darren509

Well I have to buy a new vacuum gauge tomorrow apparently the old one got borrowed. I keep coming back to wondering my TPS and upstream o2 will not trigger a CEL. Something about that just bothers me. Anyone know which connector on the ecm has the tps wires and or O2 ? Maybe a link to a diagram ? Im at a loss as to what is going on with this thing.

CherokeeChick

I know on the xjs there is a way to make codes display on your gage cluster, there may be a way to do this on the gc as well? Worth a search! It really helps if you know what cylinder(s) is/are misfiring

97grand4.0

Are you loosing your +12 volts to the coil when this happens? monitor it with a voltmeter..

darren509

Just so I'm clear you want me to check for 12v at my c oil after I unplug my tps while it's running? And I can read codes but not while the car is running. It will not communicate while it's running.

97grand4.0

No dont unplug anything, just monitor the voltage to the ignition, coil...and see if it's dropping out when the vehicle acts up. Id also be checking the backside of the alternator b+ terminal. this is just basic stuff before you move on to dissecting your pcm without any reason

darren509

Thank you and I will check it tomorrow. I thought that an unplugged tps and o2 sensor not throwing a code would be reason to suspect a problem with ecm or the wiring to it ?

darren509

Do you know off hand which wire I s the coil 12v ?

dave1123

This might be a mute point, but the original post says this is a 98 Cherokee. Nowhere in the posts does it say it's a Grand. What is the story because we may be dealing with something other than what we believe is involved. He was asking which wire was the 12v supply to the coil and I was going to look it up BUT I realized I don't have diagrams for an XJ, so which one are we talking about.

This is why it's important to be clear about which vehicle we are dealing with, and IF it's an XJ, why is it still posted in the Grand forum? The wiring color codes may be the same, but we don't know for sure. If it were mine, I'd just backprobe the connector to find out.

97grand4.0

Point taken, I would backprobe the coil B+ right at the coil instead of elsewhere or the pcm.

Darren, are you saying you unplug the TPS and get no code? I'm not sure if that would set a code, not really the way I would go about diagnosing a bad PCM, maybe. If you must, I would try unplugging the MAP, that should set a code. And yes, if it sets no code I would start thinking PCM. For that matter, the way you start checking all the tps, iac, map etc is backprobe each device and observe while the problem is happening. Many times one or more of these will be missing +5 reference voltage , often due to a burnt wire on the exhaust, or similar. Then if you are missing the +5 at one or more devices ( from memory they are all connected to the same +5 ref from the pcm). Then you go to the pinout of the PCM and see if you have +5 there.


That said, have you done the basics, new plugs, cap, rotor and wires?

dave1123

DOH! Why didn't I think of that. Bad MAP sensors can do weird things.

97grand4.0

Hey Dave youre right down the road from me, Im in Syracuse

dave1123

No ****? Send me a PM and we'll get together. There's another member in Canastota although I haven't heard from him in a while.

darren509

Thank you all for your help so far. I apologize if I did not provide enough information about the car. When I joined the forums I looked at the definitions under grand cherokee spot and thought it was the correct one. It is a 98 grand cherokee Laredo. And yes I was assuming that if I unplugged the tps and no codes were thrown I had a problem. The tps checked ok with a DVM and later today I will check the coil and also vacuum test.

dave1123

No, you were right to post it where you did. It just nice if you fill in that box in your header where it's grey and says Cherokee with what you really have. That way it gets shown on each post. The Cherokee one is a default if you don't put anything in. It makes it easier because we don't have to refer back to previous posts to remember what we're talking about. Basically the Cherokees and Grand Cherokees are pretty much the same, it's the "pretty much" part that gets tricky.

darren509

Ok so I tested the coil wire and it has 12.5v at idle and when I rev up the motor and crack the throttle it occasionally drops to 11.4 or so momentarily. I also checked the map sensor it appears to be working properly with the voltage at center wire 1. 64 volts jumps to 3v as I crack the throttle and then instantly drops to .089 and climbs back up correctly. Drove down the street and it bucks and jerks , came home checked the temp one inch prior to cat (464) and one inch past cat (578) I can honestly say the car did not drive this bad when it was brought to me, ever since I removed the throttle body to clean it all it has not run the same. I should note when doing so I did remove the TPS because I wanted to look inside to see if if needed to be cleaned and there is nowhere to clean there so I bolted it back up. I was told that upon the car driving to my house initially it had a hard time going over 50 mph. I am so lost right now as to what to check next.