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Unread 10-20-2006, 08:37 PM   #16
greasefingers
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1994 ZJ 
 
Join Date: Sep 2003
Location: Northern VA, USA, NJ
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I am assuming that my dash board gage or the sensor is off. I can measure the coolant temperature at the thermostat housing (this is the wrong one however) and get FSM valves.

The PCM uses the coolant temperature sensor located on the thermostat housing; for air-to-fuel ration calculations and other things. But the dash board gauge uses a different coolant temperature sensor that is located on top of the cyl head (driver’s side) and back by cyl #6. (this is for the 4L engine)

A thermostat will start to open at it’s rated temperature (195 F) and is fully open at some temp slightly higher than that.

A 50/50 mixture of anti-freeze boils at something close to 240 F without a radiator cap. With a 16 psi rated cap it increases the boiling point to something closer to 260 F (if I remember correctly)

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94 Black Laredo 4-Liter with QuadraTrac (42RE trans & NP249 TC) 200,000 + miles purchased new

88 Cherokee 2-door with 5speed manual 139K miles(deceased) purchased new

Last edited by greasefingers; 10-21-2006 at 05:34 AM..
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Unread 10-20-2006, 08:37 PM   #17
j.love
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ok, since you guys are talking about coolant temps here you got me to adress what I thought was just a bad guage reading on mine.

My guage reads realy low and I thought it was just a bad sender or a bad guage. I did a quick check on the uper rad hose and it was hot, and I was getting heat in the cab, so I just left it at that. I knew from talking to the guy who sold it to me that he put a 180 thermostat in it, and it dose have the jet (crap) chip. so I just figured it ran cold.

So I dove in a little further, My guage reads rite around the 127 deg. mark. Witch I thought was pretty imposiable. The upper rad is too hot to grab ahold of. and I'm getting 130 deg. air out of my heater vents. This is after running pretty good in and out of traffic for a good 1/2 hr.. Outside air temp was about45 deg. tonight. now the part that threw me was that the lower rad hoser was almost cold. So I wanted to pop the cap on the rad. Felt the cap and it was cold. Poped it open and droped the thermometer down into the rad fluid. was rite around 100 deg. with the motor still running and as hot as it was going to get.

So my thought is that the thermostat is stuck closed. ????
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David.
94 ZJ laredo 5.9 V8 swap D44a posi
Quote:
Originally Posted by ratmonkey View Post
I wouldn't drive down a driveway on that.
''If we do everything right, if we do it with absolute certainty, there's still a 30% chance we're going to get it wrong.''
Joe Biden
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Unread 10-20-2006, 09:21 PM   #18
greasefingers
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With the thermostat stuck wide open, it will still reach 195 F but it will take much longer. One that is stuck closed will over heat in a very short period of time.

I stuck my coolant sensor (at the thermostat housing) in a pot of water and measured the resistance with increasing temp. Then I co-plotted my data with the factor service manual data, using MS Excel. They over-laided one another. So is good.

A smarter way to it would be to back-probe the coolant temperature sensor, once the thermostat has opened, and extrapolate the temp from the FSM valves. This is how I know that my thermostat is functioning properly and that my dash gage or the dash age gage sender is wrong.
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Cheers Steve

94 Black Laredo 4-Liter with QuadraTrac (42RE trans & NP249 TC) 200,000 + miles purchased new

88 Cherokee 2-door with 5speed manual 139K miles(deceased) purchased new
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Unread 10-21-2006, 12:18 AM   #19
j.love
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I'll get out there in the morning and check the whole system from a cold start. Feel for the heat comming from the top rad hose for when the t-stat opens up, and check to see if I can see the flow at the rad cap. Maybee the thing is stuck wide open. Its been a while since I had to diagnose a cooling problem.

I'll then break out the old test meeter and see if the dash guage reads the properly.



.
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David.
94 ZJ laredo 5.9 V8 swap D44a posi
Quote:
Originally Posted by ratmonkey View Post
I wouldn't drive down a driveway on that.
''If we do everything right, if we do it with absolute certainty, there's still a 30% chance we're going to get it wrong.''
Joe Biden
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Unread 10-21-2006, 05:23 AM   #20
greasefingers
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Here is a pic of the graphs that I plotted. This tells me that the range of my sensor(at the thermostat housing) is fine, however the X/Y scales need to be blown up a bit to pick off the resistance in ohms at 195 F. I will see if I can find the original Excel data and post another graph, but it may be on my daughters computer (she's sleeping)
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Cheers Steve

94 Black Laredo 4-Liter with QuadraTrac (42RE trans & NP249 TC) 200,000 + miles purchased new

88 Cherokee 2-door with 5speed manual 139K miles(deceased) purchased new

Last edited by greasefingers; 10-21-2006 at 08:04 AM..
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Unread 10-21-2006, 05:53 AM   #21
greasefingers
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The coolant temperature sensor (at the thermostat housing) and not the dash board gage info: From 1994 FSM; both engines (4L & 5.2L) at 195 degrees F the resistance of the coolant temperature sensor should be between 860 to 970 ohms.

