I don't remember any ground wire coming out of my loom where the FSM shows G105. Since the PCM, starter motor relay and O2 sensor heaters ground at G105 if the engine starts and runs normally it safe to assume the wire is grounded (somewhere).


The FSMs are not perfect and there are a few errors over the years. If you wanted to know exactly where G105 actually is you'd have to trace the wires.

 

when I see these issues weird things I will look at the prior and following year systems.

IDK
My experience is build date might be utilizing the older or newer.
Also jeep @ this year 96/97 was switching over and my 2¢ the mickey mouse drawings these years are "lacking".

As mentioned above Jonny Jeep - it seams to be grounded.
Is A31 and A32 indicating they are grounded.

What is the problem you are chasing?

 

when I see these issues weird things I will look at the prior and following year systems.

IDK
My experience is build date might be utilizing the older or newer.
Also jeep @ this year 96/97 was switching over and my 2¢ the mickey mouse drawings these years are "lacking".

As mentioned above Jonny Jeep - it seams to be grounded.
Is A31 and A32 indicating they are grounded.

What is the problem you are chasing?

A31 and A32 both have continuity to ground.

It is a 'cranks but won't start' issue. I've gone through a long list of things already and this ground point was one of the next targets on my list.

Thank you for the feedback. I do remember seeing other diagrams online, for different year/model jeeps, where there G105 was located where I'm showing G106/107. I suppose I could disconnect that one and see if A31/32 lose their ground.

 

1997 TJ crank and NO start.

The jeep has run before- so that would have me looking at
CODES?
Fuel pressure PSI?
KOEO is there
12v at injectors?
12v at coil?

 

For a 97 I'd add cluster gauges working at ignition on? to that list. Could indicate loss of 5V supply.

 

G-105 for most years I've seen for 4.0s is a bolt coming out of the engine block more or less above the oil filter. As Jonny says, there are several things grounded there so if everything worked before other things are grounded. However there may be a separate wire for something, probably non-OEM, that is hanging there & may or may not be a grounding wire. Most OEM grounding wires are either black or black with a stripe. I think the PCM grounds are black with a tan stripe. There may be some exceptions but none come to mind now.

If it is not starting but was before, the PCM must be grounded & if you have continuity to ground you are OK.

If you have constant 12v power to PCM pins a22 & key on & start 12v power to a2 & the ground is good you need to see if you have 5v at pins a17 & b31.

One good clue is whether the cluster gauges come up when you turn the key on (not start). Especially note the volt & fuel gauges. If no gauge movement there is almost certainly no 5v supply from the PCM.






Jonny beat me to that one & it is an important question to answer...

 

This guy has a great video troubleshooting a no start condition. It is an 05, but the principles should be the same. He goes down a bit of a rabbit hole on the first video, but the trouble shooting steps are very solid.

 

Thank you all for the feedback. The cluster has always had its own issues. Fuel gauge reads whacky and bounces all over with a full tank, then settles down eventually, but remains at a fixed location that is incorrect. Odometer resets to 98 miles every time the Jeep is turned on. It will count up from there, but then start over from 98 when the key is turned off. Oddly, the trip counter works fine and does not reset until I manually reset it (thankfully, since I have to use it to know when to fuel up). These issues are/were present when the jeep ran. Right now, when I turn the key on the odometer comes and on shown 98 miles. The gauges do not come on. I do see the oil pressure gage bump a tad, but that's it. Other lights come on like normal as far as I recall. Brake light is on because e-brake is pulled. CEL comes on for a couple of sec then goes away (like normal I think).

Anyway, the other stuff I had already checked (multiple times as I tinker with things)... PCM is getting Bat Power. 5V feeds are all producing 5.15V from the PCM. I also get 5V at the sensors. Crankcase sensor gets 5V, has continuity, and when I unplug it the cluster gages still don't do anything. The two PCM grounds have continuity... using a Klein 600mm meter with the audible beep, they beeped indicating continuity but the display was still showing some amount of Ohms. I was wondering about that, and what threshold my meter is using to produce the continuity beep. I need to check again because I don't recall the exact amount of resistance it was showing.

