MM socket.