Special ways to test OEM Jeep stuff thread

This is not a tool thread, we have a forum for that. But many of us have simple low cost tricks or ways to test OEM things on our jeeps, besides the usual guess what is bad tactic, where you replace a part and hope your guess was right, which is usually the wrong guess about 6 times in a row and $400 later. Then it becomes an obvious,:twak: costly way to fix jeeps.

Obviously knowing how things work on a jeep is a great help, but once we get past that point, there are sometimes simple, easy little ways to figure out where the gremlins are hiding. These would be low cost, simple stealth methods of detecting cloaked Jeep gremlins!!!

So if you come up with one post it up in this thread and share it!!! One that I can think of that is not used often enough is thermal testing. For instance when a jeep dies only when it warms up, take a hair dryer and pre-heat the CPS to see if the jeep will start cold with a hot CPS, to see if it is the part that is dying when it gets hot. Repeat the test on say the High Voltage coil next!!!

Or if it will not start on cold mornings, use the hair dryer to heat the MAT or CTS temp sensors to see if it gets the rig to start.

I was thinking about a recent poster here that was not too savvy on wiring and electricals that had something draining his battery over night, and he was not familiar enough with multi meter use and wiring to figure out how to trace the parasitic gremlin load. I was thinking a relay might be stuck. Finally dawned on me that anything using enough power to drain a battery over night should get hot!!! So using the hand or an IR temp tester, one could shut the jeep off, come back an hour later and snoop for a hot relay, or wire!!!! Simple, quick, cheap test!!! Anything warmer than ambient temps!!!! So when you come up with one post it up here!!!

1) Some gremlins can traced to bad wire harness connectors. I found a bad coil connector on non-XJ jeep (OK I have a YJ too) by wiggling wires until I got the motor to miss.
2) Always check and clean your grounds early in the diagnosic process (right after fuses).
3) Of course, there's the swap in the horn relay trick (easy to verify if it's good or not).
4) Make a jumper from the + battery terminal to energize components that you want to test (e.g., relays, bulbs). Or make a jumper to ground if appropriate.
5) The IR temp gun can be used to find spark plugs that are not firing (cooler than the others) and verify that thermostats are opening.
6) I know this isn't about tools, but I must mention the Power Probe kit that I have. It can be used to energize components (I know, a jumper is way cheaper), run continuity checks, and it's a multimeter. It also has a short circuit detector that sends an RF signal down a wire - when you get to the short, the signal dies. This can be used to troubleshoot shorts that are under carpet or behind panels without removing anything. If you chase many gremlins, you will really appreciate this tool - it's a big time saver. It's not low cost, but your buds may bring beer after you find their gremlims!

I'm obsessed with vacuum leaks and troubleshooting vacuum problems.
So many little things have vacuum servos and work on the vac system in our Jeeps.

When my HVAC system wouldn't blow any way but up I played hell trying to find a tiny vacuum leak. I knew it was there, just couldn't find it. Using the carb cleaner trick on HVAC lines does not work. I can't afford a smoke machine BUT I can afford a cheap $1 cigar.

THE STOGEY TEST: Get a cheap cigar. Fire it up. Remove one end of the black HVAC vacuum line coming out of the firewall. Take a hit on the Stogey...blow some smoke into the line. If you see smoke, follow it to the leak. Bingo, fixed! I keep the same old disgusting thing in my shop just for this test.


Disclaimer: Tobacco is a health hazard. I don't recommend regular use of tobacco products.

XJFrankenstein said:

5) The IR temp gun can be used to find spark plugs that are not firing (cooler than the others) and verify that thermostats are opening.

Click to expand...

I like #5, have not had a not firing cylinder since I got my IRs, but that sounds like a great one. I found out my Renix radiator had several huge cold dead zones in the radiator while the engine was near boiling with my IR. Many of the tubes here and there were plugged up, and IR found the cold spots, so I can see how it would easy to find a cold cylinder that was not firing at the spark plug. Later it detected the top 1/3 of my brand new radiator was cold due to a trapped air pocket!!

After 6 years, I bumped the injector connector on #2 on mine one day, engine running, and found the occasional miss that escaped my detection for 6 years!!!

One thing that is often overlooked is just disconnecting a Renix O2 sensor briefly and letting the system run open loop to determine if the O2 sensor system or closed loop operation is part of the engine miss, wandering idle, bogg down on accelration problem. It is not 100% fool proof, but if the problem goes away by disconneting the O2 sensor, you now have a huge clue what sub system might be involved. It could still be bad cylinder -injector -plug or wire confusing the O2 sensor ECU system, so if the test works you still need to do more tests like make sure every cylinder is firing good by by pulling the spark plug wire or injector wire on one cylinder at a time to see if the loss of that cylinder has no effect (meaning it has a problem) or if misses and runs rough loosing that cylinder (meaning it is good). If all 6 cylinders are good with out the O2 sensor, then your problem is somewhere in the O2 wire, fuse, relay, or O2 sensor.... This test requires NO TOOLS expect a brain and hands, and no parts $$$$$$

This should work well with HO OBD-I rigs, but expect a code and CEL light using it on OBD-II 96-01 jeeps and the very last year Californica year rigs have two front O2 sensors to check.

One I've mentioned before, I've got a couple of leftover timing lights, that are nearly useless anymore. I hook them up to each individual spark plug cable, one at a time, and use the light shining on a flat (dark) surface to help me track down ignition misses. I call it a poor mans oscilloscope.

I was trying to find a short in my wiring harness. Every time I turned the ignition on the fuse would blow. I disconnected all of the loads on the circuit, and the fuse still blew.
I was burning through fuses and not getting any closer to a resolution....
So, I used a resettable circuit breaker in an in-line ATO (blade type) fuse holder, with spade connectors soldered to the ends inserted into the PDC relay sockets to isolate the problem circuit.
If the circuit has more than one relay cascaded in series as in my case, I made several of them.
This was very effective in isolating the circuit, and allowing me to track down the short.