Engine no-start

Hello Ken,

Thanks for your reply. The security light is not on. I have double checked all of the fuses and they are okay. I also visually inspected the relays and did not see any obvious signs of damage/burning. I also switched a few of the critical ones- no difference in the vehicle (still crank-no start). The fuel pump does prime (only for 1-2 secs which is normal from what I have witnessed on this vehicle). However there isn't any fuel pressure or spark. The current codes are p1684 (battery disconnect), p0123 (TPS high circuit fault), p0509 (IAC high circuit fault), and (AC pres sensor high circuit fault).

After my original issue, several months ago, (& before my temperature ground bypass) I also had replaced the fuel pump, fuel filter, IAC, MAP, coolant temperature sensor, crank and camshaft position sensors.

All of the above is very close to what I was getting before installing my temporary ground bypass that I mentioned before (+ other codes too). I would have just kept the bypass. It worked for 2.5 months (without any issues or codes). However, one day I went to start it and it did not start. Upon checking that failure, the ASD fuse was blown. I tried a couple more new fuses and they also blew. So, I stopped, ordered, and installed the new computer. With the new computer, I no longer have the ground issue (prior to my temp. Ground bypass- I was reading voltage on the ground side > DMM - on battery - and DMM + on the ground side of the sensors (back-probed, plugged in, and key in the on position).

Other tests completed today.
Battery check, re-verified body grounds, fuses, belt turning, and continuity between sensors and PCM connector (power, ground, and signal). All of these were okay.

Critical finding today.
Now is seems that I have an issue on the power side! DMM + on Battery + & DMM - on the sensors power side (back-probed, plugged in, and key in the on position) results in a reading of FULL battery voltage (12.5). As far a I understand this should read close to zero! For me, this means that I have an issue on the power side of the circuit. I also did a check with the DMM - on BAT - and DMM + on the power side of the sensors (back-probed, plugged in, and key in the on position). With this I had a reading of 5V. I believe that this also indicates an issue on the power side (high resistance).

Need some help to know where to go from here. I was going to see if I could try to trace the power side issue back to the battery. I have done this back to the PCM. However, from the PCM to the FCM and TIPM the wires change (color coding) and it is not clear to me.

FYI. I forgot to include the last trouble code number (AC pres sensor high circuit fault). That is p1598.

I admit I still have a lot to learn when it comes to electrical. So, I decided to start very basic. After checking battery voltage (12.51), I decided to trace the power side directly from the battery. With my DMM - at the battery positive I first placed the DMM + on the two red B+ cables coming from the battery positive (the exposed wire areas only about 2 inches from the actual connection to the terminal). The reading was as expected (0V or a few millivolts). Then I continued along the positive cable to the next exposed point. That was the B+ cable where it connects to the connector plug (the connector plug that plugs directly into the IPM/fusebox). The connector was still plugged into the IPM/fusebox (basically a back-probe except with the DMM + instead of a t-pin). This reading was NOT as expected. I also expected zero. However the reading was about 0.8V! Then, I unplugged that connector completely from the IPM/fusebox. I also back-probed the unplugged connector and I had the same 0.8V reading! This was also surprising! The B+ cable is only 12 inches long and other that being taped to the other positive cable (I assume the one coming from the alternator) this wire is not even exposed to be able to to be grounded! I also probed the blades in the un-plugged connector (one at a time) and NONE of them show voltage (0V or a few millivolts only)! However, when I probe the mating terminals (in the IPM/fuse-box- with the connector still unplugged) I get FULL battery voltage! I am not sure how ANY of these scenarios are possible? Unless maybe (at least for the last one) that somehow an unintended ground to the B+ terminal in the IPM/fuse-box is present?

Okay. Thanks! I was wondering the same. However, I am also trying to use this a a learning experience (for me and my two boys). Any other tests I can perform to confirm?

I have checked all relays and fuses again and they are okay.I did find a harness with connector just hanging out - unplugged. It appears to be the knock sensor cable/connection. I checked continuity back to the PCM to confirm. Only problem is I cannot find where it is supposed to plug back in! I have looked everywhere. I also tried to find a diagram on line. Even my Haynes manual has too little detail to discern location. Maybe you have access to something better to show the actual position of the knock sensor?

Put everything back together (except for that one wire connector that has no home #25-- Knock sensor connector). That connector is no where near the front or starter- and would never reach. Still no start. Codes 0123, 0118, and 1193. Also noticed that I have the very original concern again. #43 from PCM has no connection to negative battery terminal even with key in on position.

Trying video again.

Okay. Another update for you. I saw that the ground side of the sensor wires were getting power again. This signified a ground issue to me again. So, I tried my old direct ground bypass again. That was to connect a ground bypass directly from the negative battery terminal to the sensor ground at the PCM (#43 wire back-probed with the PCM connectors still plugged into the PCM).

This again worked? All sensor readings go back to normal again and the vehicle starts right up! I am still a bit baffled at this though and really want to understand.

So now the first big thoughts/questions.

Thoughts;
I thought that the PCM controlled when to turn on the sensor ground (#43). As such, I would again suspect the second computer as the issue again. However, that would be statistically very rare.

So now the questions ->
How does the PCM ground to the battery/body? I think wires #10 and #50 go directly from the PCM to the battery (through the TIPM). I am fairly certain of this and these two wires do definitely show continuity to the battery negative.
Should the sensor ground (#43) also normally be connected (and show continuity) directly to the battery negative regardless of the computer (after all, this is what I am doing with my bypass and everything is working properly when I do it)?
Or, does the computer normally make the connection between #10 and/or #50 to the sensor ground #43?

Based on the above information and answers to my questions what should I suspect/test next to confirm what is going on?

And one more explanation and big question.

I mentioned the ground side is now measuring normal (with the ground bypass on at least)- no voltage. I also mentioned that the normal measurement of voltage readings (DMM - at battery - and DMM + at back-probed power or signal wires).

However, when I tested the positive side of the circuit via another testing method (DMM- and battery + and the DMM+ at the back-probed power or signal wires) I also read voltage (example: Wire #61 reads 7V at the MAP sensor plugged in, back-probed, and key in on position). I thought that this type of testing method on the power side should also show measurements of zero volts (or very close to that)? Could this highlight another (or related issue to the one causing the need for the ground bypass)?