1989 YJ 2.5L Coolant Temp Sensor Troubleshooting - JeepForum.com
 
LinkBack Thread Tools
post #1 of 10 Old 11-14-2019, 02:11 AM Thread Starter
YJspin
Registered User
1989 YJ Wrangler 
 
Join Date: Nov 2019
Posts: 5
Garage
1989 YJ 2.5L Coolant Temp Sensor Troubleshooting

**Update: Title should read: Low reference voltage on 1989 YJ 2.5 L

I've tested the harnesses at a couple sensors and I'm getting a reference voltage around 2.4-2.5 volts. I'm assuming this is low and that I have a short/ground somewhere?**

Hello all,

I recently picked up a 1989 YJ 2.5L TBI. I'm currently working to get it to pass CA emissions... but that's another story.

Right now I suspect that my coolant temperature sensor is bad... or at least the wiring is. I haven't pulled the sensor to fully test it yet (cold engine and installed, I didn't get a resistance reading but I'll look into this further later). My concern is with the wiring harness (jeep side). I've searched the forums, and I don't see anything related to the reference voltage/harness troubleshooting specifically for the '89 YJ 2.5L.

So the issue: I'm getting a consistent reading of 2.5 volts as the reference voltage coming through from the harness. And so we're on the same page, I'm placing the positive terminal on the signal wire and the negative terminal on the negative terminal of the battery. Then I'm turning the ignition switch to on (not turning the engine of course). This gives me 2.5 volts at the harness. Everywhere that I read says voltage should be 5.0 volts or 12 volts. Does this suggest I have a short? The strange part is that I'm getting exactly 2.5 volts so seems like this could possibly be a legit number?

Does anyone have experience in this area that can weigh in? The bottomline is, do I have at short with a 2.5 volt reading or is that the correct reference voltage for an '89 2.5L?

Thanks!


Last edited by YJspin; 11-14-2019 at 12:43 PM. Reason: Added information
YJspin is offline  
Sponsored Links
Advertisement
 
post #2 of 10 Old 11-14-2019, 08:46 AM
fishadventure
Registered User
 
fishadventure's Avatar
1989 YJ Wrangler 
 
Join Date: Jul 2013
Location: Morrisville
Posts: 6,742
Garage
The sender is merely a variable negative(-) according to what temp it is seeing. Lower the volts the colder iirc

[size=3]Shackles & D-rings are different things.
Cranking IS turning over
fishadventure is online now  
post #3 of 10 Old 11-14-2019, 11:27 AM Thread Starter
YJspin
Registered User
1989 YJ Wrangler 
 
Join Date: Nov 2019
Posts: 5
Garage
Sorry, I think I might of confused my original post by talking about the actual sensor.

I am not testing the sensor right now (its disconnected), I'm testing the harness on the Jeep side that connects to the sensor. So, I have the sensor disconnected and my multimeter probes on the jeep side harness. With the jeep on, I am getting a reading of 2.5 volts coming from the Jeep's harness. I'm reading online that the voltage should be 5 volts or 12 volts depending what what the ECU is setup to do, but I'm only getting 2.5 volts. Just not sure if this is correct for the '89 YJ 2.5 liter.
YJspin is offline  
 
post #4 of 10 Old 11-14-2019, 07:31 PM
chesterm64
Registered User
1989 YJ Wrangler 
 
Join Date: Jul 2017
Posts: 23
I just checked mine and it is 2.435 volts. It's an 89 2.5L.
chesterm64 is offline  
post #5 of 10 Old 11-14-2019, 08:09 PM
Boojo35
Registered User
1987 YJ Wrangler 
 
Join Date: Jul 2007
Location: Beer City
Posts: 1,670
Garage
Another shorted sensor can pull down the voltage. Disconnect things like the Intake air temp, Tps, and Map, etc. and see if the voltage comes up. 89 FSMs suk and are hard to navigate. I get your question though. You need 5v to the sensor. I agree that you probably do. A lot of sensors generically share the 5v reference and a bad one can pull the reference to the others down.

If you cannot fix it with a hammer then it has to be an electrical problem.
Boojo35 is offline  
post #6 of 10 Old 11-14-2019, 10:35 PM Thread Starter
YJspin
Registered User
1989 YJ Wrangler 
 
Join Date: Nov 2019
Posts: 5
Garage
Quote:
Originally Posted by chesterm64 View Post
I just checked mine and it is 2.435 volts. It's an 89 2.5L.
Hm, that's interesting. I'm getting ~2.43 at my Coolant Temp Sensor and 2.22 at my Air Temp Sensor... so maybe this is standard or maybe you have my same issue? I haven't seen anything that references 2.5v vice 5v. It seems if the ECU is controlling the voltage, we should be seeing a reference voltage of 5v and if its a line coming from the battery (heater wire on the O2 sensor) it should be giving a readout of 12v. But interesting that we're seeing the same voltage at the temp sensor.

Quote:
Originally Posted by Boojo35
Another shorted sensor can pull down the voltage. Disconnect things like the Intake air temp, Tps, and Map, etc. and see if the voltage comes up. 89 FSMs suk and are hard to navigate. I get your question though. You need 5v to the sensor. I agree that you probably do. A lot of sensors generically share the 5v reference and a bad one can pull the reference to the others down.
Sounds like a good plan. I'll work through this soon. Of note though, my O2 sensor harness reads 0 volts... so no reference voltage is getting to the O2 sensor. The 12v heater line (of the O2 sensor) reads perfect (12.06 volts). So... that was interesting. In the meantime, I'll go ahead and read the voltage at each 5v sensor, record readings, then start unplugging things to see what happens. I'll come back with updates. If I still get sporadic readings, I'm thinking the cause would be the ECU since its the common link here.

