RPMs jumping in OD

chaselee

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On my way to work this morning, going down the highway my trucks keeps randomly jumping rpms. It is probably 200 rpms that it's fluctuating between. Just keeps jumping back and forth. This is the first time this has happened. Anyone have any ideas? Is this the torque converter?
 

chaselee

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Just dawned on me that I did replace the alternator on Saturday. I wouldn't think it's related, but guess you never know.
 

Chris2590

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It sounds strange but check all of your brake light bulbs. Try unplugging your neutral safety switch plug and plugging it back in also. Sometimes it can get moisture in it and do funky things


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chaselee

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Now that you mention brake lights. I also replaced the tail lights with led bulbs Saturday as well. Had one out so I figured why not drop some LEDs in there.

Where is the neutral safety switch?
 

cleatus12r

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It's the LED bulbs. The PCM needs to see ground through the filaments when the lights aren't on. Since there is no continuity through an LED bulb, the PCM will not allow torque converter clutch apply at less than 40-ish% "throttle".
 

chaselee

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So do I need to install resistors or what? I don't have hyper flash. Surely there's people out there running led tail light bulbs in these trucks. BTW, my lights were on, not sure if that matters. And how in the world are the tail lights and tq converter related in the pcm!? Lol
 

cleatus12r

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The PCM's primary input for torque converter clutch release when the brakes are depressed happens to be the 12-volt signal from the brake pedal switch. It's quick and dirty and it works. The reason you don't end up with the hyperflash on an OBS truck is because the filaments in the front turn signal bulbs are sufficient to load the flasher relay.

A resistor between the brake switch output and ground is all you need to fix your problem..... a better option due to heat is to simply wire in a 194 light bulb somewhere underneath the pick up or behind the back bumper that is wired to your brake light circuit.
 
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chaselee

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Wow, that's the craziest thing I've heard in a long time. Lol but I'm relieved to know that it's not my converter going out. Thanks for all the help fellas.
 

chaselee

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Ok so when wiring in the resistors being that my problem is brake light related, I guess I need to wire the resistors into the brake light wire right? I've done this before, but it's been for hyper flash so I wired it to the blinker wire.

Also anyone have any idea what size resistors I need to get?
 

gnxtc2

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Hold on before you start adding load resistors.

Did you replace all three brake light bulbs (taillights and high mount cab light)? All my lights (front and rear) on my truck are LED and I only had to install a load resistor for the high mount cab light. I did have to install a differrent flasher.

Billy T.
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gnxtc2

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I highly doubt that the taillights alone is causing your problems. When I did my LED conversion, I did the swap in stages. I did the taillights first. I went for a drive and no issues arose. Before installing load resistors, swap in regular bulbs and see if your problem goes away. Whats the point of going to LED and having to install load resistors.

Yes, I had to install one load resistor for the high mount brake light on the cab which is known to contribute to all sorts of problems when it goes out.

Do you have a mechanical or electric fuel pump?

Billy T.
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chaselee

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Every time I've done a led conversion I've had to do resistors. Anyway, I just installed some so we shall see if that's the cure.
 

chaselee

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Well you fellas are gentleman and scolars! The resistors cured the problem. I would have never dream that I needed tail light resistors for my TC to work properly but it is, what it is. Voltage can do weird things. I appreciate all the help.
 

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