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What did you do with your GMT400 today...or yesterday....

Big T said:
Arrived at Montana home and had a delivery of a new HVAC control unit for the ‘99 Suburban. Old one stopped working properly. Fan was always on, no matter the position and there was no change in speed. New one installed in minutes and back to working normal.

View attachment 99860
Click to expand...
Do you by chance still have the part number handy? It is the same controller used in the 96.5 and newer Hummers, and is part of a desired upgrade for the older ones.

My 95 uses a 93 truck controller and afaik many other related parts for the ac system.
 
Will L. said:
Do you by chance still have the part number handy? It is the same controller used in the 96.5 and newer Hummers, and is part of a desired upgrade for the older ones.

My 95 uses a 93 truck controller and afaik many other related parts for the ac system.
Click to expand...
I have it, but they have different units depending on whether you have rear window defogger, which the Suburban does. It’s a unit by SKP, so it’s Chinese.
 
iirc those controllers have a serviceable blower switch that can be replaced. I like the way GM did the older units by putting relays above the ac box for each fan speed which took the load off the blower switch. my 95 only has one relay for the high fan speed, everything else runs through the resistor and straight to the switch to carry the load.

I modified a controller from a 96 or 97 rig to work in my 95 since those are a one year only and are hard to find and high priced. it was simple to wire in after getting the connectors from the newer rig I pulled the controller from. some day I plan to wire in three more relays between the resistor and the blower switch to take the load off. same concept we use on the OPS sensor with the lift pump.
 
Well on today's episode of What did you do with your GMT400 today.......

driving the 93 black truck, got home this evening after work. I have been noticing the ac not cooling as well as it had been and figured it needed a little of a top off on R152a (office air in a can). grabbed my gauges and connected them up, fired up the truck with the ac on max and stood there watching the gauges.... pressures seemed ok but I know the front seal on the compressor is weeping. went ahead and added a 10oz can of duster to see what it did. after emptying the can pressures stayed the same running at 30 low and a hair under 200 high but cooling in the cab seemed to get better.

as I was about to shut things down and go inside for the evening I happened to glance at my fuel pressure gauge while the engine was running.... Gauge was sitting at ZERO psi!!! my first thought.... Great, Grand, Wonderful in which I spoke those words in another language!! Wonder how long this has been going on. I really need to get this gauge mounted where I can see it from the drivers seat!

with it still running I grabbed the override button on the drivers inner fender to see if the LP would work and it did. pressure came up to 9 psi. Shut off the engine, pulled the OPS connector and used a jumper across the two pins for the LP and power. the LP turned on. Well I guess my OPS went south or north for the winter lol. I wasn't planning on having to work on this but now that I've seen it, I can't really pretend I didn't see it this time!

Not having a spare OPS I opted to pull the one on my 95 and use that until I can order a new one. swapped out the OPS and started up the engine.... same thing, no fuel pressure. It would prime with the key on but wouldn't continue once the engine was running. pulled the connector out and re-tested with the jumper wire to verify I'm not senile, yep it works. fiddled around with the pins on the connector and in the OPS, re-connected it and wala.... It works.

Darn you GM and your tiny pinned connectors.

Ok rant time over, now back to your regularly scheduled program.... HAHA
 
Last edited:
dbrannon79 said:
Well on today's episode of What did you do with your GMT400 today.......

driving the 93 black truck, got home this evening after work. I have been noticing the ac not cooling as well as it had been and figured it needed a little of a top off on R152a (office air in a can). grabbed my gauges and connected them up, fired up the truck with the ac on max and stood there watching the gauges.... pressures seemed ok but I know the front seal on the compressor is weeping. went ahead and added a 10oz can of duster to see what it did. after emptying the can pressures stayed the same running at 30 low and a hair under 200 high but cooling in the cab seemed to get better.

as I was about to shut things down and go inside for the evening I happened to glance at my fuel pressure gauge while the engine was running.... Gauge was sitting at ZERO psi!!! my first thought.... Great, Grand, Wonderful in which I spoke those words in another language!! Wonder how long this has been going on. I really need to get this gauge mounted where I can see it from the drivers seat!

with it still running I grabbed the override button on the drivers inner fender to see if the LP would work and it did. pressure came up to 9 psi. Shut off the engine, pulled the OPS connector and used a jumper across the two pins for the LP and power. the LP turned on. Well I guess my OPS went south or north for the winter lol. I wasn't planning on having to work on this but now that I've seen it, I can't really pretend I didn't see it this time!

Not having a spare OPS I opted to pull the one on my 95 and use that until I can order a new one. swapped out the OPS and started up the engine.... same thing, no fuel pressure. It would prime with the key on but wouldn't continue once the engine was running. pulled the connector out and re-tested with the jumper wire to verify I'm not senile, yep it works. fiddled around with the pins on the connector and in the OPS, re-connected it and wala.... It works.

Darn you GM and your tiny pinned connectors.

Ok rant time over, now back to your regularly scheduled program.... HAHA
Click to expand...
Have you done the lift pump relay upgrade?
I consider it a must on 1995 and older

If you make your own, make it plug and play like Leroy's.
I posted a link to the connectors, years ago. Or maybe Glagulator did. I'm sure that's where I got the numbers, no matter who posted them.

Use 14 ga. Wire. To make life easier, I've gone to ordering color appropriate wires.

You can check the lift pump plug with the truck running to see if you have voltage.
I've had luck just adding the lift pump relay to vehicles with an OPS that wouldn't run the lift pump, but would trigger the relay to run the lift pump.

I never did figure out how to have the prime feature with only one relay. I always used 2 relays. You're better at this sort of thing than me, so I'm sure you can figure it out.

I think the 93 has the prime feature already, so it might be easier wiring it.

I prefer mountable relay sockets to mountable relays.
The last mountable relay I used, I changed out to a mountable within a few hours, because I wanted to pull the relay for some reason.
 
Yes the relay mod has already been done by the PO. fter I bought it I had to clean up the wiring for it and mount the relay on the fender. I don't think this truck needed the relay mod though since there is already a relay and fuse next to the terminal block on the firewall, but it's now got two relays though I think the PO took out the factory relay from the wiring.

I installed the prime feature with my time delay relay I added for the GP controller. I swapped the old factory GP controller out for one out of a 96+ controller a while back. the newer units have a single trigger wire that would go to the PCM which I connected to a time delay relay and set it for 8 seconds on time each time the key is cycled. when the GP controller is energized, the LP runs to prime the system.

The PO had a 1/4" live fuel line running into the cab and connected to a fuel pressure gauge which I promptly removed and relocated that gauge under the hood for safety. I need to see if I can move that gauge to the wiper cowl where I can see it from inside the cab or spend the extra for an electric gauge that I can put in the cab so I can see when issues like this arise. one if these days LOL
 
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