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Old 06-26-2012, 07:07 PM   #1
Schurkey
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4WD doesn't engage Thermal Actuator

1988 K1500

Had 4wd towing the boat toward the boat launch, did not have 4WD afterward.

Looking back, 4WD has been very slow to engage for over a year--although it crept up on me so slowly that I never really noticed a problem 'til it quit working altogether.

Tested voltage for the thermal actuator at the two-wire Weatherpac connector on the front axle. Disconnect and probe chassis-side connector. There are two, two-wire connectors side-by-side on the front axle. On my truck, the connector was on the right side, and led directly to the actuator. The other one (on the left) is for the switch on the axle mechanism that turns on the 4wd indicator light on the shift console.

Had voltage (very bright test light) from pin "B" to ground; and had voltage from pin "B" to pin "A". If the voltage is correct (mine was) the service manual says to replace the "Front Axle Solenoid" although in another place it calls this part the "Thermal Actuator", and I prefer the second name.





Before re-connecting the electrical plug, I unthreaded the Thermal Actuator from the axle (only a few drops of gear lube came out) then I started the engine, engaged 4WD, and re-connected the actuator plug. The original thermal actuator only got about 1/8" longer in thirty seconds.

Went to NAPA, purchased p/n BP 600-2271 for $90 + tax. Comes with a "Lifetime" warranty and the box says that it's made in the USA. There's a slightly less expensive version (MPE FWD12SB) with a one-year warranty, but that one wasn't in stock. Although not an exact visual match to the original, the new part seemed to be a reasonable replacement. An ohmmeter test of the new actuator showed about 1.5 ohms through the two-wire connector. Just a little bit higher than the original, which was 1.1 ohm. I connected the new actuator to the vehicle's electrical connector, started the engine and engaged 4WD. The new actuator grew about 1/2 inch or more in length. Damn thing is like an electric penis. Turn it on, it gets l-o-n-g-e-r.

Disconnected the wire harness, put Loctite/Permatex 592 PST thread sealer on the threads. Screw it in (16 ft/lbs) and re-connect the two-wire harness. Secure the connector body on the little bracket so it doesn't flop around. DONE. Tested a couple of times, 4WD now engages in about four seconds by my stopwatch (85 degree day, it'll be slower when it's cold) 4WD hasn't worked that good in a very long time!

Problem is that I was so quick to screw the thing into the axle assembly, I forgot to take a photo of the new actuator at full extension. Suffice to say that it's 1/2 inch (or more) longer than the one in the photo above when powered-up and extended.



I found out AFTER I had the new actuator in place that GM has issued a revised design, uses a small electric motor instead of the thermal element. This requires a revised wire harness, so it's not exactly a drop-in replacement--but it's pretty close. I have no idea how much that costs, or if the parts are still available from GM. Seems that NAPA lists a special wire harness, and a couple part numbers for a "5-pin" actuator. I suppose that's the electric-motor version.


GM bulletin #76-43-01A - (09/18/1998) REVISION: 09/17/98
as shown in this thread:
http://www.automotiveforums.com/vbul...t=4wd+actuator

Supposedly the electric-motor style is much faster to engage when cold.

Just though folks might like to know how simple this repair is; to provide some insight into normal resistance of the thermal actuator, and a confirmation that the thing is supposed to "grow" something like half-an-inch if working properly.
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Last edited by Schurkey; 06-28-2012 at 12:08 AM.
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Old 03-14-2019, 12:46 AM   #2
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Re: 4WD doesn't engage Thermal Actuator

Update: 6 1/2 years later, the actuator has failed again.

This time I did no wiring diagnosis. I spent more time lifting the front of the truck, placing jack stands--and then after the job was done, removing the jack stands and lowering the vehicle than I spent unscrewing the actuator (1" wrench) and unplugging/re-plugging the single two-wire connector. A guy could drive the truck so that the passenger-side tires were up on a curb, slide under the vehicle, swap actuators in ten minutes--maybe less. 'Course, all the curbs around here are buried under three feet of snow at this point.

The actuator failed just ahead of what the National Weather Guessing Service is predicting to be a "Historic" blizzard, so this thing had to be fixed and functional ahead of the snowfall.

Yup--the actuator was in fact faulty, Yes, NAPA made good on the "Lifetime" warranty, Yes, the replacement 600-2271 Thermal Actuator is still "Made In USA" according to the label on the box. Yes, the price has gone up considerably--from about $90 in 2012 to $125 plus tax today (except I didn't have to pay, they accepted my warranty return.)

The only extra time I spent on this was to take the photo I missed last time--the actuator plugged in electrically, and fully extended (electrically "live" for about thirty seconds or so--just long enough to pose the photo.) Compare to the photo in the first post to see the range of motion you should have (fully retracted to fully extended.)

I will pull the fill plug on the differential to verify that it's got an appropriate level of axle grease, sometime AFTER I've dealt with the blizzard. Not one drop was spilled changing the actuator this time--so I'm a little concerned that the grease level is low.



NAPA (and I suppose every other parts-house in North America) can get the updated electric-motor actuator (p/n 600-3921) instead of the thermal actuator. The price of the updated actuator is not much higher than the thermal actuator (about $135)--but NAPA wants $80 for the matching wire harness (p/n 600-2944) needed to retro-fit it to these older trucks
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Old 03-16-2019, 09:42 AM   #3
j cAT
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Re: 4WD doesn't engage Thermal Actuator

that thermal actuator will not work well as it gets damaged if the penis gets stuck on its erection ..
I would change all the drive train fluids .. cold weather and old fluid will cause longer delay on 4WD engagement.

I check all the fluids when the eng oil changes are done .. condition and level.
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Old 03-17-2019, 02:23 PM   #4
Schurkey
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Re: 4WD doesn't engage Thermal Actuator

Quote:
Originally Posted by Schurkey View Post
NAPA (and I suppose every other parts-house in North America) can get the updated electric-motor actuator (p/n 600-3921) instead of the thermal actuator. The price of the updated actuator is not much higher than the thermal actuator (about $135)--but NAPA wants $80 for the matching wire harness (p/n 600-2944) needed to retro-fit it to these older trucks
Dorman 600-600 (Harness) and 600-101 (Actuator) on Amazon or Summit Racing cost just over half what the NAPA-branded parts go for.
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