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Discussion Starter #1
I have a 2001 Voyager with 3.3 flex fuel and 130k miles. In my shop manual, it mentions that the following are used to determine the fan speed:

•Engine coolant temperature
•A/C pressure transducer
•Ambient temperature from the body controller
•Vehicle speed
•Transmission oil temperature
The PCM uses these inputs to determine when the fan should operate and at what speed​

I have been getting code 1491 off and on for several years. It usually goes away in the spring and fall when the AC and defroster are not used.

A week ago, the code was set and the defroster was on. It was freshly set as I started the van (double ding), Because it was really cold (polar inversion of Jan 2014, near zero degrees in Phila), the finicky power steering pump was causing the serpentine belt to squeal like a stuck pig for several minutes. I was hoping the newly set light was just a code indicating loss of alternator power, but when I read the scanner, I found I was wrong.

I am pretty certain that the code will only get set on my van when the AC clutch is engaged, based on experience. Does anyone know the test algorithm that the PCM applies to the 4 input values above, to set the code, vs clear the code?

I have seen this code clear itself if I don't use the AC or defroster for a few weeks. problem is, I only have 3 weeks before the inspection is due on this van, and it won't be possible to drive it in January without the defroster. If I understood how the test that sets and clears this code, I might actually be able to get to the root cause, and pass inspection.

One theory I have is that the code might get set if the A/C pressure transducer does not report a high enough pressure in fixed time window after the AC clutch is engaged. That would make sense, because the belt was slipping for at least 90 seconds. Last summer, I noticed that van was a little low on freon, so I topped it off. I am certain that I have not seen the P1491 between the topping off of freon last summer, and last week.

I should also mention that the 1684 code is also displayed (battery power loss in last 50 starts). This makes sense, as I had to replace the battery in November.
 

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Clean you battery terminals. Do not use battery terminal spray which can cause your problem. Use dielectric grease or vacuum grease.
Check your battery ground if terminals are clean.
Check whether the circuits share same ground or harness. If same ground is shared, check the ground. If same harness is shared, look for the places where insulation of inside wires can be broken (usually places suffering heat and vibration).
 

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Discussion Starter #4
Clean you battery terminals. Do not use battery terminal spray which can cause your problem. Use dielectric grease or vacuum grease.
Check your battery ground if terminals are clean.
Check whether the circuits share same ground or harness. If same ground is shared, check the ground. If same harness is shared, look for the places where insulation of inside wires can be broken (usually places suffering heat and vibration).
Please ignore the 1684 code. I replaced the battery less that 50 trips ago, so I would expect the 1684 code. The 1684 code won't set the CEL anyway, and would not cause me to fail inspection.

My problem is the P1491 code. I am convinced that on my particular van, it is only getting set when the engine is first started, and the AC compressor is engaged, What I am looking for is a description of the "low speed fan circuit" test that the PCM uses to when it sets the P1491 code. Other posts hint that the Freon pressure is one of the variables used in the equation used to determine the fan speed, so it might be part of the test of the fan circuit.

This code causes the CEL to light up, and in my state, I cannot pass inspection if the CEL is lit.
 

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Discussion Starter #5
This happened again in 2015. The van needed a lot of work to pass inspection, and I could not afford to fix it. The code eventually went away for a while, then came back again. The van never overheats, and I topped of the AC just to be sure.

I went to clear the code again, and my Actron reader was completely dead. I bought one of those Bluetooth OBD2 readers, and I installed Torque Pro on my phone.

Of course the 1491 code only gets set when my wife drives it, and I find out a week later. Of course she doesn't remember when it got set. Recently, I got her to notice the conditions when the code was set. It was raining hard (wipers on), and the air temperature was in the 60s. She had driven to a store 5 minutes from our house, and the code set when she returned to the van and started the engine to go home. I assume the engine had barely enough time to reach operating temperature.

The next evening, I was driving the van with the 1491 code, not yet knowing when it had been set. The air temperature was a damp 50 degrees, and the Defroster and AC was on. When I got to my destination, I cleared the 1491 code from my phone, and it would clear (Key On Engine Off). However, as soon as I started the engine, the code would immediately set again.

I opened the hood, and with the engine running, the fan was running at full speed, The engine had just reached operating temperature, and it was 50 degrees F outside, so the fan would not normally be running at full speed. I assume that when the 1491 condition is active the PCM grounds the cable (to force 100% fan speed) as a fail-safe. Better to have too much fan, than too little fan.

I went into my doctor appointment, and when I returned 30 minutes later, the van had cooled off. I was able to clear the code, and it has stayed clear ever since. I am just waiting for all the registers to set, so I can get my emissions sticker.
 

