jwillman

My AC clutch is not getting power. Compressor clutch works when hooking 12V direct to the clutch power wire.

AC switch on the HVAC controller is working.

Seems to be no power coming out of the relay.

928 618 105 00 relay is NLA. Any way to test or alternate relay solution?

SeanR

Open the hood and pull the cowl off, find the freeze switch. One of the leads on the freeze switch should be approx 12v. If not then you have a HVAC head unit relay issue. As for fixing it? yea, that's a bummer. I usually order a rebuilt from Rog.

jwillman

My 79 does not have an HVAC relay on the controller Simple HVAC on off switch triggered by the slider in the AC and defrost positions. That switch is functional.

The wiring diagram does show a "temperature switch, evaporator" between the AC switch on the HVAC controller and the AC speed relay. Did not know I had one of those so guess I need to go find it

dr bob

That's the one Sean described. There's also a pressure safety switch in that same circuit, opens when freon pressure gets too low. If you see battery voltage to ground at both connections on the freeze switch, the pressure safety switch is telling you there isn't enough freon left in the system. Voltage on one side says the freeze switch is open, from too cold or from failed switch. If there's no or low voltage at either terminal, work back towards the head unit.

jwillman

Spent some time today troubleshooting.

Now the fan won't run at all. I swapped relays between the horn and the blower relay and both relays work.

When I jump pins 30 and 87 on the fresh air blower the fan works.

I assume that means that what ever energizes that relay is missing.

Where does trace 31 get its signal / power? It drives all the relays in this circuit.

dr bob

31 is Ground.

Circuit for that starts at 30 (always battery voltage) through terminals 30 and 87 at the freshair blowe relay (XXI), then out thru CE J1 to the dash AC switch, thru the evaporator temp (anti-freeze) switch that Sean described, before going through the speed relay you highlight. 31b on the speed relay gets tach pulses to tell the system the engine is running, uses those to close the contact between 30 and 87 on the speed relay. The circuitcontinues from speed relay 87 out thru CE Q2 to the low pressure switch. Thru the low pressure switch to the clutch coil, and to the idle air bypass valve (called "supplementary air valve for AC" in the wiring diagram.

Electrical troubleshooting:

Check fuse 17 as it protects the supply going to the blower relay (and the defroster relay). Blower relay should pull in if the fan is in any 'on' position. Verify voltage at CE J1. If none, check for power at fresh air blower terminal 30 socket. If none, recheck fuse 17. If power there but none at J1 with the relay installed and blower switch in any 'on' position, check relay socket 86, should have battery voltage from the blower switch. If yes but none at CE J1 .and. yes at relay socket terminal 30, relay is bad.

The speed switch should have battery voltage at terminal 30, via the blower relay, the AC switch in the dash, and the freeze switch. If yes, look for voltage at CE Q2 with the engine running. If no, the speed relay may be bad. It expects to see pulses pulled low by the tach circuit. Testing that usually demands an oscilloscope, but if your tach is working you generally have a good signal to the speed relay. The same pulses serve the fuel pump relay too.

If there's battery voltage at CE Q2 with engine running, AC switch on, yet your compressor clutch doesn't engage, the low-pressure safety switch is the next suspect. You'll need to test that circuit for continuity, easiest from a freeze switch terminal to the 14-pin by the jump post. Diagram shows this as pin 9 in that connector. Pull the engine side off, test for continuity from the car side pin 9 to either connection on the freeze switch. If none, it's likely the low pressure safety switch is open. You can pull the connector off the switch and test continuity through the switch itself. If none, it's quite likely that there isn't enough freon to close the switch. Folks replace those switches thinking they might be bad, but I've never run aross an actual bad one. Put gauges on the system to verify pressure. If the switch truly is bad, you can change it since there's a Schraeder valve in the port it screws on to.

dr bob

OB Experts -- Is the "AC Speed Relay" the same type relay used for the fuel pump? Pinouts and function look identical. If so, it would be good to know for roadside emergency relay swap.

jwillman

Fuel pump relay and speed relay are not the same - unfortunately.

Fuse 17 is good. I jumped pins 30 and 87 on the fresh air blower CE panel and got normal blower function, i.e. blower works and responds to the blower speed switch so i have good wiring from J1 threw blower switch and resistor unit to motor.

