Check engine light, Code P1491

Tiny
DKMACHADO
  • MEMBER
  • 2000 DODGE DAKOTA
  • V8
  • 4WD
  • AUTOMATIC
  • 160,000 MILES
I own a 2000 Dodge Dakota and I have had the check engine light on for a while and I have not been able to locate the problem. The code P1491. It says there is an open circut. I replaced the Relay and the fan and that didn't help. I can reset the the light with an OBD II Sensor and fan will turn on but it will fry the Relay when I do that. I traced and replaced all the wires to the PDC and still no luck with the light shutting off. Any Ideas what it might be?
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Thursday, June 2nd, 2011 AT 7:59 PM

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Tiny
CARADIODOC
  • EXPERT
1491 is listed as "P1491 Malfunction In EGR System". There is a black plastic disc on top of the valve that typically causes a problem from moisture condensing inside. Used to be that you couldn't buy it separately; you got it with a new EGR valve.

What do you mean by "frying" a relay? Does the coil burn open or do the contacts become overheated? Both circuits should be protected by fuses.
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Thursday, June 2nd, 2011 AT 8:40 PM
Tiny
DKMACHADO
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The code P1491 comes up as radiator fan delay circut open for Dodge/Chrysler. I mean "frynig" as in the Relay gets so hot that it starts to melt the plastic on it and the (PDC).

"P1491 Malfunction In EGR System" is for Honda and Acura
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Friday, June 3rd, 2011 AT 1:56 AM
Tiny
CARADIODOC
  • EXPERT
You're right, darn it. Since the '96 model year, everything is supposed to be standardized among all manufacturers of cars sold in the U.S. I looked it up in the Chrysler service manual and you have the correct description.

Now that we're in the same circuit, understand that it is physically impossible for a relay to overheat, ... Except for one uncommon condition. The coil that activates the movable contact is designed to run on 14 volts which is system voltage when the alternator is running and charging the battery. The only way that coil can overheat is if it is partially shorted already. We can eliminate that because you already replaced the relay.

The second and more common cause of overheating is resistance in the circuit. Just like water flowing in a garden hose, there is a pressure drop at any restriction, as long as water is flowing. Voltage is electrical pressure. Resistance is that restriction. The goal is to have no resistance anywhere in the circuit except for the load which is the fan motor. If the contacts in the relay are burned or pitted there will be a little resistance between them. That resistance drops a little of that 14 volts so not all of it appears at the motor. The important part is when there is voltage, (lets say 1 volt), dropped across the contacts, and there is current flowing THROUGH the contacts, there is power dissipated at them. Voltage times current equals power which is heat. Heat buildup in the contacts promotes more burning which increases resistance and causes even more heat buildup. It's a vicious cycle. The cure can involve sanding the contacts clean and smooth but obviously replacing the relay will also take care of that problem.

But, ... Along with that, there's one thing you didn't change and is just as likely to cause the problem. That is burned terminals on the relay where they plug into the socket. Look at the old relay's terminals to see if two of them are blackened. If they are, replace the terminals in the socket too. Squeeze them a little tighter if necessary so they make a solid connection.

A tight fan motor will slow down and draw higher current which will aggravate the condition. Partially shorted windings in the motor will also cause an increase in current even though the fan spins freely. Now that you have a new motor installed, the current flow might be down to normal, but overheated relay terminals will still cause trouble and continue to get worse.

If by some chance you don't find overheated terminals, something else has to be drawing heavy current if the new relay continues to get hot. I can walk you through a test procedure to measure the current, but I've never had to do it myself so I don't know what a normal value is. The motor is protected by a 30 amp fuse, so taking the safety margin into account, I'd expect to see around 20 amps. To test it you'll need an ammeter that can measure at least up to 20 amps DC.
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Friday, June 3rd, 2011 AT 4:16 AM

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