...unless someone else can confirm mine and chesterm64's voltage readings of 2.43 volts at the Coolant Temp Sensor?

Last edited by YJspin; 11-14-2019 at 10:42 PM. Reason: added comments; spelling
YJspin is offline  
post #7 of 10 Old 11-15-2019, 12:01 AM Thread Starter
YJspin
Registered User
1989 YJ Wrangler 
 
Join Date: Nov 2019
Posts: 5
Garage
Disregard my previous voltage numbers from above. I just got back from pulling reference voltages for my Coolant Temp Sensor, Air Temp Sensor, MAP, TPS, and O2 Sensor with the following readouts:

Cool Temp: 2.38v
Air Temp: 2.38v
MAP: 4.96v (back-probed signal wire which showed 4.68v at zero vacuum and responded well when vac applied)
TPS: 4.96v (back-probing the signal wire didn't work out so couldn't test sensor response)
O2 Sensor: 0.01v (12v wire checked good)

As you can see, Coolant Temp and Air Temp are possible issues at 2.38v. I will say that the wiring diagram in Chilton shows the two sensors sharing a ground wire (possible clue?). The definite issue is on the O2 harness with ~0v. So maybe two problem areas? Would this kind of behavior point to the ECU? I can check stuff, but I'm not great with electrical so not great with diagnosing. Any help would be great. Thanks!

Also, anything else I should check?

Last edited by YJspin; 11-15-2019 at 12:03 AM. Reason: clarity
YJspin is offline  
post #8 of 10 Old 11-15-2019, 05:49 AM
chesterm64
Registered User
1989 YJ Wrangler 
 
Join Date: Jul 2017
Posts: 23
Quote:
Originally Posted by YJspin View Post
Hm, that's interesting. I'm getting ~2.43 at my Coolant Temp Sensor and 2.22 at my Air Temp Sensor... so maybe this is standard or maybe you have my same issue?
Mine seems to be running fine right now, goes to closed loop at about 135 degF, which is what I think it should do.
chesterm64 is offline  
post #9 of 10 Old 11-15-2019, 06:31 AM
chesterm64
Registered User
1989 YJ Wrangler 
 
Join Date: Jul 2017
Posts: 23
These are my numbers after driving to work this morning. About 8 miles and 29 degF here.
Scanner numbers

Last edited by chesterm64; 11-16-2019 at 12:31 PM. Reason: fix image url
chesterm64 is offline  
post #10 of 10 Old 11-15-2019, 07:21 AM
fishadventure
Registered User
 
fishadventure's Avatar
1989 YJ Wrangler 
 
Join Date: Jul 2013
Location: Morrisville
Posts: 6,742
Garage
Quote:
Originally Posted by YJspin View Post
Disregard my previous voltage numbers from above. I just got back from pulling reference voltages for my Coolant Temp Sensor, Air Temp Sensor, MAP, TPS, and O2 Sensor with the following readouts:

Cool Temp: 2.38v
Air Temp: 2.38v
MAP: 4.96v (back-probed signal wire which showed 4.68v at zero vacuum and responded well when vac applied)
TPS: 4.96v (back-probing the signal wire didn't work out so couldn't test sensor response)
O2 Sensor: 0.01v (12v wire checked good)

As you can see, Coolant Temp and Air Temp are possible issues at 2.38v. I will say that the wiring diagram in Chilton shows the two sensors sharing a ground wire (possible clue?). The definite issue is on the O2 harness with ~0v. So maybe two problem areas? Would this kind of behavior point to the ECU? I can check stuff, but I'm not great with electrical so not great with diagnosing. Any help would be great. Thanks!

Also, anything else I should check?
The thing with sharing common neg(-) wire is that “voltage” isn’t really a power level, sortof more of an indication of a difference in potential (or ‘pressure’) between two points in a circuit. You can measure voltage but it won’t show you if there’s only milliamperes of current or 20 A of current

But you can jumper the neg(-) at two points along the suspect wire and test to see if/where it’s compromised if the jumper makes things better.

You can also test voltage close to the ECU- or not so much test voltage but put one lead of a multi meter on the voltage available close to the ECU and then on that same wire but close to the sensor put the other lead of the multi-tester. If that doesn’t show like .01 difference in voltage but is much higher then you can probably suspect a compromised wire. The voltage drop test is valid on either (+) or (-) side of a circuit

[size=3]Shackles & D-rings are different things.
Cranking IS turning over
fishadventure is online now  
Reply

Quick Reply
Message:
Options

Register Now



In order to be able to post messages on the JeepForum.com forums, you must first register.
Please enter your desired user name, your email address and other required details in the form below.

User Name:
Password
Please enter a password for your user account. Note that passwords are case-sensitive.

Password:


Confirm Password:
Email Address
Please enter a valid e-mail address for yourself.



Email Address:
OR

Log-in











Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)
 
Thread Tools
Show Printable Version Show Printable Version
Email this Page Email this Page



Posting Rules  
You may post new threads
You may post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are On
Refbacks are On

 
For the best viewing experience please update your browser to Google Chrome