No 4WD and no dash light - Diesel Forum - TheDieselStop.com
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99 & up 7.3L Power Stroke Engine and Drivetrain Discussion of the 99 & up 7.3L Power Stroke diesel engine and drivetrain in the 1999-Up Super Duty trucks and Excursions. No gas engine discussion allowed except on transmissions and drivetrain that pertain to all models. Please confine discussion of topics in this forum to those items that are specific to the 7.3L Power Stroke engine.

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Old 11-08-2012, 04:59 PM   #1 (permalink)
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No 4WD and no dash light

I'm sure this is related to the issue I had this summer when my trans/transfer case wiring harness dropped onto the front drive line while my hubs were stuck and the harness got torn up. Yeah it was a ****ty day. Anyway I cleaned it up, soldered in, and shrink tubed the wiring repairs. I'm feeling confident that the wiring repairs are good. It was summer and I didn't think about checking the 4wd.

Well now it's snowing and I find that I have no 4wd. No light on the dash even. I understand about checking the t-case motor for voltage, but if that's the problem should the dash light still work? I replaced the two under-hood relays in July, and the fuse is good. But some of the wires did short out to the truck. Maybe the wires shorting out fried a relay, the motor, or the GEM? Just wondering before I start replacing stuff, what component could have gone bad that would make the dash light not work?

Was going to go hunting this weekend too. Might have to skip that.
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Old 11-08-2012, 05:09 PM   #2 (permalink)
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When my GEM burnt I could smell it if I got down by the fuse panel. I isn't too bad to get to but usually other options like power door locks are effected at the same time. I soldered it and put superglue over that and it held. Not saying that's the problem, but it's a possibility.
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Old 11-08-2012, 05:38 PM   #3 (permalink)
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The position switch on the t-case motor reports back to the GEM and it turns on the light on the dash. So the issue could be at the t-case or at the GEM. My guess would be at the t-case. It could be that the motor is fried and can't shift the t-case (and turn the position switch).
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Old 11-08-2012, 06:40 PM   #4 (permalink)
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I didn't and don't smell anything. The windows and door locks work.

That makes sense about the light. It's not connected to the switch directly, it's responding to the t-case engaging. That's why the light doesn't come on right way going to 4LOW, because that takes longer to engage.
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Old 11-11-2012, 08:35 PM   #5 (permalink)
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I just checked the two-pin connector to the t-case and get no voltage when moving the dash switch. Tonight I swapped the t-case relays with the other two right next to them but still no voltage with the switch.

Then I applied battery voltage to the motor's pins and I can hear the motor moving. Switch polarity and I hear it move again (back?). I don't know if it's putting the t-case in 4wd. However I get no dash light in either case.

Not sure what to check next.
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Old 11-11-2012, 10:37 PM   #6 (permalink)
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Originally Posted by tykesplace View Post
I just checked the two-pin connector to the t-case and get no voltage when moving the dash switch. Tonight I swapped the t-case relays with the other two right next to them but still no voltage with the switch.

Then I applied battery voltage to the motor's pins and I can hear the motor moving. Switch polarity and I hear it move again (back?). I don't know if it's putting the t-case in 4wd. However I get no dash light in either case.

Not sure what to check next.
When you put battery power it is probably switching the motor, but no signal is going back to tell it to turn on the dash light. Did you have power to the switch? Is there power to the relays?
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Old 11-12-2012, 12:58 AM   #7 (permalink)
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I haven't checked power at the switch. How to do I check power at the relays? Any of the slots in particular?
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Old 11-12-2012, 01:25 AM   #8 (permalink)
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What year is your truck?

Post that and I'll dig up a wiring diagram for the ESOF to help you troubleshoot.
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Old 11-12-2012, 02:29 AM   #9 (permalink)
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Thanks! It's a 99.5.
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Old 11-12-2012, 09:35 PM   #10 (permalink)
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I just had a thought. The voltage at the two-wire connector. Are both hot, just at different times depending on which way the motor being turned? When I checked voltage it was across those pins. If both are positive but only hot one at a time, I don't think I would read volts, correct? Should I instead test positive with the multimeter on each pin separately with negative to a ground?
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Old 11-12-2012, 09:39 PM   #11 (permalink)
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The two relays work together to switch polarity of the two wires powering the shift motor. One connects to ground, and the other connects to 12V. They may just both be grounded normally, and the one or the other pulls in to supply 12V to the motor to run it in either direction. So putting your meter probes in both wires should work, or you could put the black probe on a ground point.

I'll get that wiring diagram to you a bit later tonight after I get home.
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Old 11-13-2012, 12:54 AM   #12 (permalink)
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Here ya go

99 electric shift control wiring.pdf
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Old 11-13-2012, 10:52 AM   #13 (permalink)
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Thanks for the diagram. Very helpful. It's looks like the GEM is right in the middle of everything.

Here's what I see with the t-case shift relays on Page 1. The diagram says "the t-case shift relay controls the operation of the t-case from L2H and H2L through inputs from the GEM". Both the Low/High and High/Low relays have 5 pins each.
  • Pins 85 and 87 are DG/LG from the dist box, fuse 17, and should be hot. So I'll check those to make sure power is coming in.
  • 87A should be to a good ground.
  • Pins 30 are O and Y wires to the t-case motor, but only look to be hot if activated at the relay. Otherwise they're ground. If I test continuity from those wires to ground that means the relays are in the position as shown in the diagram. I think these wires could have been damaged in my harness incident so I'll also check their integrity between the relays and the t-case motor.
  • Pins 86 are BR/W and O/LB coming from the GEM relay controls. I'm thinking these control the t-case relays and would only be hot if triggered by the GEM.
So do these relays only control L2H and H2L? Meaning the t-case motor doesn't move the t-case from 2WD to 4WD? Just between 4H and 4L when triggered by the GEM which responds to the 4WD mode switch? When I was stuck in 4L this summer I replaced these relays and could then get out of 4L, so that seems to be true.

Here's what I see with the 4WD mode switch on page 3. It is connected directly to the GEM and has Off/High/Low positions. I don't see how the GEM knows what position the switch is in though. Varying resistance in the switch position maybe? Also the GEM is connected to the t-case through the 7 pin connector. Is that how the t-case is moved between 2WD and 4WD? Again this is the harness that was damaged. And finally the dash indicator lights are hot from the dist box fuse 19 and are are triggered by the GEM.

So, even if I can confirm the integrity of all the t-case shift relay wires, the t-case motor, fuses 17 and 19, and the t-case relays, I think that only tells me about the mechanical functionality of L2H and H2L. Not 2WD to 4WD. Is 2WD to 4WD internal through the t-case contact plates as shown on page 3? Again these are connected to the GEM.
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Old 11-13-2012, 02:02 PM   #14 (permalink)
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More info. I'm shopping around for a GEM just in case that's the issue and found this one at AutoPartsCheaper.com. I called a tech to make sure I was looking at the right one. I also asked about the 4WD functions. He confirmed that the t-case motor only engages/disengages 4L. Moving between 2WD and 4WD is engaged inside the t-case and is controlled directly by the GEM. He said this is why you don't feel anything when engaging 4H. Almost like a clutch. However 4L is a mechanical engagement triggered by the motor and therefore the clunk when going to 4L.
They're on back order for that part until March or so. I hope that I can find it somewhere else.
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Old 11-13-2012, 02:31 PM   #15 (permalink)
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No 4WD and no dash light-1210111230.jpgHere's the one I fixed. Sorry kind of blurry when enlarged.
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