Sordid Tale of a "Service Engine" Light

Maintenance/Repairs
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161

Hellokit,

You are correct about the 12 volts power to the heaters. I looked over the drawing again and refreshed my memory on how things are connected. I had forgotten how the circuit was designed. Thanks.

I think the PCM is going to be the problem here since Matthew proved there is 10 ohms resistance between pins 94 and 71 though I am just assuming that is the proper resistance for the heater. Since the rear heaters appear to working ok then the supply voltage ‘should’ be present to all the heaters. It would be good to prove that though, as you suggested.

162

Cougar is right. It could be a problem inside the PCM (engine computer); but, let’s first check for input voltage to the oxygen sensor heaters, and for grounds.
With the PCM disconnected, check the wiring harness ground wires for continuity to ground. Different ground wires provide ground for different circuits within the PCM; so, check ALL the grounds. The ground wires are terminals: 24, 25, 51, 76, 77, 103. Check each wire terminal to the car body ground. Make sure the ground contact point, with the black probe tip, is a good clean one.

163

I’m sure you are busy as an achiever, this morning. On checking the grounding wires from terminals PCM 24, 51, 56, 76, 77, 103, the wiring diagram shows all the wires as coming together on a strip, and then being grounded. You could (physically…by hand and eye) follow these wires from the PCM to the grounding point, disconnect, and reconnect, each wire. If there is a screw, or nut, loosen and re-tighten same. This action can help to insure a good electrical connection to ground. PCM 25 is a separate grounding wire for “case ground”.
Erase DTCs and re-scan the oxygen sensor heaters.

Hellokit

164

OK, I accomplished most of the tests:

You need to see what the voltage is anywhere on the red wire, It should be the same as battery voltage, minus a few tenths volt. You can take voltage readings at the idle air control valve, engine coolant temperature sensor, intake air temperature sensor, etc.

  1. Measure battery voltage b[/b].
  2. Disconnect the electrical connector from any of those I just listed.
  3. Turn ignition switch ON.
  4. Read voltage on red wire terminal (11.61V on MAF)
    Result: If voltage is low, a resistance somewhere has lowered it. The lowered voltage will affect the sensors, etc.

There ISN’T a DTC set for O2 sensor stuck lean, now, is there? NO. No mention of lean or rich or anything.

To read supply voltage: Turn the ignition key OFF. Disconnect the PCM electrical connector. Turn ignition key to RUN (ON). Take voltage readings on the wiring harness electrical connector side (NOT on the PCM). Example: Touch red probe of multimeter to pin 71, and black probe of multimeter to ground. Result: 12 volts (hopefully) (11.48V… note that I did this at night and the longer the battery was on, the longer the lights were on, so the battery was slowly draining). Move red probe to pin 94 (11.3v and then 5 minutes later 9.2v)(black probe on ground). Result: lower voltage because of B2S1 heater resistance. Compare voltage values to those on pins 93 b[/b], 95b[/b], 96 b[/b] (O2 sensor heaters).

Different ground wires provide ground for different circuits within the PCM; so, check ALL the grounds. The ground wires are terminals: 24 b[/b], 25 b[/b], 51 b[/b], 76 b[/b], 77 b[/b], 103 b[/b]. Check each wire terminal to the car body ground.

Now, hellokit, when you told me to follow the ground wires 24, 25, 51, 76, 77, 103, which are all black and white, to the place where they all connect to the car somewhere to ground… I tried. BUT, it’s kind of impossible to keep track of them. From the PCM they go (with a ton of other wires) through engine (not really, but you can’t see them) and then they are split up into other bundles (apparently) and go all over the damn place. I’ll try again tomorrow.

Cougar, you said I didn’t mention checking 93 and 71 before, but I did. 93 was 0.6ohms and 71 was 0.7ohms. Also, I want to be to make sure I understand exactly what you said: I should put one probe lead on 71 (using the backprobe technique), and the other probe lead on 93. Correct? I’m making sure I understand what you say when you use the phrase “common point” because I haven’t done this technique measuring volts before.

165

After looking at your voltage readings it appears to me that the wiring to the sensor heaters and the grounds is all ok. It still looks to me that the problem on the front heaters is due to to the PCM.

As far as pin 71 being the common point what I mean is that one lead of all the heaters are tied together. They are also tied to pin 71 of the PCM and they also tie to 12 volt power through fuse #28. All these points are joined together and it looks to me that they are ok going by your readings. This means power is getting to the heaters on one side and it is also getting to the return sides on pins 93,94,95,and 96. The problem is pins 93 and 94 are not going to ground because of a problem inside the PCM, not allowing current to flow through the front heaters. The ground side needs to function for current to flow.

