Preheating system acting funny again....

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Yeah, this happened last winter, and I thought I fixed it. No matter how cold it is, the WAIT TO START light stays on for the same amount of time... 11. 5 seconds to be exact. I've been recording outside temperatures during different starting periods. 45 deg. , WTS light on for 11. 5 seconds. -15 deg!, WTS light on for 11. 5 seconds. And that time, it took some cranking to get it going. I'm finding my self having to cycle the heaters 2 or sometimes 3 times to get a good preheat out of it when its -15 outside. This summer, I replaced the air temperature sensor in the intake, and I thought that did the trick. Shouldn't the amount of preheat time be dependant on the ambient air temp? Cause if thats so, my truck is stuck on the 45 deg. preheat time... even if the air out side is -30 with the winde chill. Any ideas? When I can't plug it in, its getting hard to start. Thanks for any input.

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'91 D-250, Cummins, 3 speed auto, 165k, Hurculiner bedliner, after market bumper,custom one tone metallic blue paint job (sorry no pics) and completely stock... . for now anyway

 

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Hi jaynes.

The '93 service manual says above 59F-no heat, 18-59F-10 sec. , 1-16F-15 sec. , -15-0F 17. 5 sec. , below -15F-20 sec.

In '93 the heaters are controlled by the PCM. If your '91 is the same and your sensor is good it may be the PCM. If you don't have a PCM then it may be whatever timing circuit does control the heat cycle.

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'93 W350 Club Cab, Bright White, 5 sp. , 136K, Factory Options: LE package, 4. 10 Limited Slip, 7600 lb. rear springs, tachometer, front stabilizer bar. Mods: Banks stinger plus, Linex bedliner, 25,000 lb gooseneck ball, 10,000 lb receiver, Tekonsha brake controller, POWER WAGON injectors-WAY faster than stock!.

 

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Your preheat time duration should be controlled by your pcm located in the fenderwell next to your battery. The pcm could be shot, but I would recommend pulling the whole thing out for a general condition assement and clean-up. Pay attention to the connector and wires; look for frays, corrosion or any other sign of wear. I know this sounds like a joke, but I lost a bet and an alternator to my dad over this issue this year. Oh also, if the the pcm sounds like a baby rattle when you move it, you might have another $250 investment headed your way. Just kidding.

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1992 W-350 Club Cab Dually (All quiet for now)
1992 W-350 8' contractors dump with 3. 5" exhaust(don't ask)

 

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Do you guys know if the preheat system is the same for the pre 91. 5's or is it different. Cause if its different, it might work differently. I've heard that PCM and or wiring is completely different from pre91. 5s and post 91. 5's (91. 5 and up. ) So I was just thinking that. I'll have to check the PCM and the relating systems... see if everything is clean and intact. Thanks guys.
Josh

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'91 D-250, Cummins, 3 speed auto, 165k, Hurculiner bedliner, after market bumper,custom one tone metallic blue paint job (sorry no pics) and completely stock... . for now anyway

 

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I am not sure about the pcm differences between the years. I have two 92's, but my dad just bought a nice 91. 5 intercooled w-250. Sooo, I'll look out under the hood and see if anything is different visually, but my service manual is only for the 1992 year. Sorry
Russell

 

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Thats ok, I think there is a bad component somewhere, its just a matter of finding it. Like this morning, the temp here was about 12 deg, and it should have preheated for about 14 seconds if I remember right, but it didn't. Sooo, it comes down to trial and error now... I'll take it in as a last resort. Otherwise I can just keep cycling like I have been. Thanks!

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'91 D-250, Cummins, 3 speed auto, 165k, Hurculiner bedliner, after market bumper,custom one tone metallic blue paint job (sorry no pics) and completely stock... . for now anyway