Swapped around all relays (ASD, Horn, Fuel). All fuses are good and all are getting battery power. With key on, I can hear/feel the relays click when I insert them. 85 and 86 are getting battery power (with key on). I previously checked the resistance following instructions from the FSM and got within the 75 Ohms +/- 5. Terminal 30 has power. What is weird though, is that even though 85/86 are closing and showing battery power when I test (while grounded to bat-), pin 87 is still not getting power. This is with the key on, probing from the underside of the PDC touching the terminal (and the green/orange ASD output wire). Also back probing green/orange wire at the PCM shows no power.

Making this even weirder (to me) is that for the ASD relay, on my 97 JT 4.0, the middle terminal apparently isn't used (the center one in the row of 3, marked with the No. 4). There is no wire connected to that cavity. Apparently, it has always been this way and only 4 of the 5 relay terminals are used in this relay. However, with the relay on and closed, that Middle terminal gets power, while the 87 does not.

Another weird thing is that the fuel pump wasn't getting powered up either. I jumped it and it came on. Then, something I must have done in the past few days, has caused it to come on with key on. Unfortunately, it stays on (as best I can tell). The relay gets hot and I can hear the pump running. This happens the entire time the key is on.

I should also mention that I don't think the ASD relay was closing either when I first began investigating my issue. The only things I have 'changed' are cleaning a few of the main grounds, and spraying contact cleaner in several of the various related contacts.

Oh, lol... and I jumped the gun a bit and put in a 'new' PCM purchased from FS1 supposedly programmed with my VIN. I did this early on after checking a few things and becoming impatient. I installed it and nothing appeared to change. I wish I had made better notes instead of relying on memory, but I'm almost certain that the relays were still not closing after the new PCM was installed. This was a very recent thing and surprised me to hear them clicking closed after messing with stuff for so many days (weeks).

Also, regarding the data codes, the reader is unable to connect to the computer.

One more oddity - since I keep having to top off the battery charge due to testing/cranking, one day the battery was charging and I was working in the garage and the Jeep horn started spontaneously blasting. I've unplugged them at the horns, but they remain powered. The relay clicks closed the instant it is inserted (key off or on).

My original post about the G105 ground was because I had been analyzing wiring diagrams and noticed the horn and data connector shared a common connector that appeared to be grounded to G105 - which appeared to also ground several important items involved with starting the vehicle. Logic and the laws of statistics led me to target some things where different issues/observations appeared to have some overlap.

Just to be sure, I went to an auto store today and asked them to test the battery. He said it looked fine and his device did not indicate any bad cells.

So that's my story up to this point. I know I still have a long list of things to check, and every now and then I learn that I have been misinterpreting some things or checking pointless items. The more I learn about vehicle circuits and power, the more I realize I don't know.

I keep hoping I'm overlooking something dumb, but so far that obvious thing hasn't popped up. And as far as I know, it has gas in the tank...

 

Crankcase sensor gets 5V, has continuity

This sticks out. How are you measuring this continuity? is it the sensor or the wires you are referring to?

When the sensor is unplugged and tested between the B and C terminals it should be open circuit.

The odo behaviour is odd to say the least. Don't think I've come across that one before. If the cluster had an internal fault it should display all 8s on the odo.

There was a recall for the fuel sender units in 97s for showing incorrect fuel level. I had it done in mine. Might be worth checking, but I don't know what they'd do for a replacement these days if the recall was outstanding on yours.

Although fuel ASD and horn relays are changover relays they are all just used to switch on circuits when the coil is energised.

 

OK, a lot to digest so here goes.

When the gauges don't come up with key on it almost always indicates no communication from the PCM which you have, as expected. However most of the time that is caused by either no 12v power to the PCM or no 5v output. You have checked both & both are good so not a "normal" problem. On your '97 data is sent from PCM through a "twisted pair" of wires so there could be a problem there but seems doubtful given the other problems you are finding.

