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Old 06-05-2020, 04:55 PM   #1
Second Series
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4x4 engages in 2wd at start. 1990 k1500 5-speed manual.

4x4 engages in 2wd at start. 1990 k1500 5-speed manual.
The 4x4 light comes on right after I start the truck, stays on for a couple minutes. The front axle seems to be engaged because I got it up to 50mph and the truck was shaking. I was going to turn around and go home, but the light went off so I went onto the freeway and did 70mph smooth. This is with the floor shifter in the 2WD position.
I am able to shift into and out of 4WD with the light turning on/off with the typical delay. I’m looking at the light by the floor shifter.
I just got the truck back from the shop and trying to troubleshoot, if I can fix it myself that saves me from taking it back to the shop. The work done: Clutch, so the transmission and transfer case were removed and re-installed. The front Diff was leaky so that was R&R’d also. Since both ends of this system were worked on I don’t know to look at one or the other.
Last night I started to look at it. I disconnected the actuator, and put a test light on the connector. Start the truck in 2WD. A moment later the test light came on, and stayed on for a minute or two and then turned off.
Is there a relay between the T-case switch and actuator? I think the next thing I’ll look at is unplug the T-case switch and check the switch with an ohm-meter to see if it is changing when I start the truck. Does anyone have any idea what could be causing this?
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Old 06-05-2020, 05:05 PM   #2
1976gmc20
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Re: 4x4 engages in 2wd at start. 1990 k1500 5-speed manual.

Well I don't know what your electrical issue is, but even if the actuator was engaged all the time you would not have any problem. Mine was not working when I bought the truck so I shimmed it so that it is engaged all the time since it's a "ranch truck" but I do drive it 25 miles to town once in a while. It's no different than driving a solid axle 4x4 with the hubs locked in and in 2wd.

It's certainly not going to cause shaking as long as your lever is still in 2wd. Not even then really, but it will bind on turns and generally wreck the transfer case if you use 4wd on dry pavement.

PS: I guess if there was something wrong with the front driveshaft (out of balance) then that might cause shaking when it's turning (CAD engaged) at highway speeds. Anyway mine doesn't shake and the front driveline turns all the time when the truck's moving.
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Old 06-10-2020, 11:09 AM   #3
stockypoo
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Re: 4x4 engages in 2wd at start. 1990 k1500 5-speed manual.

Start at the transfer case switch. It closes and supplies power to the front axle actuator. 1990 4wd Electrical description: "When the ignition switch is in RUN and the four-wheel- drive control lever is engaged, the transfer case switch closes and battery voltage is applied across the front axle actuator. The front axle actuator energizes and the four- wheel-drive gears are meshed. This meshing action of the four-wheel-drive gears closes the front axle switch, and battery voltage is applied through the WHT (156) wire across the four-wheel-drive indicator lighting the bulb. Battery voltage is also applied through the front axle antilock brake module. This will keep the antilock braking system disabled in 4WD mode. The in-line resistor (auto* matic transmission only) prevents the transmission con* verter clutch solenoid from engaging when the front axle switch is closed"
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Old 06-11-2020, 04:50 PM   #4
Second Series
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Re: 4x4 engages in 2wd at start. 1990 k1500 5-speed manual.

I had been looking for a wiring diagram, but found a few that did not include the differential part of the circuit. I finally found a schematic that shows everything, and I see no relay. It looks simple enough. I’m thinking either the transfercase switch is faulty, or the linkage needs adjusting. I can disconnect the front axle actuator, and pull the 25A Heater A/C fuse and connect an ohmmeter to those two places to see if the transfer case switch is connecting when I start the truck. I would think if the linkage needed adjustment, the problem wouldn’t go away after a couple minutes. I just hope there isn’t something loose inside the transfer case causing the switch to throw. I may be overanalyzing, I should probably just replace the transfer case switch and see if that fixes it. I’ll check it with the ohmmeter, the switch will not be connected to power during this test, if it throws then it’s a mechanical issue. My thought here is the switch is worn out and at power on the switch arcs causing the issue.
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Old 10-06-2020, 03:47 PM   #5
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Re: 4x4 engages in 2wd at start. 1990 k1500 5-speed manual.

I pulled out my trailer and attempted to back up hill on gravel, no-go. I shifted into 4x4, but the light didn’t come on. I did put a deep rut in the road. I got out my test light and crawled under the truck. Power to the transfer case switch, no power to the diff. I jumpered the connector at the T-case switch and got the trailer moved. I finally got the T-case switch out, and found a replacement at a local store. I decided to test it before putting in. The old one tests open at all times. The new one tests closed at rest, push the ball in some and it tests open, push the ball in all the way it tests closed. So this new switch seems to be a normally closed, closed-open-closed switch, single pole-three throw. Not what I would expect. I put it in, and the truck shifts into 4x4 as it should. Now it’s not shifting out of 4x4. With truck off, but key on, I take the test light to the diff connector and see voltage. I can grab the t-case linkage and move it through several positions, and power remains on the harness. Is the switch bad, or is something inside the t-case bent or broken?

I went to my flaps and bought another switch, tested it with ohm meter there. Same brand switch, this one is n/c and reads open when pressed. I just noticed the drawing shows it closed in the schematic I posted above. So I put it in and the 4x4 works as it should. I checked the first new one and it is closed at all positions, so I'll take that back to the other parts store. Now it's time to drive!
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'47 Panel to '88 K2500 Frame Swap
Mechanical Speedometer Drive Solution
1947.2 1 ton Chevy Panel
1955.2 Chevy 6700 Bus/RV
1990 Chevy K1500

Last edited by Second Series; 10-07-2020 at 12:11 PM. Reason: Problem solved
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