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Unread 07-10-2013, 07:48 PM   #1
RamboPants
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1987 XJ Cherokee 
 
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Low voltage when A/C is on.

My voltage gauge stays above 13 most of the time but when i turn my A/C on it drops to almost 8 at Idle in P or D, when Im moving it stays normal until i slow down and stop again. Anyone know where i should start? I have a New A/C wiring harness, relays, checked and cleaned every ground...

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Unread 07-10-2013, 08:25 PM   #2
XJaySC
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It could be a bad clutch on the a/c compressor. I've seen weird things happen when those start to go bad. If it slows the belt down just enough it can cause some strange issues.
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Unread 07-10-2013, 08:29 PM   #3
sAe23
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Have you checked the tension on your serpentine belt?
When was the last time you cleaned your battery terminals and ground wires?
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Unread 07-10-2013, 08:39 PM   #4
RamboPants
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The battery is less than a year and I just cleaned the terminals and grounds Sunday. I'm gonna try a new belt first and maybe a clutch.
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Unread 07-10-2013, 09:13 PM   #5
chris87xj
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Does your aux fan operate when the AC comes on?
Try unplugging the power feed to it and see if anything changes.
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Unread 07-10-2013, 09:27 PM   #6
RamboPants
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Yes it comes on.
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Unread 07-10-2013, 10:59 PM   #7
wgirvine
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If I understand you correctly, the voltage is fine when the engine is reved up (driving) but at slower revs (idle) the voltage drops. Might be one or two windings in the alternator aren't working, usually because of open diodes. Have the alternator checked...
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Unread 07-11-2013, 12:32 AM   #8
5-90
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Quote:
Originally Posted by RamboPants View Post
My voltage gauge stays above 13 most of the time but when i turn my A/C on it drops to almost 8 at Idle in P or D, when Im moving it stays normal until i slow down and stop again. Anyone know where i should start? I have a New A/C wiring harness, relays, checked and cleaned every ground...
1) Any suspect IP gage readings should be verified independently before ANY further troubleshooting is undertaken.

2) I believe I covered this - here - within the last month. Check.

3) While you're searching, look for a writeup I put up on testing primary electrical. Run through it.

I'm going to put my money on either the alternator starting to fade on you, or you're right at the edge of its idle output BEFORE you turn the aircon on, and switching it on draws from the battery. It should be borne in mind that you do NOT get full output from an alternator at idle speed, which is why the PD/PS XJ models got an "EXT IDLE" switch to keep the lights and such running - idle got bumped up to ~1200rpm, so they could get full output.
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Unread 07-11-2013, 06:50 AM   #9
cruiser54
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http://www.cherokeeforum.com/f2/batt...or-test-78125/ is 5-90's write-up.
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If you own a Renix Jeep, you'll want to check this out. >

An index to over 25 write-ups can be found in Post #1.

Cruiser's Mostly Renix Tips
My Jeep Technical Photos

http://www.cherokeeforum.com/g/album/1725214
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Unread 07-11-2013, 07:40 AM   #10
RamboPants
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I'll get my alt checked out and let you guys know. Checked my belt this morning think is ok can advance or autozone check my alt while its in or do I need to pull it?
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Unread 07-15-2013, 10:47 AM   #11
OhioXJ2010
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Alt store check

Quote:
Originally Posted by RamboPants View Post
I'll get my alt checked out and let you guys know. Checked my belt this morning think is ok can advance or autozone check my alt while its in or do I need to pull it?
Autozone can check the Alt "in vehicle", but since the voltage regulator is in the PCM, the Alt readings you get will only tell you what is passing through the Alt.(for example, mine was 5.0 amps from Alt, but they couldnt say if the failure was in the PCM or Alt.) I ended up taking mine out and had them check it inside the store with their table top tester. Bearing was going out,also found some bad/corroded wiring. Replaced all wiring with 4 & 8 gage and purchased a 160 amp Alt from LActrical http://http://www.amazon.com/OUTPUT-160AMP-ALTERNATOR-CHEROKEE-WARRANTY/dp/B00BXR5SEM/ref=sr_1_429?m=A1VKPCUL86V2OF&s=automotive&ie=UTF8 &qid=1373903070&sr=1-429 I hope to finish up all wiring and fire her up later today! Good Luck!
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Unread 08-10-2013, 03:53 PM   #12
RamboPants
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Got the alt tested and its good, I have cleaned and tested every ground. Voltage drops when i turn the headlights on also and the wipers are slow as ****...
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Unread 08-10-2013, 05:01 PM   #13
cruiser54
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Originally Posted by RamboPants View Post
Got the alt tested and its good, I have cleaned and tested every ground. Voltage drops when i turn the headlights on also and the wipers are slow as ****...
Have you added any grounds, refreshed all the grounds, added an instrumental panel ground?
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Quote:
Originally Posted by chris87xj View Post
If you own a Renix Jeep, you'll want to check this out. >

An index to over 25 write-ups can be found in Post #1.

