How to Test Your Cherokee Coolant Sensor and Wiring

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The Coolant Sensor Bad Light

Coolant Sensor Bad Light
Is the coolant sensor really bad? Troubleshoot. photo

If you own a 1990s Jeep Cherokee there's a good chance you've become intimately familiar with the Coolant Sensor Bad light on your dashboard. What does it mean, anyway? And almost as important, why in the world did Jeep decide that you needed a Coolant Sensor Bad light inside the car to haunt you while you drive? Not only do you have to look at that annoying glow, but in some vehicles the glow is accompanied by a seriously annoying beep. It's unbearable, enough to make you park the car and walk away from it for good!

The good news is you don't have to live with that dreaded light or the beeping. There is a simple solution to this problem in most cases. It's just a matter of figuring out what part of this weird warning system is causing the light to activate, and deciding what you are going to do about it.

The Coolant Sensor Bad light is supposed to tell you when your coolant sensor is malfunctioning, and this protect you from an overheating situation that you can't anticipate, followed by a warped head or other heat related breakdown that can be very expensive to replace. Your Jeep's coolant sensor (or at least the one that annoying light is referring to) is located in your coolant overflow reservoir. There's something about the design that is almost guaranteed to cause a failure of some sort. Sometimes it's the wiring, other times it's corrosion at the plug. Then sometimes the sensor really us the problem! Your problem is figuring out which one it is. Many people report that they have replaced the Coolant Level Sensor in the recovery tank and the Coolant Temperature Sensor in the thermostat housing but that did not fix the problem. At this point these drivers are very frustrated. You can avoid this frustration by doing some basic and simple tests to determine what the real problem is ahead of time. No money lost, less time broken down, and an easier day for everyone.

Before you start, you might want to brush up on some electrical troubleshooting.

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Testing the Coolant Sensor

VIC wiring chart
Wiring diagram for the VIC on a Cherokee or Grand. About.com

The dash light is telling you one of three things. Number one: there is a short in your wiring. Number two: you have a bad VIC (also known as the Vehicle Information Center). Number three: you actually have a bad coolant sensor. Unfortunately there's a number for that they didn't plan on, which is a corroded sensor plug that is causing the error message. If you test the sensor and it's good, you know that the problem is some place in the wiring harness and you can begin the process of cleaning and/or replacing any part of the harness that is suspect. To properly test the coolant sensor, you'll need a multimeter that can test for resistance, most do.

Attach one lead of your tester to the black wire, and the other lead to ground. Remove the coolant sensor from the reservoir tank. With the float all the way down, the resistance across the contacts should be zero. Now raise the float up as if you had a full coolant tank -- the resistance should be approximately 3.3 ohms. If the sensor reads ok, you have a wiring problem. Reinstall the sensor. Pull the orange and white connector in your under dash harness and locate the pink/black wire (the same color you saw going into the connector). Attach your lead to this wire, and the other end of your test lead to ground. You should read the same 3.3 ohms for a full tank. If you don't, you have a bad wire and you need to replace it. If it's good, the problem is likely a bad solder joint inside the VIC. This means repair or replacement of this part.

Troubleshooting is that simple. Well done!