Why won’t my front diff engage?

[Chillin Dylan]

Hey all. I’ve just got my truck back on the road after a drivetrain swap and I’ve gotten stuck in a barely wet yard twice in 24 hours now. My 4x4 isn’t working at all. It’s a floor shifted unit behind a stick shift. No light on the floor even. Low or high. The transfer case is out of a 97, the truck is a 95. Both had the same transfer case in them. Both trucks’ 4x4 worked great. Everything is plugged in on the transfer case and the front diff. I’m not using the stock TBI ECM anymore, I’m running a Holley sniper. What all can cause the front diff to not engage? Like I said both trucks’ engaged the front diff no problem right before the drivetrain swap, so it’s hard to believe the actuator just decided to stop worked but I suppose it’s possible

 

[Schurkey]

Thermal actuator, or the newer stepper-motor actuator?

Wires connected at transfer case? Wire harness not beat-up, frayed, pinched, broken insulation?

Fuses good?

The usual failure mode of the thermal actuator is that it gets slower and slower to engage, and then finally doesn't engage. As the time to engage varies with ambient temp anyway, this is kinda hard to tell until it's too late. It seems that around here, they generally fail in the Dead of Winter which makes changing them even more fun. My '88 had a thermal unit (the original???) when I bought it at 180K miles, and I've put two on since then. The second was a "warranty replacement" through NAPA, so at least it didn't cost me anything but time. Truck has ~320K on it now.

If there's a "usual failure mode" for the stepper-motor style, I don't know about it. It's probably worth converting the thermal style to the stepper-motor style; I wish I'd known about that when I did mine the first time.

 

[454cid]

If there's a "usual failure mode" for the stepper-motor style, I don't know about it. It's probably worth converting the thermal style to the stepper-motor style; I wish I'd known about that when I did mine the first time.

When mine stopped working it was my own fault. I had unplugged the relay on the firewall for some reason, and when I put it back together I bent a pin.

 

[Chillin Dylan]

Thermal actuator, or the newer stepper-motor actuator?

Wires connected at transfer case? Wire harness not beat-up, frayed, pinched, broken insulation?

Fuses good?

The usual failure mode of the thermal actuator is that it gets slower and slower to engage, and then finally doesn't engage. As the time to engage varies with ambient temp anyway, this is kinda hard to tell until it's too late. It seems that around here, they generally fail in the Dead of Winter which makes changing them even more fun. My '88 had a thermal unit (the original???) when I bought it at 180K miles, and I've put two on since then. The second was a "warranty replacement" through NAPA, so at least it didn't cost me anything but time. Truck has ~320K on it now.

If there's a "usual failure mode" for the stepper-motor style, I don't know about it. It's probably worth converting the thermal style to the stepper-motor style; I wish I'd known about that when I did mine the first time.

How can I tell which is which? I tried to find threads on the conversion but had no luck. The search thing on here kind of sucks. Either that or it’s an ID10T error lol

 

[1997]

i know on the 95 ECU there is a connection to i think the VSS and 4x4 switch, reason when you put it in 4 low it changes the output to compensate for low range gearing, allowing the speedo to work correctly.

not much help, but might be worth looking at that wiring circuit.

 

[Schurkey]

How can I tell which is which? I tried to find threads on the conversion but had no luck. The search thing on here kind of sucks. Either that or it’s an ID10T error lol

Go to any on-line parts store, look up the actuators. The photos of the different kinds should make it clear.

i know on the 95 ECU there is a connection to i think the VSS and 4x4 switch, reason when you put it in 4 low it changes the output to compensate for low range gearing, allowing the speedo to work correctly.

The VSS is on the transfer case output. No need to "change" anything based on high/low range. Driveshaft turns are driveshaft turns as far as the VSS/speedo/odo/cruise/ABS are concerned.

 

[1997]

right, i probably explained it wrong. Here's the 95 ECU pin out.

F-8

GRY/BLK

4WD Switch

 

[GoToGuy]

My 95 K2500 is electric, floor transfer case shift. My 93 K1500 is or was thermo converted to manual select.

 

[Chillin Dylan]

Go to any on-line parts store, look up the actuators. The photos of the different kinds should make it clear.

It looks like both of my trucks have the thermal actuators because the have the weird looking rubber sleeve on them. I took the actuator out of the 97 and tried to put it in the 95 diff and it wouldn’t fit. It was like the rubber sleeve was a little too big

 

[Chillin Dylan]

It looks like I need dorman 600-600 and 600-101 to do the full electric actuator upgrade. I haven’t had a chance to check fuses or anything since I made this post so I’ll do that when I get off work tonight. But I want to do the upgrade either way