Strange carb cooling fan issue. (1 Viewer)

[Last Flight Out]

Need some help on this one and looked at every archived topic around Cooling Fan Sensor issues. I have a 1981 US Spec FJ40 for reference that is de-smogged but retains the cooling fan and relay/ecu and original crusty sensor. I have reached out to a couple of my "go-tos" like @GA Architect ... but gotta get this figured out.

The issue: Cooling fan was working fine a few weeks ago but is no longer coming on when the temperature threshold is complete (185-200 degrees) to close the small bimetallic disc inside the sensor (disc closed=fan comes on to complete circuit, etc). Giving me crazy vapor lock and need to fix soon. 5Am fuse under dash is good an tested. Fan works when jumped. Have not looked at the Cooling Fan relay/ecu 85927-60040, but it is there and all connected.

Tracing down the issue:

1) "Bench-Test Method": Removed the sensor + mounting plate (sensor still attached to plate) from stock manifold location and kept connected to the single wire factory lead. Turned ACC on in truck, draped the sensor over engine and used a heat-gun to heat up sensor and thermo-gun to measure sensor to 200 degrees. Shut ACC off and fan does not come on.

2) Did same bench-test again (as above), but this time ran a jumper ground wire to the top of sensor and the other end to chassis (kept sensor still connected to the original plug/wire as well). Temp hits 200 according to my thermo-gun, switch ACC off and fan comes on. Fan stays on until temp on sensor his about 165 and shuts off (would normally take 18 mins or so with hood closed and in stock mounting location)

3) Reinstalled everything back to manifold mounting location and tried again with grounding jumper wire - and fan works when installed. Remove jumper wire, can cuts off, put back jumper wire - fan comes on until temps lower to correct amount. See video below.

What's next: The easy way out of this is I can just run a small ground jumper from sensor to another part of chassis or engine and I assume it will work in this method. But why should I be doing that - this is a simple circuit and something has clearly failed.

What is preventing that sensor with the original wire where it is no longer coming on and will only operate correctly (185 fan comes on, 165 fan goes off) when I use ground jumper wire. When grounded in this way, the sensor is sending that signal through the factor harness, then into the relay/emissions ecu and operates normally.

Suggestions for how to trace down a bit more?

 

[Engineer8000]

The sensor "body" has to be grounded to work. The controller is looking for about 3.8K ohm resistance to ground through the sensor to work. If it is open or higher resistance, the fan will not come on. I would remove the sensor from the bracket and clean up the mating surfaces to be sure you are getting a solid ground through the mounting. Grounding the lead from the sensor to the controller will trick it into thinking it has a high temp.

 

[Last Flight Out]

The sensor "body" has to be grounded to work. The controller is looking for about 3.8K ohm resistance to ground through the sensor to work. If it is open or higher resistance, the fan will not come on. I would remove the sensor from the bracket and clean up the mating surfaces to be sure you are getting a solid ground through the mounting. Grounding the lead from the sensor to the controller will trick it into thinking it has a high temp.

Thanks for the detail. Was literally cleaning up the plate and sensor and will refit. I was under the impression that I could simply heat up the sensor bottom and use a multimeter for continuity (flat part/bottom to top nipple threaded part) once it reaches the 185-200 mark....but didn't work that way....was thinking when the metal diaphram in the sensor closes, then would have continuity between the two spots. But maybe my thinking is wrong here.

Is there anyway to test the sensor off the truck like with a multimeter?

 

[Engineer8000]

That is a NTC thermistor. The early trucks (76 and 77) that had carb cooling fans had the temp switch, after that they went to the thermistor. The controller is looking for a specific resistance, below which it will turn the fan on.

 

[45Dougal]

So set the multimeter to measure resistance in the 20k range and you should see the reading change as it warms up.
Sounds like it's going to work fine now the bracket is shiny, but watch out that you maintain continuity if you paint it.

Once again, another demonstration of why the continuity setting on your meter is not always helpful - measure resistance instead and get the full picture.

 

[Last Flight Out]

That is a NTC thermistor. The early trucks (76 and 77) that had carb cooling fans had the temp switch, after that they went to the thermistor. The controller is looking for a specific resistance, below which it will turn the fan on.

Well @Engineer8000 - Update. After cleaning bracket, sensor and engine mounting location, I mounted it temporarily at front of engine - zapped it with heat gun and it came on (with engine off).

So installed it back factory location, with end of sensor pointing to manifold, started, let it idle for a few and then drove it for 7 mins - came home, parked it and turned off - no fan. Checked temps and my thermo was reading above 200.....So Im bewildered.

Tried it again, this time I zapped it with gun and it came on.....this time in the normal location. With gun and heat - comes on. Drove it to what I though was hot enough - does not come on.