I did not check the AC operation with the fresh air blower jumpered. That would validate operation of the AC circuit to the compressor.

The wiring diagram shows a "temperature switch, evaporator". There are two switches on the receiver dryer but nothing I see up by the evaporator or in the parts manual which shows a low pressure switch and thermostat on the dryer. Is the temperature switch, evaporator the thermostat?

I swapped relays with horn and windows and they work so what I need to figure out is why with the fresh air blower relay when installed in the CE panel does not allow blower to run.

According to the wiring diagram the fresh air blower relay is activated through J4 to pin 86 from the " heater control assembly" which I assume means the HVAC control head? Blower should run on low anytime ignition is on according to the owners manual and high anytime you select defrost. Does the HVAC controller energize the defroster and blower replays through the lower slider and the traces on the controller card?

dr bob

Jim, do you have power on CE J4? From the HVAC control head in any but the 'off' position on blower speed switch, or maybe the top function slider. Power for the blower switch shows up from the next page of the wiring diagrams, from 15 (key in 'run' or 'start' position), unfused, thru CE H7. (Which also feeds central warning unit and the alternator idiot light in the dash.)

[edited]: I don't know the internals of the early HVAC controller, but the diagram on sheet VII shows the path through a slider or maybe part of the rotary blower switch. Drawing convention has a dashed line linking the speed switch with the other function as if they are the same switch. That is there, so it's quite likely that the blower relay does not depend on a trace under the function slider. Verify that the edge connector on the HVAC controller is plugged in. After that, you will want to get into the HVAC control head to see what's keeping the blower relay excitation from coming through.

jwillman

Thanks Dr. Bob. I need to check for voltage at J4.

I have a pretty flaky ignition switch, i.e no real return from start position. I have a replacement ignition switch so perhaps this a good time to replace it.

dr bob

Jim -- check CE H7 with the key in run position. If your ignition switch is flaky, you'll know it by no/low voltage there.

jwillman

I checked for voltage at H7 last night and only had about 5 V. I had also been having some weird fuel pump behavior. Fuel pump was not energizing momentarily when going to start so I replaced with another relay which ran all the time when going to start. Replaced with a new relay which never ran until I started the engine so more than one symptom that my ignition switch might be the problem.

I replaced the ignition switch last night but did not get to do more than test start. My fuel pump that never ran now comes on momentarily as it should and I have actual positions on the ignition switch.

I plugged in the electrical connectors for the HVAC control head and have some intermittent conditions.

1. I move the slider to AC and one time get fan, AC compressor to run and put out cold air and both electrical cooling fans run. Switch to off and back to AC and get nothing. Try a couple more times and on occasion get AC operation.

2. When I move the selector to Defrost fan runs at high and AC compressor runs as well as both cooling fans.

3. When sliding back out of defrost the fan will sometimes continue to run and other times immediately stop running. AC compressor switch is working as the compressor stops once moved out of the defrost position.

This points to a controller issue to me. The slider traces were peeling off in a couple of places when I first got the AC running and I used a silver trace pen to redraw the traces but it is acting like the slider contacts / traces are not working properly.

Thoughts? Any one rebuild these 78-79 controllers?

jwillman

bump. Anyone have troubleshooting for electrical traces in the 78-79 HVAC controllers?

jwillman

I checked voltage at J1 and J2 and although not Battery voltage of 12.75 its close at almost 11V.

I am not getting voltage at Q2. I jumped pins 30 and 87 and get compressor operation and good cooling.

I have normal tach operation and get the fuel pump to run momentarily when the key is turned on before starting so speed signal seems good.

AT this point speed relay appears top be the issue. As the AC speed relay is NLA I will used a switched jumper in the time being.

Any better long term solutions welcome.

dr bob

Jim, I'm away from the drawings so no detailed help.

But....

The function of the speed relay is to disengage the clutch unless the engine is running. Functionally that's what the x-bus does, sort of. It's true when the key is in 'run' position only, drops out during starter time. If the speed relay truly is unavailable I would look for a way to get x-bus power to relay socket 87, or modify the CE socket so 86 gets X-bus power and 30 stays connected to the 30 bus.

Perhaps my first effort would be to fix the old relay. It's relatively simple to use a common (Radio Shack) 555 IC timer to trigger a common relay on tach pulses. Look under the cover of the old relay and see what pieces have given up some factory smoke recently.