Cruiser's Mostly Renix Tips
My Jeep Technical Photos

http://www.cherokeeforum.com/g/album/1725214
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Unread 08-10-2013, 05:48 PM   #14
RamboPants
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I haven't added any grounds, i have refreshed every ground in the engine bay though. Cleaned the c101 twice and got rid of some factory splices. The voltage is fine when with my foot on the pedal a few secs after i stop it drops. I think whatever causes my low idle is causing my voltage problem. By the way what is the bare wire that goes from the c101 to the knock sensor? There is like 2 wires for the knock sensor then a bare wire with no insulation I noticed it years ago and just grounded it to the block between the knock sensor, the wire is partially wrapped in foil also.
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Unread 08-10-2013, 07:09 PM   #15
cruiser54
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Add a couple per my writeup below. Do the instrument panel ground upgrade, too.



Renix Ground Refreshing

The Renix era XJs and MJs were built with an under-engineered grounding system for the engine/transmission electronics. One problem in particular involves the multiple ground connection at the engine dipstick tube stud. A poor ground here can cause a multitude of driveabililty issues, wasted time, failed emission tests, and wasted money replacing components unnecessarily.

The components grounding at the dipstick tube stud are:

Distributor Sync Sensor, TCU main ground, TCU “Shift Point Logic”, Ignition control Module, Injectors, ECU main ground which other engine sensors ground through, Oxygen sensor, Knock Sensor, Cruise Control, and Transmission Sync signal. All extremely important stuff.

The factory was aware of the issues with this ground point and addressed it by suggesting the following:

Remove the nut holding the wire terminals to the stud. Verify that the stud is indeed tightened securely into the block. Scrape any and all paint from the stud’s mounting surface where the wires will attach. Must be clean, shiny and free of any oil, grease, or paint.

Inspect the wire terminals. Check to see that none of the terminals are crimped over wire insulation instead of bare wire. Be sure the crimps are tight. It wouldn’t hurt to re-crimp them just as a matter of course. Sand and polish the wire terminals until clean and shiny on both sides. Reinstall all the wires to the stud and tighten the nut down securely.

While you’re in that general area, locate the battery negative cable which is fastened to the engine block just forward of the dipstick stud. Remove the bolt, scrape the block to bare metal, clean and polish the cable terminal, and reattach securely.

Another area where the grounding system on Renix era Jeeps was lacking is the engine to chassis ground. There is a braided cable from the back of the cylinder head that also attaches to the driver’s side of the firewall. This cable is undersized for it’s intended use and subject to corrosion and poor connections at each end.

First off, remove the cable end from the firewall using a 15mm wrench or socket. Scrape the paint off down to bare metal and clean the wire terminal. Reattach securely.
Remove the other end of the cable from the rear of the head using a 3’4” socket. Clean all the oil, paint and crud from the stud. Clean the wire terminal of the cable and reattach securely.

A suggestion regarding the braided cable:
I prefer to add a #4 Gauge cable from the firewall to a bolt on the rear of the intake manifold, either to a heat shield bolt or fuel rail bolt. A cable about 18” long with a 3/8” lug on each end works great and you can get one at any parts store already made up. Napa has them as part number 781116.

A further improvement to the grounding system can be made using a #4 cable, about 10” long with 3/8” terminals at each end. Attach one end of this cable to the negative battery bolt and the other end under the closest 10mm headed bolt on the radiator support just forward of the battery. Napa part number 781115.

For those of us with Comanches, it’s very important to remove the driver’s side taillamp assembly to access the ground for the fuel pump. Remove the screw holding the black ground wire. Scrape the paint from the body and corrosion from the wire terminal. Reattach securely.

If you want to upgrade your grounds and battery cables in general, contact Jon at www.kelleyswip.com. He makes an incredible cable upgrade for a very reasonable price.


Revised 03-04-2013

Improving the Instrument Panel Ground

The ground point for the complete instrument cluster on your XJ or MJ is located up under the driver’s side dash. If you lay on your back and look up under there with a flashlight, without wearing a hat, you will see a black wire attached to a shiny piece of metal almost directly above the hood release knob. The screw will have either a ¼” or 5/16” head on it.

This ground point is responsible for handling the ground circuit for the following items: Dome lamps, Seatbelt and key warning, trans comfort switch, wiper switch, headlamp switch and delay module, fog lamp switch, cargo lamp switch, all instrument panel grounds and illumination, power windows and door locks, cruise control dump valve, and a few more things.

The problem is that where the ground point is located does not have a good contact with the chassis where the ground should be. The solution is simple.

Make up a jumper wire with #10 gauge wire about 10” long. On one end, crimp on a ¼” round wire terminal. On the other end, crimp on a 3/8” round wire terminal.

Remove the screw from the existing ground wire and attach the small terminal of your jumper so that the original wire and your new jumper share the same attaching point, one over the other.

Look above the driver’s side plastic kick panel just forward of the top of the hood release knob. You will see an 8mm stud there. Attach the large terminal end there with a washer and nut over it tightened securely.

**Special note for Comanche owners: Make your jumper wire 12” long and attach it on the driver’s side kick panel close to the fusebox on the 8mm stud.**


Revised 11-29-2011
ip-ground-location.jpg  
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Quote:
Originally Posted by chris87xj View Post
If you own a Renix Jeep, you'll want to check this out. >

An index to over 25 write-ups can be found in Post #1.

Cruiser's Mostly Renix Tips
My Jeep Technical Photos

http://www.cherokeeforum.com/g/album/1725214
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