The relays. To begin you should not be testing the relay's coil input/output from (under) the socket when the relay is plugged in. You first need to check the coil terminals (85 & 86) with the relay pulled. And you certainly should not test the coil ground (or any grounds) with a volt meter when the component (relay in this case) is connected into the circuit. You can normally test for ground with the relay plugged in using a 12v test light but better to pull the relay & check from the exposed terminal side. You say you have 12v at both 85 & 86 which is almost certainly not actually what is happening. If you pull the relay & check, I think you will find you have 12v with key on to one of the 2 & no 12v OR GROUND at the other. If true, the relay is not closing which even though you hear it click is proved by the fact that you are not getting 12v power out at terminal 87.

Explanation, when you use a meter to check for grounds in a "active" circuit & the grounding wire happens to have either high resistance or is open rather than getting continuity as you should you get a voltage reading which is from a small current passing through the component (relay coil) & passing through the meter. If you remove the voltage source TO the component before checking you can then get an accurate resistance reading. To properly test without pulling the relay you can use a test light connected to the battery + post. Being only connected to the + voltage source blocks any current getting through it & the light stays off unless it has a low resistance path to ground.

So assuming you are not getting the necessary grounding from the PCM to actually close the relay that explains why no 12v at 87. Also, when the relay is open the other terminal (87a) gets the 12v from 30 since 87a is the normally closed terminal & 87 is normally open (lacking energizing through the 85/86 inputs to it's coil).

Both the ASD & FP relays need coil grounding from the PCM to close & since you tried another PCM & assuming that one did not happen to have the exact same failure as your original (not likely) you should carefully check the grounding wires from relay terminals to PCM terminals looking for a break, corrosion or other damage. Since the FP has apparently been somewhat intermittent I would say that is a good possibility. The FP wire is brown & the ASD is dark blue/yellow.

Before getting into tracing the wires you can use a couple of jumper wires to jump each relay from terminal 30 & 87. Then try starting it since the FP & coil & injectors should now be getting power.

It still may not start because of lack of communication but at least you have bypassed the relay problem so you can check further to try getting it to start.

 

OK, a lot to digest so here goes.

Rubi,

Thank you for taking the time to read through everything I posted and the time to respond. I need to read back through your response a couple of times to process things.

Prior to removing the PDC from its case and exposing the underside, I had only been checking the terminals from the top with the relay removed. Instead of relying on memory, I just need to go back through these steps and make notes. That said, I feel certain that everything I checked at the time made sense. As in, with the relay out and key on, I was getting power at 30, and 86. There's nothing to check in cavity 87a though. I guess that is normal since 30 does not need to power anything while it is open and connected to 87a.

At some point I was thinking the ground signal from the PCM might be shorted, so I checked and was getting continuity between 86 and back-probing blue/yellow at the PCM.

However, at this point, I'm questioning my use of the multimeter. My errors most likely are how I was checking voltage, but I still want to recheck everything on your list to be sure.

I think I understand your point about checking the ground side of the coil from the underside while plugged in with the key on. I just figured the PCM was grounding 85 (blue/yellow), thus closing the circuit. I thought it made sense that I would see the same voltage at 85 and 86 (blue/yellow and green/red) with the coil closed.

I believe the root of my confusion is the role of grounding in DC circuits. I need to read up on some basics before making assumptions on a more complex setup.

I also think I understand my confusion with terminal 4 having power with the relay 'closed'. Realizing now that the center terminal (no 4) is actually 87a, it had power because the relay was not actually closed.

By the way, last week hacked together a little fused jumper and put it across 30 and 87 in the ASD relay socket and the FP socket (not at the same time though). The jeep did not start in either scenario, but the FP did start running with the FP jumper in place.

I will not be able to do any additional testing until tomorrow evening at the soonest. I'm eager to go back and check some things based on these responses. I will post as soon as I have more info.

Thank you again.

 

One more oddity - since I keep having to top off the battery charge due to testing/cranking, one day the battery was charging and I was working in the garage and the Jeep horn started spontaneously blasting. I've unplugged them at the horns, but they remain powered. The relay clicks closed the instant it is inserted (key off or on).

