alg

help, I have a 1999 grand cherokee with this problem. it started and ran fine. then the next time i went to use the car once it started it misfired and had no acceleration at all. Codes told me fuel injector 1 and 3 were bad. replace them and all the plugs. still same problem. checked voltage at battery found it was hi. 18.4 volts. disconnected the control wire at the alt and the car ran fine again. replaced the alt. with a new one not rebuilt. connected all again the same problem comes back. disconnected the control wire on the alt and it runs fine again. what else can cause this issue. PCM?

dave1123

IDK if this is the problem, but underneath the battery there is a temperature sensor. The PCM uses this temp to determine the charge rate and if it's getting bad information, can cook the battery. Most people don't ever know it's there. Voltage fluctuations can do strange things.

97grand4.0

Yes I second this, the battery / ambient temp sensor is VERY important, sets the charging rate, high resistance tells the computer its colder, wont let the OBD2 monitors run or let you into closed loop.
That being said I would then take it to AZ for the free alt check, THEN I would check the volts on the field terminals, you should not see full bat voltage here, if you do its telling the alt to charge full blast, and you have to find out why..suspect a short to B+, OR, it's back to the pcm/batt temp sensor.

alg

thanks for your insite, is this sensor you speak of located on the pan that the battery sits on. or do you speak of the engine temp sensor?

dave1123

It's pressed into the plastic battery tray and has an electrical plug underneath. Just pry it out, pull it up and disconnect the plug. DON'T LET GO OF THE PLUG or you'll be taking the tray out to retrieve it. Tie it off somewhere. It sends temperature info to the PCM to control the charge rate. Battery gets hot, PCM cuts back the charge rate. This information is also used by the PCM to control the leak detection pump, the canister purge solenoid, and the 02 sensor heaters. All this sensor is is a thermistor or temperature sensitive resistor, same as the engine coolant temp sensor, outside air temp sensor, intake air temp sensor, and transmission temp sensor.

alg

Thanks for your support, I found the sensor you speak of, I disconnected it to ohm test it, it shows completely open (infinity resistance) is that what I'm to see with this out of the circuit? or should I get some kind of reading. I did try to start the car, and again with the control wire connected to the alt. it goes to rough engine idle with no acceleration at all. i don't mind buy a sensor, (cheap enough) but PCM is not something I want to buy unnecessarily. any thoughts on this, thanks for your input

97grand4.0

There is a chart for resistance which I can't seem to find, but, infinity high ohms would correspond to high charge rate.

18 VDC is VERY high charge. I mistakenly put in a 50k ohm resistor last year, trying to boost the battery charge, (50 below 0f) and even still, it only charged to about 14.4 vdc. (also wouldn't let the obd2 monitors run and failed inspection but that's another thread).

So, if you are getting 18vdc, 1) is the battery connected? You could have a bad terminal. You may well need a batt temp sensor, replace it if you are sure it is reading open (high resistance). Anything above 20k is surely too high with ambient temps above 80f here.
That being said, 18vdc is more like the max output of the alternator with the field connections wired right to B+ on one, and ground on the other (full output). So I'm thinking alternator, pcm voltage regulator is fried, or the alternator wire from the pcm to the field is getting fed from the car B+ and not the wire from the pcm.

alg

Hi and thanks again for you advice, so here is what happened. I pulled the batt sensor started the car and all wires connected to alt. had the same engine issue. I then took a paper clip shorted out the plug and wala the car ran........no misfire and accelerator was working. checked charging voltage it was at 15v. my idle was a bit hi but it came down some. still a little hi. but all is now working. the sensor is out of the cir. at this point. So I dont think its the pcm at this point but fingers point to this sensor. Your a life savor. I would be ordering a pcm if not for your great advice. I'm leaving for the beach for a few days was hoping to take this car but I will not push it LOL. nor do I want to push it.......Again thanks, I did replace the alt. with a new one not rebuilt. with the hope that was the problem. along with fuel injectors and coils and plugs.
thanks

97grand4.0

"The end justifies the means" "To the Victor, belong the spoils." "A bird in the hand is worth two in the bush" "A spell checker is worth 1000 words."

dave1123

Try sticking a 12K resistor into the plug. That should tell the PCM that the battery temp is around 68*F. If your volts drop down to around 13, you'll know the PCM is okay.

97grand4.0

Sounds right from what I recall, Dave.
The problem is where to get a 12k resistor, or any resistor, now that Radio Shack is gone. Not only that but, as in my case, the pcm regulator is known to undercharge and there are workarounds involving tacking in a regulator from older dodge trucks, kits on ebay etc. I was up around 20k and still not charging above 14, ever. But I definitely put the batt sensor back in once I learned the obd monitors weren't running because the pcm wasn't seeing ambient temps between something like 20f and 90f.
No problems starting this summer, but we will see once winter comes whether I replace the pcm (NOT likely) or opt for the workaround.

As an afterthought, I suppose I could always try buying a new batt temp sensor. ROFL!

alg

I will just get a new sensor, its cheap enough 30.00 The other strange thing i noticed is, the dash board gauge for the amp/voltage gauge is reading 0. I dont remember that happening before this whole thing. Where is that connected to? PCM? or perhaps i have something unplugged somewhere?
Since I am leaving today I will order a new sensor and it will arrive by the time I am back, I will post my results. again thanks to all of you that have helped me out. Even my friend who is a real good mechanic could not come up with this solution. He thought it was the pcm. When I told him what I did with the paper clip. He said I most likely reset something and that's the reason its working now

97grand4.0

I'm still undecided about this one. 0 volts on the dash is not good. I would make sure i had a good 9v battery in my voltmeter first, then do some checking under the hood, and let us know.

alg

Thats my ? where to look, where dose the source for the gauge come from is it related to some sort of plug that I might not have reconnected or it came loose. the Battery is new, the alt is new. thanks for the intel.

alg

I am getting charging voltage at the batter BTW its at 14 to 15