There is one last step I would like you to do. Reconnect the PCM and check the voltage of the pins while the circuit is live. All you need to do is place the meter ground probe to a good chassis ground on the car. Turn on the ignition then check the voltages, one at a time, on pins 93,94,95,and 96 with the other meter probe. I suspect you will find pins 93 and 94 are going to around 12 volts at those pins and the other two are going to be near ground level, less than 1 volt. If that is the case then the PCM is bad.

166

I see something different than Mr. Cougar on pin 51. You used the voltage setting on your multimeter to measure the ground wire resistances on terminals 24, 25, 51, 76, 77, 103. That can be done; but, let’s use the ohms setting, ok? Tiny millivolts are just electrical noise…essentially, zero volts.

When you check the ground wires (24, 25, 51, 76,77,103), multimeter on “ohms”, pay particular attention to pin 51 to ground resistance. I think it may have resistance. It should have less than 1 ohm resistance to ground.

I thought the ground for the PCM would be near the PCM. It isn’t, is it? alldata.com has wiring harness for grounds drawings. They are shown in a ghost car, so you can see the actual layout. It’s not strictly necessary to find a ground; a new ground can be made.

167

Cougar,
Your idea is good; BUT, you’re forgetting that the PCM grounds the heater circuits for only a few SECONDS to heat the oxygen sensors. After those few seconds, the PCM opens its internal grounds for the heaters. I don’t know HOW the PCM decides to turn the heaters ON. You?

168

Hellokit,
Thanks for the heads up. You bring up a very good point that I didn’t think about. I really don’t know for sure how the PCM decides on when to turn the heaters on or off. I was just going on the assumtion that the heaters would turn on at startup and stay on until the PCM detected the engine was at normal operating temperature. Assumtions can eat your lunch sometimes.

I saw your comment about pin 51 and surmise that is related to the front heaters ground or possibly for all the heaters. Unless I can think of something real significant to help further with this issue and since you are giving Matthew such very good advice, I think I will back off on further posts here so not to confuse the testing. The trouble is pretty narrowed down now anyways. Hopefully the PCM isn’t the trouble here.

Thanks for your insight,
Cougar (Glen)

169

EVERYBODY,
Please don’t hesitate to post, at ANYTIME. ALL viewpoints are needed. Mr. mfaerber has other issues (I mean, with his car); so, we ain’t done, yet.

The clue I saw on the wiring side of pin 51 (a ground wire) is a voltage of 3.2 v. It should be O volts. Tiny millivolt (mv) readings are, effectively, O volts, as on the other ground wires.
A voltage reading on a ground wire means that there is a resistance, as from a poor connection (from corrosion, rust, or a loose connection), somewhere from the point the voltage reading is taken to where the wire attaches to ground (the car body, engine body, or transmission body).

To isolate the problem further: turn the ignition key OFF, disconnect the wire (connector) from the PCM. Set multimeter on ohms, with either probe firmly attached to a good, clean, ground, move the probe along the wire (touching the copper of the wire, or a terminal) while observing the meter. Where (if) there is a change in the ohms, the area just passed is where a poor connection is.

170

Damn, I’m awfully sorry guys, the 3.2v on pin 51 was a typo. I should have been 3.2mv… Also, it took me a while to get those readings because the meter is SO sensitive. I would put the negative lead on the car body ground and wait or adjust it until the meter read pretty much 0.000 and then slowly and repeatedly touch the pins…

I will do the rest of the tests a little later today.

Again, thank you all so very much.

171

Could this all simply be tied to a air leak in the evap system somewhere… Should I go to a party store or something, buy a can of fake smoke, and look up the instructions on how to use a smoke machine to see where to hook up?

172

Well I misread the reading for pin 51 appearently because I thought all the readings posted were in the low millivolt range, which now we find out really were, so I wasn’t concerned about a ground problem. If I had noticed the 3.2 volt measurement I sure would have been concerned also.

Matthew, you need to understand that the heater problem and the fuel vent problem are two entirely different problems. The only relationship they have together that I can think of is they both tie into the PCM. I suppose it could mean the PCM is the trouble in both cases but that hasn’t been proven yet. Lets stick to the heater problem until it is fixed and then see what happens.

I still would like to measure the voltage on pins 93,94,95,an 96 while all the connections to the PCM are tied into it. Make the meter measurements with common meter probe (black lead) tied to chassis ground. I want to have you do that just to see what readings you get (though I think we know now) at each of the pins. Since Hellokit mentioned that the heaters were only on for a few seconds then you may need to connect the meter up before you turn on the ignition. Then turn it on and take a meter reading. Repeat this sequence for each heater pin. It would be interesting to see how long the heaters stayed on so if you want to start on a working heater circuit as a reference then check pins 95 and 96 first.

