re-installing AC (from scratch) on my mk3

burpod

teh stallionz!!1
Joined
Nov 27, 2004
Location
cape cod, ma
TDI
82 rabbit vnt ahu, 98 jetta vnt ahu, 05 parts car, 88 scirocco.. :/
big question: does the stock AHU (or ALH for that matter) AC/fan setup disable the coolant fans from running when the car is travelling at higher speeds? looking at the mk3 bentley wiring diagrams, i don't see any possible way it could do this but maybe i'm missing something. so is it correct that no matter what speed the car is travelling, the coolant fans are always running when the AC is switched on?

what i would like: does anyone have any suggestions to add a speed sensitive switch/relay so that the fans don't run constantly? i would think that going 40+ mph, the running fans are totally unecessary. seems like a waste, especially on long summer highway drives - plan to take my car on a cross-country roadtrip in june. i found this- lingenfelter speedbased switch. it's a bit pricey but looks like it would work. any other ideas that might be cheaper? perhaps some sort of cheap mechanical switch that activates a basic relay. i'm going to assume i can find a way to do this and leave an accessible provision for it in the wiring so i can finish up and get the car drivable again (gotta put the engine back together too lol). BTW, i have no plans of re-installing the stock fan control module, see below.

history:
when i rebuilt the car about 9 years ago, i completely removed AC - everything, wiring and all. never figured out exactly why, but it wasn't working at the time and i didn't care) i sort of "custom wired" the CE2 fusebox and redid everything. at the time, i rarely ever used AC and installed an fmic instead and simplified the wiring. a lot has happened since then and now i want my AC again - got two kids now (almost 2 and almost 5) and it's essential :oops:

so i'm in the process of installing it - with my current FMIC that is a great fit and a 12x24 universal parallel flow AC condenser. i'm going to have to make a couple custom lines, but that should be relatively easy if i can get someone to weld a couple fittings on aluminum pipes. the HVAC box with new evaporator is back in the car now, as well as working recirc flap.

i modified the fuse/relay-box so that i could use #1 relay position (AC) to run the high speed fans - flexalite twin fan. i believe low speed runs one fan (no relay), and high temp/speed activates the relay (a #100 with 2 87 outputs) to run both fans. i also had previously wired the AC button to manually run both fans - in the event the coolant fan speed switch failed at an inopportune time. so i don't use the big honky vw coolant fan control module, just a single relay in the fusebox.

 
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burpod

teh stallionz!!1
Joined
Nov 27, 2004
Location
cape cod, ma
TDI
82 rabbit vnt ahu, 98 jetta vnt ahu, 05 parts car, 88 scirocco.. :/
fmic:



my fusebox:



relay #1 (200) controls high-speed fan (both fans running)
seat belt warning position is now for lift pump
relay for fogs, relay for horn
and use old-style single high-amp relay (lower right) for single-wire cylinder glow plug system
above the box, i have two heated-seat relays, and relays for each headlight.
hanging down to the right, i also have a high-amp relay for simply coolant glow plug control (all on or all off)

i have a mystery relay on the upper left that i can't figure out what i put it there for, other than possibly for a future use. so going to remove that and use that for my AC relay.

thinking i may also delete my alarm box and power door locks, as i rarely use it. i have a secret jumper i can quickly unplug if i want to make my car un-startable :)
 

burpod

teh stallionz!!1
Joined
Nov 27, 2004
Location
cape cod, ma
TDI
82 rabbit vnt ahu, 98 jetta vnt ahu, 05 parts car, 88 scirocco.. :/
i thought i understood this, but now i'm confused about the pressure switch F129 - i thought it was two switches in one - a low pressure and a high pressure. i've read a number of AC threads but can't find the exact answer, but based on DanG144 thread it looks like pins 1-2 have continuity when pressure is within a certain range? so it's open (no continuity) when pressure is too low or too high? then what are pins 3 and 4 for?

i don't want to use the stock fan module so i'd like to get my AC clutch to work with a simple relay.
 

burpod

teh stallionz!!1
Joined
Nov 27, 2004
Location
cape cod, ma
TDI
82 rabbit vnt ahu, 98 jetta vnt ahu, 05 parts car, 88 scirocco.. :/
found this info that explains it.

Switch part between terminals 1 and 2 switches off the A/C clutch -N25- when the system pressure is too high or when there is not enough refrigerant in the system . Switch part between terminals 3 and 4 switches coolant fan -V7- (via the coolant fan control module -J293-) to the next higher fan speed when system pressures increase. Switch opens when the pressure is below 1.2 bar and closes again above 2.4 bar (switch point). Switch opens above 32 bar and closes again below 24 bar (switch point). Switch closes above 16 bar and opens again below 12.5 bar (switch point).
so i'm thinking maybe i don't need a speed sensitive switch. perhaps it would be adequate to simply trigger my high-speed fan using pins 3-4 on the pressure switch and call it good. if i'm moving down the highway and the fans kick on, so be it. but guessing they probably wouldn't unless i'm going slow or stuck in traffic on a hot day. thoughts?
 
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