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Unread 08-20-2008, 02:29 PM   #1
fgcox1
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The Ultimate Coolant Temp Sender/Sensor Thread Once and For All

As I was researching to solve a temp gauge problem on my '93 4.0 I have come across a lot of confusion concerning the coolant sensors on various years and engines of the YJ. The '93 FSM is not very clear on this.

I am posting what I have found and am looking for verification on it. We can collaborate and the info might become an FAQ item.

As one would expect, with some YJ's having carbs and others being fuel injected, there are some significant differences.

For example, a computer controlled fuel injection setup needs an input to allow for computer awareness of the engine state (ie:temperature). YJ engines use a sensor (ECT - engine coolant temperature) located in the thermostat housing to tell the computer what is what. As the engine temperature changes the sensor varies the resistance to let the computer know what the temp is. The sensor has a 2 wire connector on it. Some earlier models may have used a switch to let the computer know when it has reached a preset temperature.

A carbed engine would not need this and would use either a switch for an idiot light or a thermistor (a variable resistor that changes resistance depending on temperature) for a gauge. This is usually a one wire hookup as the body of the sensor is grounded and the gauge reads the resistance in reference to ground. It also uses the ground as a return path.

So now we know that a fuel injected engine with a gauge will have 2 sensors/sending units on it.

Now we come to the location of these sensors which is where most of the confusion comes from.

All 2.5 engines and 4.0 engines up to '90 or '91 (not sure of specific years) have the temp gauge sending unit installed in the head on the drivers side rear of the engine right next to the valve cover.

A change was made in 1992 (?) that moved the gauge sender to the drivers side of the 4.0 engine block, near the block drain plug, under the intake and exhaust manifolds and about two inches above the bottom of the block. That is why you will see folks swearing they do not have a sensor on the rear of the head and being very confused!.

I personally still haven't been able to locate the gauge sender on my '93 due to the air cleaner box and the motor mount blocking the view of the area where the 93 FSM says it should be. I hope to do that this weekend. I will update this thread on what I find.

Any of you guys with a 92 and later model 4.0 that have dealt with this can hopefully verify the location.

It is very important to qualify what engine and year a person has when they ask about this to help clarify where the sensor is and which one it is.

Hopefully this thread will help folks on resolving temp gauge problems. I do not claim to be the source of this info, just the reporter so if some of it is inaccurate please let me know.

Tnx Fred in Mesquite Texas

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Unread 08-22-2008, 10:49 AM   #2
fgcox1
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'93 4.0 owners, help needed!!!!!

Now I think I am going nuts!

I pulled the airbox for access and there isn't a sensor on the side of the block as indicated by the FSM.

If you own a '93 4.0, you could do the entire Jeep world a huge favor.

At the back of the engine, drivers side next to the firewall there is a small square, grey, one wire connector coming out of the harness just behind the last injector.

Mine has the remains of a wire connected to it.

If someone could trace that wire, not the harness end but rather the other to the sensor location, we could put this to rest for good.

Anyone willing to help?

Tnx Fred in Mesquite, TX
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Unread 08-22-2008, 12:23 PM   #3
muddy93
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I've also got a 93, 4.0HO, fuel injected, and my temp guage sensor is on the top, rear, drivers side of the head, next to the valve cover(same location that you mentioned for earlier moddles in the first post). I don't have any pics of it at the moment, but I'll try to get one this weekend.
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Unread 08-22-2008, 12:37 PM   #4
jeepster93
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'93 4.0 liter.
In the front of the engine.

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Unread 08-22-2008, 12:58 PM   #5
muddy93
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Quote:
Originally Posted by jeepster93 View Post
'93 4.0 liter.
In the front of the engine.


that's the temp sensor for the computer. there is a seperate sensor for the temp guage. the location of the second/guage sensor is the one in question.
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Unread 08-22-2008, 07:37 PM   #6
backinthegame
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x2 with muddy93. We had to replace ours a few months ago.
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Unread 08-22-2008, 07:44 PM   #7
Brandon_YJ
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What, you don't like the 258ci (4.2L) engine or something?
Mine is located on the passengers side of the engine block at the rear of the block. Resembles a "bell" with a wire sticking in the top of it.
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Unread 08-22-2008, 08:21 PM   #8
fgcox1
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Temp Gauge Sending Ubit

Muddy93,

If you (or anyone else) can get a picture it would surely (don't call me shirley!) help. I swear there isn't anything there.

I will attempt to post a close up of the location. I have circled my connector to no where. If you can make out the detail, just above the connector you can see the bolt with a nut on it at the very back of the head.

I also have attached a page from the 93 FSM that purports to show the placement on the side of the block but there is nothing there either


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Unread 08-22-2008, 09:45 PM   #9
timatoe
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Quote:
Originally Posted by Brandon_YJ View Post
Mine is located on the passengers side of the engine block at the rear of the block. Resembles a "bell" with a wire sticking in the top of it.
That's oil pressure, engine coolant temp. is, driver side rear corner, next to the valve cover.

fgcox, the sensor may only appear to be a stud sticking up from the head.
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Unread 08-22-2008, 10:03 PM   #10
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Quote:
Originally Posted by timatoe View Post
That's oil pressure, engine coolant temp. is, driver side rear corner, next to the valve cover.

fgcox, the sensor may only appear to be a stud sticking up from the head.
Oops, I knew that, just forgot what the thread was about.
No, seriously, I knew that... had to replace it already.
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Unread 08-22-2008, 11:41 PM   #11
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My carbed 4.2 has a temp sending unit and (had) a coolant temp sensor. The temp sending unit for the gauge is at the rear of the cylinder head ( red in the pic) and the temp sensor for the pulse air system is on the intake manifold ( in yellow). I didn't mean to confuse but clarify that 4.2L's have 2 also.
temp-sending-unit-sensor.jpg  
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Unread 08-23-2008, 04:48 AM   #12
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92 4.0 Mine is drivers side top of the head in the back. one of the bigger differances is the unit them selfs. In the early 90's they seem to have changed almost every year. for mine I couldn't get the correct sender from the parts store. Went to the local Jeep guru and found out the above info.
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Unread 08-23-2008, 06:01 AM   #13
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93 YJ 4.0L
Mine is on the drivers side rear of the head.


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Unread 08-23-2008, 07:20 AM   #14
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89 YJ, 99 4.0, 95 OBD1 harness and electronics.
Hillbilly tee in the heater inlet hose with mechanical gauge sender

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Unread 08-23-2008, 10:09 AM   #15
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Quote:
Originally Posted by fgcox1 View Post
Muddy93,

If you (or anyone else) can get a picture it would surely (don't call me shirley!) help. I swear there isn't anything there.

I will attempt to post a close up of the location. I have circled my connector to no where. If you can make out the detail, just above the connector you can see the bolt with a nut on it at the very back of the head.

I also have attached a page from the 93 FSM that purports to show the placement on the side of the block but there is nothing there either


the pic pf your plug looks like the plastic plug part has seperated from the metal part that screws into the head. what's left is probably going to just look like a bolt head on top of the head.
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