So either my thermo gun is not accurate, the sensor is going bad or I need to find a sensor that comes on like 10 degrees lower.

 

[Last Flight Out]

So set the multimeter to measure resistance in the 20k range and you should see the reading change as it warms up.
Sounds like it's going to work fine now the bracket is shiny, but watch out that you maintain continuity if you paint it.

Once again, another demonstration of why the continuity setting on your meter is not always helpful - measure resistance instead and get the full picture.

Where would I measure resistance? It only has the one wire tap on top. + lead to that and - lead to....the bottom of the sensor?

 

[Engineer8000]

Measure from the lead to the bracket or the outer body of the sensor. The book says 2.0 to 3.8 kohm at 194 to 230 F.

 

[45Dougal]

Now that you've got it installed, I'd measure it with the red probe to the wire connection on the sensor, and then the black connection to each of the points shown, and a 5th reading to the engine block, and a 6th to the battery negative post.
They should all read about the same, but you may find there's some corrosion adding some resistance at one of the interfaces.

 

[Last Flight Out]

Measure from the lead to the bracket or the outer body of the sensor. The book says 2.0 to 3.8 kohm at 194 to 230 F.

I tested at 200-210 via the pot of boiling water and saw .3-.4 mohm or 3-4 kohm (if I am converting correctly)

 

[Last Flight Out]

Now that you've got it installed, I'd measure it with the red probe to the wire connection on the sensor, and then the black connection to each of the points shown, and a 5th reading to the engine block, and a 6th to the battery negative post.
They should all read about the same, but you may find there's some corrosion adding some resistance at one of the interfaces.

View attachment 3496934

So I’m looking for consistent resistance at these points, at a stable temperature, correct? Thank you for helping me out….its really frustrating.

 

[Engineer8000]

Yes anything in the 3-4k range when hot is good. There is a bit of hysteresis or dead band where it comes on and shuts off.

 

[Last Flight Out]

Yes anything in the 3-4k range when hot is good. There is a bit of hysteresis or dead band where it comes on and shuts off.

So the question is what has changed? Why is the fan not coming on when at temp - after driving? I guess I will drive some more and see, but something has changed as i am struggling to restart when hot - when that fan used to come on to reduce temps down.

 

[45Dougal]

So I’m looking for consistent resistance at these points, at a stable temperature, correct? Thank you for helping me out….its really frustrating.

Yes. The sensor checks out ok on its own, but if you have extra resistance further downstream in the ground path then it will cause a problem.
You know what the sensor reads. The screw joint to the bracket shouldn't add more resistance, nor the next etc etc

 

[Engineer8000]

ground the lead to the sensor, fan should come on.

 

[Last Flight Out]

Now that you've got it installed, I'd measure it with the red probe to the wire connection on the sensor, and then the black connection to each of the points shown, and a 5th reading to the engine block, and a 6th to the battery negative post.
They should all read about the same, but you may find there's some corrosion adding some resistance at one of the interfaces.

View attachment 3496934

@45Dougal did a quick check this am on cold engine at all these points and finally at neg terminal of batter and all reads the same around 84.5 kohm. So I’m not sure how I can activate the fan to turn on with heat gun and not with regular engine heat. I may try doing a simple ground jumper from the top of the sensor and just attaching to another ground. Will test later to see what happens.

 

[mattressking]

The bracket you're using is for a 3F-E, not for a 40/60 intake manifold. The correct bracket places the sensor closer to the exhaust manifold...perhaps try that before pulling your hair out? My assumption is that where it is placed is not hot enough and mixing in the heat gun is getting the sensor hot enough.

 

[Last Flight Out]

The bracket you're using is for a 62, not for a 40/60 intake manifold. The correct bracket places the sensor closer to the exhaust manifold...perhaps try that before pulling your hair out? My assumption is that where it is placed is not hot enough and mixing in the heat gun is getting the sensor hot enough.

View attachment 3497489

Yeah I saw that picture yesterday when researching this. I’ll try to bend the bracket later so that it tucks up near the exhaust side more to see. I’m also testing a ground jumper (see pic, blue connector) to see if this helps.

And no, I don’t think you sold me the sensor. This was reinstalled by ACC Garage here in Atlanta when I had the truck desmogged. I guess they used something from their old parts bin.

 

[mattressking]

Grounding the sensor wire bypasses the sensor completely so it should fire the fan immediately after cycling key on/off.

 

[Last Flight Out]

Grounding the sensor wire bypasses the sensor completely so it should fire the fan immediately after cycling key on/off.

So Matt… Big thank you to you for reminding me about that bracket. I haven’t tested it yet but I remember when I had my 40 D-smogged, I had a huge tub of leftover parts, and sure enough there was the bracket stuck to a exhaust port. Cleaning it up now, and will reinstall the way it was to see if this helps. More to come.