98 K2500 - HVAC Control Module Issue

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Burban Man

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Hey all,

Long time lurker very infrequent poster, I have searched this forum and others high and low and apologize in advance if there's an answer in another thread somewhere, if there is I couldn't find it.

Truck is a 1998 K2500 ECLB, originally a 454 truck but has a 6.0 swapped in. 4l80e, floor shift 4wd. Pretty cool truck, was really rough when I got it and I'm still ironing out little things as I go.

I'm not getting any heat from my vents. I was looking up at the blend door actuator under the dash and when I turn the knob it doesn't react. At first I believed it to be dead or that the blend door was stuck or broken, but then I did some testing and I believe it's working normally. I've got power on the brown wire, ground on the black wire. I jumped 12v from the brown wire to the light blue signal wire and i watched the actuator immediately roll over and had nice heat coming from the vents. Took away 12v and it went back to full cold. This tells me that the actuator responds to voltage on the signal wire, the gears inside are not stripped, and the blend door is not broken or stuck.

This is where it gets weird, I took the dash bezel off and pulled the HVAC module out, and testing at the light blue wire on the large flat connector on the right side I'm getting 2.6v no matter what position the temp knob is in. This is the signal wire from the module to the actuator and I'm assuming that it's supposed to output 0-2v ish in cold, up to 12v in full heat. Maybe it's 0-5v, I can't find any info on this on alldata or anywhere else. I have 4 or 5 spare HVAC modules from random junkyard trucks that I use for parts or as backups and have plugged all of them in with no change, still get 2.6v on the light blue wire no matter what position the temp knob is in.

So, either I'm completely misunderstanding how the HVAC module controls the blend door actuator, I'm misunderstanding how the blend door actuator works internally and it's actually bad even though I can force it to work correctly, or I have 5 HVAC modules and they all have failed in the exact same way and have a dead chip or something that controls the blend door output??? Any input is appreciated.

On another note, I just finished the engine swap in October and so far so good, truck has been super reliable and I've used it full time as a work truck. All factory gauges work, A/C works, 4wd works in hi and low. Can't complain too much, just wish I could have some heat :flamingdevil:
 

east302

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Your reasoning looks sound. Not sure what the signal voltage should be on the temperature actuator, but think the mode door actuator (gas pedal) has a 0-5V range.

Probably doesn’t matter what the range is since should be a varied signal voltage, regardless.

Just to confirm…

When you jumped 12V to the signal wire, was this was done at the actuator or the panel connector?

Backprobing the light blue (733) signal wire at the panel gets a constant 2.6V?

This was done with ignition on?

Do you have 12V upstream on brown (141) at the same panel connector?


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Burban Man

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Your reasoning looks sound. Not sure what the signal voltage should be on the temperature actuator, but think the mode door actuator (gas pedal) has a 0-5V range.

Probably doesn’t matter what the range is since should be a varied signal voltage, regardless.

Just to confirm…

When you jumped 12V to the signal wire, was this was done at the actuator or the panel connector?

Backprobing the light blue (733) signal wire at the panel gets a constant 2.6V?

This was done with ignition on?

Do you have 12V upstream on brown (141) at the same panel connector?


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Thanks for the reply,

When I jumped 12v to the actuator it was done at the plug on the actuator. Just used a couple t pins and jumped the brown wire labeled #10 B+ to the #8 SIGNAL wire. I did check at the HVAC panel itself and I'm not getting a signal of any kind out of that on the LT blue wire, just the same 2.6v

Yes, I actually had the truck running during most of this testing but when it wasn't running the ignition was always on.

It looks like according to that diagram and the redrawn Alldata one i've been using that the mode door, temp door and recirc door actuators are actually not powered by the HVAC panel connector that the signal comes from, but they get power directly from the HTR A/C fuse and there are 2 splices, S268 and S243 in between where power splits off to go to each actuator. I'm sure I could find these on Alldata and get a description about where they are but idk if it's worth looking for them because I'm getting 12v at the brown wire at the connector on the temp actuator and the other functions work okay so I'm gonna assume that the circuit is intact.

What jumps out at me about that diagram is the fact that the temp dials and mode dials are rheostats, and they show power coming in, going through the resistor strip and out on the signal wire, and the other leg of the circuit is grounded. The temp door portion just shows a ground symbol on the diagram, but it's not actually connected to the black ground wire on pin #2 on the module like the mode door dial is (at least according to the GM drawing which I always take with a grain of salt). The fact that I have several HVAC modules and none of them will power the temp actuator specifically makes me wonder if for some reason the ground leg for the temp dial is supposed to be grounded through a different wire that isn't listed, but that wire is open somewhere?

For instance, there's a plug on the passenger side firewall that's right under the coolant expansion tank that I had a couple run ins with since this is an LS swapped truck. In order to get my factory gauges and PRNDL display to work I had to tap into a couple of these wires, and I had to ground one of them. The GM pinouts for this connector and also the other firewall connector C100 on the other side both leave a lot to be desired, and some of the descriptions are complete nonsense. There's a pin on the passenger side firewall plug that is labeled "ECM ground" or something along those lines on the GM diagram but the wire is actually not connected to the ECM at all, it's one of the ground wires for the gauge cluster, specifically the one that grounds the tach and the PRNDL display. The only reference to this is in factory diagrams which I was able to use to match up the circuit # but there's no other information about it. Took me a considerable amount of time to decipher the oem pinouts even with LT1swap.com walking me through a lot of it.

Anyway, I'm just rambling at this point. I've got the LT Blue wire jumped to 12v still under the dash just to get me by, I live in Phoenix so it's not nearly cold enough to tear my hair out trying to completely resolve this. I'll just take the T pins out when it's not "cold" anymore and move on. If I eventually get bored and tear it all apart and figure it out I'll come back and give an update, maybe someone will stumble upon this thread and it'll help them with a similar issue :cool:
 
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