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On a 4th gen, I haven't noticed High or low, just on and off. Some new blower motor relays now have a replacememt wire harness to cut off old and splice in new and if not included, they are readily avaiable as a separate item. That said, it could be the issue.
Pressure Transducer could be the issue. Fans do not operate constantly when AC is on. As High side pressure increases, the temp of the freon Increases and I beleive at a set pressure the Transducer calls for Fan, to aid in lowering the pressure. I base this on what happened when I recharging an empty system on a 2003 DGC. I observed ths low side being under charged, while the high side hit an overcharge level, like 250psi plus, fan was called for and the pressure dropped below the correct need pressure, the fan was turned off.
So suspects are bad pressure transducer since it calls for fan or its wiring and a slight chance of cooling fan relay, but very slight chance.
I would like to know what your operating pressures are, both low and high sides.
 

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I found this in a old post:

My chjeck engine light is on (goes back on even after I reset it) and I'm getting a P1491.

Here's what my Service Manual says about the code:

P1491:

Rad Fan Control Relay Circuit - An open or shorted condition detected in the low speed radiator fan relay control circuit.
OR
Speed Control Solenoid Circuits - An open or shorted condition detected in the Speed Control vacuum or vent solenoid circuits.
OR
A/C Clutch Relay Circuit - An open or shorted condition detected in the A/C clutch relay circuit.

I'm not overheating and my AC gets cold so that leaves me with the Speed Control Solenoid Circuits.

I never use my cruise control (not enough open road here in Manila, Philippines) so I don't know if my cruise works.

Anybody else ecperience this?

Thanks,
Philip


Also, most states require that you only spend a certain amount of money trying to fix your vehicle. Ask.
 

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Discussion Starter #8
As for what 443l3.3 said:

I haven't noticed High or low, just on and off

The fan definitely runs as continuously variable speed. The relay is solid state monster bolted to the steel part of the bumper. The relay is controlled by a "Pulse Width Modulated" signal from the Computer, and allows the fan speed to be set to any speed between 1% and 100%. When the fan is turning slowly, you might not be able to hear it. When I was unable to clear the 1491 code last week, the fan was running at 100%, as if it was 100+ Degrees with the AC on, and sounded like a plane about to take off. 100% fan was definitely not expected because I had only driven 7 miles in 50 degree wet conditions. the next day, When I was able to clear it the next day, the fan was running at maybe 20%.

As High side pressure increases, the temp of the freon Increases and I believe at a set pressure the Transducer calls for Fan, to aid in lowering the pressure.

I have no way to get the High pressure on the A/C. I have one of those do-it yourself charging kits that have a gauge on the can, and connect to the low side of the A/C system. The 1491 code did set back in January, in the rain and just above freezing (defroster A/C on). The AC worked fine last summer, but I topped the freon off this spring. I have been getting the 1491 code randomly for several years. The problem might be AC might be related, but my experience last week in the mid 50's has looking elsewhere.

I am interested in what marvinstockman found:
Speed Control Solenoid Circuits - An open or shorted condition detected in the Speed Control vacuum or vent solenoid circuits.

I assume this is the "Cruise Control"? My van is a "lo-line" base van, which did not come with Cruise Control. I added it using the factory kit back in 2005. It is possible that a wire is loose. I don't think the 1491 cruise is related, because the code seems to get set when it is raining, and during short trips around town. The cruise control only gets used on the highway.

To restate: I get the code randomly, my AC appears to work fine, and the code gets set without Cruise control. It seems to get set when it is wet, and cool, or cold.
 

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Discussion Starter #9
With the code cleared, I ran the van as normal, so the "emissions readiness" registers will get marked as "passed". After a 5 days, they all said "Pass" and I was ready to make the appointment for inspection the following morning. That night, I was in the car with my wife driving, and we hit a heavy downpour. The p1491 code got set again. The next morning it cleared fine.

I guess I will have to only drive in dry conditions, until I can get the van inspected.
 

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Discussion Starter #10
SEVERAL YEARS LATER.....

It seems that my P1491 problem was with the fan relay connector. The fan relay has two thin wires (input) and two fat wires (output, that connects the fans). The thin wires are used by the PCM to Pulse Width Modulate the power to the fans, making the fans variable speed. Periodically, the fans are started at a low speed to test the circuit. If the PCM detects that no current is drawn by the thin wires, it assumes the relay is bad, and throws the P1491 code. I suspect the fans can run at full speed, even if a thin wire is broken.

My AC was being worked on, and the fans were not coming on with the AC. They would come on when the coolant temperature got high enough. The mechanic showed me how to get to the fan relay. I bought a relay, but it was one of the thin wires on the connector. Apparently one of the input wires had broken. The AC fans run at a variable speed, based on the high side pressure transducer of the AC. I returned the unused relay, and found the connector at good price.

All works now. The wires look like they were undersized, and intermittently had been open all these years.
56715
 
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