Since we are working on the ground side of the heater circuit you should see a very low voltage on the pins while the heater is ON. When the circuit turns OFF the voltage will go up to around 12 volts, for a working circuit at least. A bad circuit will always show 12 volts at the PCM since no current can flow and cause a voltage drop across the heater element.

173

Thanks for clearing up the typo. What we’ve tried to establish is: that there is 12 volts through the oxygen heaters to the PCM pins (93, 94, 95, 96). It seems that there is. On the ground side of the PCM, we want to check for ohms from the PCM pins (24, 25, 51, 56, 76, 77, 103) to ground. That test is inconclusive. The voltage test may have been sufficient. If these grounds check out, it, apparently, leaves us with a PCM which needs to be tested using the test procedure of Pinpoint tests H36 and H37, minus using a breakout box (which we don’t have). Who knows how to do tests H36, and H37, without a breakout box?

The oxygen sensor heaters aren’t dependent on the EVAP SYS being 100%.

174

I don’t know what the test prodedures for H36 and H37 are or have access to them are but as a guess I would say that they applying power to the heater pins through a current limiting resistor built inside the test box to see if current will flow through the PCM and to ground when the ignition is turned on. If that is true then it is doing about the same thing as my test does. If the procedures mention a particular grounding pin to check on the PCM for the front heaters that would be good to know. If all of the heaters are grounded to a common point inside the PCM we are flogging a dead horse here, or is that PCM? I jokes.

175

OK, this might be a clue: I could only do one of the tests (hellokit’s), because my battery needed to be recharged (thankfully I have a charger). The lights in the dash were starting to dim and the battery read 6v when the car was on (ignition off).

So I didn’t do the volt tests, but I did the ohms tests (I took my time and did each one several times):

24: 0.5
25: 0.4
51: 0.4
76: 0.4
77: 0.4
103: 0.4

EDIT: I let the battery charge for a while and put it back in the car. Then I took voltage readings the instant the car was turned on (engine off) using backprobes (er… long sewing needles) on the PCM wiring harness while it was plugged into the PCM itself:

PIN 93, 94, 95 and 96 all read 11.7v.

176

Good job on the readings Matthew. You have verified that the grounds are good to the PCM by the low resistance readings. The voltages for the heaters is a different story. The high voltages mean that none of the heaters were on at the time you made the checks. I’m not sure what to tell you about that. The PCM may turn them on later after the car has started. I just don’t know for sure. I assume the rear heaters are still functioning ok. You may be able to figure out how the cycle works by monitoring the voltage on one of the rear heaters that works and see when the voltage drops after you start the car.

177

Thanks Cougar. I seem to remember reading that the O2 sensors do not start doing what they are supposed to do until the engine reaches a certain temperature. So I guess that could be it. Tomorrow after work I try doing those readings again the second I get home and everything is still hot. I have a feeling hellokit will know if that would work or not.

178

Good job on getting the oxygen sensors’ heater voltage measurements. Those voltages were ok; but, dog-gone-it, there were no voltage drops which would indicate that the PCM closed its switches to energize those heaters! We expected the rear oxygen sensor heaters to be turned on, and their voltages to drop to near zero. Why didn’t they? Had the ignition key already been turned ON, prior, which heated the heaters; then, the PCM wouldn’t (I’m guessing) turn the heaters on until they cooled, or something.

The ground wires were checked good. What we can’t check is if the PCM ground pins are making contact in the female terminals in the wiring connector. The connector terminals may have been enlarged with a probe (inadvertently) and the PCM pin aren’t making contact with the female terminals. A careful visual inspection can determine this, one way, or the other.

Does the engine have to be running for the PCM to turn on the oxygen heaters? If you think that you can SAFELY check for voltage drops on the heaters, with the engine running, it may yield an answer.

179

You very well could be right about them turning on at a certain temperature. Once the engine gets to normal temperature though I would think the heaters would be turned off by the PCM and let the exhaust temps keep the sensors working like they should. I don’t have a lot experience with the heater operation cycle unfortunately.

180

The oxygen sensor sensing element starts producing a voltage when the whole sensor reaches operating temperature. The oxygen sensor heaters were incorporated to make that happen much sooner. If the oxygen sensor is already hot, the PCM may not even turn the heater on. So, cool oxygen sensors (a cool engine) would be the best place to start checking for heater operation.