Don’t forget this sensor may not read correctly until the thermostat opens up and floods the thermostat housing. In other words it may not be submerged in fluid until then and will not read correctly until then.
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Cheers Steve

94 Black Laredo 4-Liter with QuadraTrac (42RE trans & NP249 TC) 200,000 + miles purchased new

88 Cherokee 2-door with 5speed manual 139K miles(deceased) purchased new

Last edited by greasefingers; 10-21-2006 at 08:05 AM..
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Unread 10-21-2006, 08:02 AM   #22
greasefingers
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The second coolant temp sensor, for the dash board gage, is of a different type. They are both thermistors and decrease in resistance with increasing temperature. Since they are thermistors, they are not linear and the data points should be plotted with a second degree polynomial curve fit for interpolation. The dash board gage will be sent a resistance between 245 to 255 ohms when the temperature of the coolant is 195 F.

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94 Black Laredo 4-Liter with QuadraTrac (42RE trans & NP249 TC) 200,000 + miles purchased new

88 Cherokee 2-door with 5speed manual 139K miles(deceased) purchased new
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Unread 10-21-2006, 08:20 AM   #23
j.love
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Steve, The second graph for the coolant temp sensor would indicate that the guage would read an inacurate reading or more properly by less sensitive reading, as the tempiture rizes past somewhere near the 200 deg. mark. Would I be correct in that asssumption? It would make sence to me that it dosent need to me as acurate once the temp. got to that point.






.
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94 ZJ laredo 5.9 V8 swap D44a posi
Quote:
Originally Posted by ratmonkey View Post
I wouldn't drive down a driveway on that.
''If we do everything right, if we do it with absolute certainty, there's still a 30% chance we're going to get it wrong.''
Joe Biden
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Unread 10-21-2006, 10:02 AM   #24
greasefingers
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Yea those data points come from the FSM, so the thermistor is mostly linear until it gets really hot then the slope changes and almost goes flat. But it is hard to comment on something with three sets of data points unlike the first graph that has statically a sound number of points to accurately define the curve.
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94 Black Laredo 4-Liter with QuadraTrac (42RE trans & NP249 TC) 200,000 + miles purchased new

88 Cherokee 2-door with 5speed manual 139K miles(deceased) purchased new
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Unread 10-21-2006, 10:23 AM   #25
greasefingers
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Here it is blown up a bit more
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94 Black Laredo 4-Liter with QuadraTrac (42RE trans & NP249 TC) 200,000 + miles purchased new

88 Cherokee 2-door with 5speed manual 139K miles(deceased) purchased new
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Unread 10-21-2006, 11:26 AM   #26
97GrandCherokee
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mine runs between 205 and 210. Is that bad?
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Unread 10-21-2006, 12:16 PM   #27
j.love
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went out there this morning and did the hand check for the thermostat. waited till it started to warm, and then I did get a rush of hot coolant running threw the top hose. So, I do know that the thermostat is opening and closing at temp. but what temp I do not know rite now. ( The neibors were looking at me funny as it was. had a cup of coffe in one hand and the ther firmly griped to the upper hose. "morning dave" I reply with a raise of my mug and a head nod.)

ok steve, I'm ready to start to pull some ohms reading. maybee later tonight when the motor is good and cold again. i'm just gona try and back probe the temp sensor while its on the motor and i'm not gona try the boiling water trick just yet. so my question would be is there a diferance between a 4.0l sensor and a 5.2? I am running a 5.2.

Now i'm wondering if the reading for the two sensors should be relitivly the same ohms readings, and since the two on the 5.2 are in the same general location wouldnt they be reading the same temp. or is one inlet temp and the other out let temp? (or before and after thermostat) Gota look that up in my book.




***97grand- Your starting to creep up there in temp but your fine for now. I wouldnt worry untill it gets past the 215-220 mark. If your worried about it you might want to do a flush of the system and clear the radiator of debree. ( If I remember corectly things start to go drasticly wrong at about 230? )






.
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David.
94 ZJ laredo 5.9 V8 swap D44a posi
Quote:
Originally Posted by ratmonkey View Post
I wouldn't drive down a driveway on that.
''If we do everything right, if we do it with absolute certainty, there's still a 30% chance we're going to get it wrong.''
Joe Biden
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Unread 10-21-2006, 12:21 PM   #28
j.love
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ok edit that , they are two diferent ohms scales. one for each sensor. sorry too much coffee!
But. I still have a question about if from 4.0 to 5.2 are they the same ohms?





.
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David.
94 ZJ laredo 5.9 V8 swap D44a posi
Quote:
Originally Posted by ratmonkey View Post
I wouldn't drive down a driveway on that.
''If we do everything right, if we do it with absolute certainty, there's still a 30% chance we're going to get it wrong.''
Joe Biden
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Unread 10-21-2006, 12:32 PM   #29
97GrandCherokee
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its never gone past 210 at all and plus i live in minnesota and its cold out here like 30 degree weather now and maybe its how my jeep runs cuz i live in a colder state?
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Unread 10-21-2006, 03:14 PM   #30
greasefingers
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205 to 210 F is normal for many and both jeep engines. The ohms is the same for both engines, however the dash gage sensor only has one wire, so I do not know how to measure it.
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Cheers Steve

94 Black Laredo 4-Liter with QuadraTrac (42RE trans & NP249 TC) 200,000 + miles purchased new

88 Cherokee 2-door with 5speed manual 139K miles(deceased) purchased new
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