You have one of the following problems here:

1. the horn relay contacts are stuck/fused closed
2. the RD/YL relay ground/trigger wire is shorted to ground
3. the clockspring is shorted internally to ground
4. the horn switch in the steering wheel airbag cover is stuck closed

If your clockspring is internally shorted, this can screw up the signals from most any of your engine sensors since it shares their common sensor ground. You may want to remove the horn relay to see if that has any positive effect upon your other issues -- if nothing else, it's a better way of disabling the horns than unplugging the horns themselves.

 

Thank you for this info. I do have the rd/yl wire highlighted in my FSM pdf as something to check. I haven't gotten to that yet, but I did inspect the ends at the relay and in connector C-7 (as best I could without taking it completely apart).

I've swapped all relays around so I don't think its stuck. It did not appear that any wires or terminals on the socket side were touching - but kind of hard to see.

Interesting comment about the clock spring. I will figure out how to access it and check it out.

Thank you.

 

Many apologies, I missed an important step when checking the power/grounding for the relay coils (85/86).

After checking for 12v with key on connect the test light (or meter) to battery + & move to the other 85/86 terminal. Put the probe into that other (non 12v) terminal & from key off turn the key on. The test light should light for about 2-3 seconds then go off. If you then start cranking the engine with the starter the test light should come back on & stay on as long as the engine is being cranked. This applies to both relays.

This is how the PCM controls both relays. The PCM needs a signal from the crank sensor to ground the relay's coils as when starting & the engine is running.

If you are not getting that grounding & 12v to 87 you will not have power to FP or coil/injectors. If not, you can still jump the 2 relays & try starting.

Sorry for missing that most important step earlier.

 

You are doing great so far. It is a lot to process & most of what you have done so far is correct. You did the relay terminals correctly except the coil ground which needs to be check right at key on & when cranking anyway.

Checking grounds with a volt meter confuses a lot of guys. You thought there should be power on the ground side of an active (powered) circuit as in both 85 & 86 but actually that only applies if the ground is open which will show mostly equal voltage or if there is high resistance which will show less than input voltage & how much reduction will depend on the resistance (higher resistance, greater voltage drop). As I noted in the later post the ground was probably actually (correctly) open if tested with key on after 3 seconds & not having the engine cranking which I assume was how it was checked & normally would be but not in this circuit. So it is easiest to check that terminal for ground with relay out & engine cranking. That is well demonstrated in the video posted above even though it is a 2005 so some differences from your 1997. I really spaced & failed to put that important fact in the original process. You may well find the PCM is actually grounding both relays but it will not start based on the other issues mentioned in your earlier posts. On that note you really need to jump both relays at the same time because my experience is that in rare cases where the PCM fails to supply grounding it happens to both. I'm pretty sure they are on the same driver. If it happened to only one I would think it to be a wiring issue. In your case it still might be since your FP has been intermittently pumping/not pumping so who knows. If you jump both you can then verify fuel pumps which you have done & also check for 12v to both the coil & injectors. That will be helpful info if it fails to start when jumped.

Are your gauges still not responding?

If it fails to start with jumpers in I think you should recheck for 5v output. Just to one sensor should be OK & the cam sensor may be the easiest is access.

If 5v is good see if you get a pulsing ground signal at the grey coil wire then the various injector signal wires, all the not dark green/black (12v) wires as you crank the engine.

 

If 5v is good see if you get a pulsing ground signal at the grey coil wire then the various injector signal wires, all the not dark green/black (12v) wires as you crank the engine.

Is there a way to check a pulsing ground with a digital meter? Thought I recall seeing people having to use an analog meter or oscilloscope since the pulsing is too fast for a digital meter to display accurately. I think I have an analog meter somewhere. But that might be the one I accidentally blew up, now that I think about it.

Thank you again.

 

It depends on the meter & how often it refreshes. You may be able to see it fluctuate a bit even if it will not fully go from open circuit to full continuity (0 ohms). Since the engine is cranking at a very low RPM you should be able to get enough action to tell if it is pulsing on & off. A noid light is the tool usually used for checking injector pulses but no need to get one for a one time (hopefully) test.