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Which coolant temp sensor
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Author:  diy_darryl [ Sun Jan 23, 2011 9:42 am ]
Post subject:  Which coolant temp sensor
There are two coolant temp sensors near the front of the head, three if you count the pneumatic switch doo-dad.

Which of the two electrical (resistive) units drives the instrument panel meter?

One has a threaded terminal on it and the mounting threads are smaller, it seems to ohm out ok, about 200 ohms at room temp, can't remember exactly.

The larger one is my problem......... terminal reads shorted to ground with the wire disconnected (at room temp of 70 deg in the garage). It has a plug like a spark plug sort of.

The problem is that the parts houses online do not show this sensor as applicable on the /6, it says it is for the 318 V8? All of the larger temp sensors for the /6 1984, D150, show a two-wire sensor. I can see if someone in the past installed the wrong sensor but NOT when the wiring harness matches up also.

I pulled a bonehead move after replacing an exhaust manifold and forgot that I drained the coolant. I ran the motor for a few minutes before realizing that the temp gauge was pegged. I shut it down, filled up the radiator and all is well except now the meter only moves to the first tic mark when the engine is fully warm.

Should I just buy the sensor that 'looks' right but says it is for the 318?

Here is a pic (if it works):
Image

Image[/img]
Author:  diy_darryl [ Sun Jan 23, 2011 10:13 am ]
Post subject: 
Took a look in my Chilton's manual and did find some info. The wite color at least, on the sensor in question is blk/red which goes to the spark control computer.

The other temp sensors wire color is org/blk which according to Chilton's goes to my temp gauge.

Now.......how to figure out what I did to my truck exactly...... not enough info as to resistance readings etc in the manual that I have.
Author:  wjajr [ Sun Jan 23, 2011 10:38 am ]
Post subject: 
Set down the Chition, turn around, and walk away from it. Get on line and order a Factory Service Manual.
Author:  diy_darryl [ Sun Jan 23, 2011 11:10 am ]
Post subject: 
LOL!!!! I'm well aware I need a factory service manual but the fact is that I currently do not have one.

Here are my latest findings.......

Small temp sensor (org/blk wire) reading about 59 ohms with engine slightly warm, new in box sensor about 200 ohms at room temp, hit it for a few seconds with a heat gun, it read 69 ohms and resistance raised as it cooled. I am thinking there is no problem with sensor. It all worked until I overheated the engine and pegged the meter movement.

The OTHER sensor (blk/red wire) that goes to spark control computer is shorted when cold and was open with the engine slightly warm. Does this sensor just provide an open/close input to computer?

Now.....how do I verify accuracy of temp gauge (for those of you with a service manual that might find more info than I can currently get my hands on?

Is the temp gauge movement removable from the back of instrument panel for a possible junk yard replacement or do I have to replace the whole inst. panel?
Author:  diy_darryl [ Sun Jan 23, 2011 11:23 am ]
Post subject: 
Author:  Reed [ Sun Jan 23, 2011 1:28 pm ]
Post subject: 
What year is your truck? I have 83 and 84 FSMs and I can scan and post the relevant wiring diagrams or other sections.
Author:  diy_darryl [ Sun Jan 23, 2011 2:00 pm ]
Post subject: 
Quote:
What year is your truck? I have 83 and 84 FSMs and I can scan and post the relevant wiring diagrams or other sections.
Thanks Reed,

My truck is a 1984, 2wd, D150, /6, w/ automatic trans.

To recap...... I overheated the engine after running for a few minutes with no coolant (forgot I drained it). I pegged the temp gauge before I noticed it. The temp sensor resistance seems normal as I have a new one to compare to.

After overtemp, shut truck down, filled w/ antifreeze, restarted, temp gauge only moves to first tic mark when fully warm. It worked normally before the 'incident'. :oops:
Author:  Reed [ Sun Jan 23, 2011 7:51 pm ]
Post subject: 
According to the 84 FSM, the wire going to the temp sending unit is orange. The lean burn component is called the "water temperature thermistor" and has a black wire with a red tracer.

The sensor for the lean burn should read"continuity with a resistance less than 100 ohms" when measured below 60 degrees. When hot, the sensor should read no continuity.

The coolant temp sensor should read between 500 and 1100 ohms when the motor is cold and the ambient temperature is less than 90 degrees. When the motor is hot, the sensor should read greater than 1300 ohms.
Author:  emsvitil [ Sun Jan 23, 2011 8:28 pm ]
Post subject: 
The thermostat could be stuck open.
Author:  diy_darryl [ Mon Jan 24, 2011 7:52 am ]
Post subject: 
Quote:
According to the 84 FSM, the wire going to the temp sending unit is orange. The lean burn component is called the "water temperature thermistor" and has a black wire with a red tracer.

The sensor for the lean burn should read"continuity with a resistance less than 100 ohms" when measured below 60 degrees. When hot, the sensor should read no continuity.

The coolant temp sensor should read between 500 and 1100 ohms when the motor is cold and the ambient temperature is less than 90 degrees. When the motor is hot, the sensor should read greater than 1300 ohms.
Everything looks good with the leanburn sensor.

What I find odd with the gauge sensor specs are that I have a new one in the box that agrees with the one that is installed i.e. approx 200ohms at 70 deg room temp, DECREASING to approx 60 Ohms when slightly warmed up, which is OPPOSITE of what the manual says?????
Author:  diy_darryl [ Mon Jan 24, 2011 7:55 am ]
Post subject: 
Quote:
The thermostat could be stuck open.
LOL!!!!!

You might have hit the jackpot, what was I thinking????? That is a 'classic' symptom. :shock:


Another thought........ I do not drive this truck and maybe I am ASSUMING that the gauge read in the 'normal' range.

I asked my 17 year old son where the temp gauge normally read and he said "WHERE'S THE TEMP GAUGE?" :roll:
Author:  diy_darryl [ Tue Jan 25, 2011 6:33 am ]
Post subject: 
Drained coolant down, thermostat was NOT stuck open. Changed it anyway and also put new coolant temp sensor in. Started truck and now it indicated HOT, almost full scale but motor temp seemed normal. Could even hold my finger in the hot antifreeze without too much discomfort.

Drained back down and put original temp sensor back in and gauge now reads one tic mark below mid-scale deflection.

Not sure what the deal was with the new temp sensor, defective???? Not the right one? (although it appeared identical and resistance readings agreed mostly with a few spot checks at unknown temps)

Thanks for the help guys!
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