Temperature Sensor looking wacky

[BillyB]

Okay - out in the woods a few weeks ago my water pump took a dizzump. We got a new one - went out there and swapped the new one in.

On the way home - temp would sit at what used to be normal (bottom 1/4 of the green stripe on the gauge), but them jump up to the highest part of the green.

So I'm thinking - thermostat shot. After several attempts - I got the thermostat replaced and and sealed. (my book said tighten to 30 ft/lbs - so cracked a few housings before I found 13 ft/lbs was the 'right' torque).

So after that - now my gauge will jump to 'pinned' on occasion - when starting and definately cold. Even before I fire up the engine. Sometimes it reads cold, after driving a while. Other times it bounces around.

I'm thinking the sensor may be bad.

I was just looking through my trusty 'Chilton' and saw that the proper fill on the coolant system is an 3/4 to 1 1/4" from the bottom of the filler neck. (of course - same book that said 30 ft/lbs on the thermostat housing bolts).

If I overfilled with coolant - would that cause the problem? Should I drain some out?

I think I saw a thread on how to check the sending unit - I'll look that up again also. (found this: http://home.sprynet.com/~dale02/coolant.htm)

Any thoughts appreciated!

Todd

 

[priceg]

Two things come to mind, one, you possibly have air in the system, which when passing by a temp sensor. causes incorrect output, two, your gauge wire or temp sender grounding intermittently.

 

[BillyB]

Okay - following the idears on the coolant temp switch page -

Unplugging the harness while engine running = no change.

Shorting the terminals of the harness connection = no change

When hot = 0 ohms across the switch.


Would I diagnose correctly that the switch is okay - and my problem is in the wiring between the harness and the gauge?


By the way - when it was warming up - and got to running temp - the gauge was reading correctly. I turned it off so I could unlock my hood. To get in for some tests. When I turned er back on - pegged back up to 'overheating' on the gauge.

Any thoughts?

Also - when I unplugged the harness - didn't change the gauge - still pegged to overheating.

 

[BusaDave9]

So if the gauge jumps to max and also down to min then it sounds like a bad sensor or gauge. That link you have talks about a coolant switch. Not sure what application he is talking about. A CJ sending unit is like a resistor that changes resistance according to temp. It's not a switch. Check out this link and get out your meter:
Jeep CJ Gauge & Sender Diagnostics
As for the coolant level. Don't worry about over filling. Any extra coolant will go out the overflow tube. If your gauge is jumping around quickly it's an electrical problem. Your engine cant change temp that fast.
Also look over the wires between the sending unit and the gauge. Look for damage (short or open wire)

 

[BillyB]

If I have air in the system (likely given the number of times it was drained and refilled) -

How would I go about purging the air?

Thanks!

Todd

 

[priceg]

Elevate the radiator fill neck by jacking up the front of the jeep, you don't need alot, you just want the filler neck to be the highest point of the cooling system. Run the engine, with the cap off untill both radiator hoses are the same temp. Also turn your heater on.
You can easily test your gauge and wiring by grounding your temp gauge wire. When grounded, the gauge should full range to hot. If it passes, the only part left is the sender. Good Luck.

 

[IOPort51]

The temp sending "thing y" is on top of the back of the manifold, you can loosen that and "burp" the coolant system.

If I have air in the system (likely given the number of times it was drained and refilled) -

How would I go about purging the air?

Thanks!

Todd

 

[BusaDave9]

my gauge will jump to 'pinned' on occasion - when starting and definitely cold. Even before I fire up the engine. Sometimes it reads cold, after driving a while. Other times it bounces around.

Todd

It doesn't sound like air in your cooling system is causing your problems. That can't cause your gauge to peg before you even start your engine.

Electrical problem with gauge or sending unit.

 

[IOPort51]

^^^ X2^^^

 

[BusaDave9]

Wiggle Test

Time for the wiggle test

Turn on the ignition and wiggle the wire under the dash going to the gauge.
Next have someone wiggle the wire going to the sending unit.
Look for the gauge to jump. If it does there's your problem.
If it doesn't then get out your multi-meter and check out this link:
Jeep CJ Gauge & Sender Diagnostics
Scroll down to the section called "Temperature Gauge"

 

[BillyB]

The temp sending "thing y" is on top of the back of the manifold, you can loosen that and "burp" the coolant system.

Okay - for the LIFE of me - I can't find this "thingY".

This page Jeep CJ Gauge & Sender Diagnostics says about the same thing:

The temperature sender on AMC 232 i6 and AMC 258 i6 / 4.2l I6 engine is located at the top rear of the engine head. It is near the last head bolt and next to the valve cover on the manifold side of the engine right near the firewall. It is upright and cylindrical with one wire attached to its center post. The sensor's probe extends into the head's water jacket.

I've been looking at the manifold temperature sensor (258 Manifold Temperature Sensor?? - JeepForum.com)

Been searching for a picture of where the temp sending unit is. I can't find one. There is such a tangled mess of vacuum hoses, wires and stuff over the manifold I can't get a clear view of anything back there. Does anyone have a picture of the temp sending unit on the AMC 258 i6 / 4.2l ? Or have a link?

Think I found one: http://home.roadrunner.com/~john.strenk/Gauge web page/CJSpeedometerGaugeQuickTest.htm

 

[dslywalker]

Get down on your knees and look strait under your Master Cylinder to the intake manifold and you'll see it.It will have a wire hooked to it.It's about 3/4" in diameter.If it isn't there look for a plug.It might be a purple wire.Make sure the gauge is grounded also to the dash thru the mounting screws

 

[IOPort51]

the one on the back of the head is the one I was thinking of , my apologies.

Okay - for the LIFE of me - I can't find this "thingY".

This page Jeep CJ Gauge & Sender Diagnostics says about the same thing:

The temperature sender on AMC 232 i6 and AMC 258 i6 / 4.2l I6 engine is located at the top rear of the engine head. It is near the last head bolt and next to the valve cover on the manifold side of the engine right near the firewall. It is upright and cylindrical with one wire attached to its center post. The sensor's probe extends into the head's water jacket.

I've been looking at the manifold temperature sensor (258 Manifold Temperature Sensor?? - JeepForum.com)

Been searching for a picture of where the temp sending unit is. I can't find one. There is such a tangled mess of vacuum hoses, wires and stuff over the manifold I can't get a clear view of anything back there. Does anyone have a picture of the temp sending unit on the AMC 258 i6 / 4.2l ? Or have a link?

Think I found one: http://home.roadrunner.com/~john.strenk/Gauge%20web%20page/CJSpeedometerGaugeQuickTest.htm

 

[BillyB]

I found it. Covered with oil and stuff. Cleaned the area up. Pulled it. Replaced it.

Gauge running normal I think. I'm running a bit warmer than I used to - likely the new thermostat I put in there. I hope.

Seems to be running like I would hope!

Thanks to all!

Todd