Calling all Electrical Gurus - HELP PLEASE!

[gshiwota]

Electrical Gurus - please help!

Hi I'm reposting this since I'm getting desperate for help. I have a bunch of money sunk into my AC upgrade and I can't use it b/c I can't figure out this electrical problem. Help, help, help!

I have owned this car for about a month and when I initially bought it, the AC blower seemed dead. However, about two weeks ago I decided to try it again while driving and it worked - all three settings blew air - the problem fixed itself I thought. Now I just upgraded all AC components (evap, cond, pro cooler, compressor, hoses) to the Rennaire system and went down to the local AC shop to charge the system. Once again, when I turn the AC Blower switch, nothing happens - no compressor clutch engagement / evap blower / condensor blower. The AC tech then jumped the AC Compressor wire to the battery to engage the clutch, and everything worked (compressor engaged, cond blower, and evap blower worked).

I have spent the better part of the day trying to figure out the problem with my AC. I have verified that the AC blower switch is working fine.

I have tracked down the problem to the AC Relay (I have verified the relay is good) not getting power from the ignition switch. According to the diagrams in the Bentley manual the AC Relay should have power from the ignition switch at poles 85 and 86 (assuming I'm reading this correctly). When I check the voltage at the relay block, I only have power at pole 30 (always hot). Poles 85 and 86 are 0-volts regardless of ignition key position.

Unfortunately, I cannot trace down these wires easily since they are taped tightly into the wiring loom. Before I try cutting this up, I'm hoping that someone out there knows where these wires come from so I can trace down the connection. Better yet, has anyone experienced this?

I'm certain this is the problem b/c I ran a "short" wire across poles 30 and 86 and the AC system worked perfectly (all 3 fan speeds, cond blower, and AC clutch engaged).

Help! Help! Help!

Thanks again!

Gabe
'82 911SC

[Patton]

Terminal 30 should always be hot as you have tested at the AC relay. I don't have a electrical diagram of the ac system for an 82 sc but I'm relatively sure that in between the ignition switch and terminal 86 of the ac relay there is a fuse in fuse box 1


above is from a 84 911 and clearly would result in your symtoms should the fuse be bad.
Hope this helps,

[Wil Ferch]

Quote..." According to the diagrams in the Bentley manual the AC Relay should have power from the ignition switch at poles 85 and 86 ..."

Without going into the details of your electrical problem, let's just review the "standard" pin-outs on a typical relay. A relay has a "control circuit" ( think of this as an electromagnet, that when energized, moves a rod or actuation mechanism) . The "actuation mechanism" then pushes on another mechanism that has two contacts.... like a knife switch has.... that connects to the two power leads. So...the electromagnet can be low power ( simply to energize the movement of the rod)...such that it forces closed ( or open) the knife switch that is connected to the real electrical "load" we're trying to control....like a high amp or high voltage motor.

The conventional pin-out is that the control circuit...electromagnet... has its two ends labeled 85 and 86. ONE of these is always ground. Probably 85. The numbers 30 and 87 are the OTHER TWO contacts across the hypothetical "knife switch" that make or break the connection with the main power line to our "load". You should NEVER expect power at BOTH pins 85 and 86, they are the end-points of the control power circuit on the relay, with one end always at ground. When the control circuit is activated, by putting power to EITHER 85 or 86 ( the OTHER one is ground)...then the electromagnet is energized which then sucks-in the contacts between 30 and 87 which completes the power delivery to the load. One of 87 or 30 gets connected to the battery (or a switchable source from the battery) and the other ( 87 or 30), goes to the "load" ( motor, etc). The motor, or load, will have another wire that goes to ground.

Draw it out, and it'll become a bit more clear.

Pin 30 is typically "hot" as this is the incoming power supply for "load". Pin 87 will only show voltage if the conacts are closed..and the contacts GET closed when the control circuit is powered up. From the sound of what you're saying, if 86 AND 85 never show power in any key position, then the control circuit is never being activated. Maybe you're not getting power from wherever pin 86 should get power...the ignition switch? When you jumper 30 and 86, it seems 86 gets "hot" from the 30 is incoming power ( for the "load" circuit)...so jumpering these two causes power circuit to be "complete" and the blower works.

My suggestion is to noodle a bit more to find out why the control circuit is not getting power. Determine which is ground ( 86 or 85)...then trace the OTHER one to its power source and verify why it's not getting power. Is there a fuse in this circuit?

Anyone else?.....what is Porsche's convention on 85 and 86 ( which is ground? They are bi-directional as far as functioning is concerned...but Porsche wires up only one way...right?). I think Porsche uses 85 as ground.

[gshiwota]

Patton - Thanks for your input. I have a really dumb question - where (physically) is fuse box #1? Are you referring to the fuse box in the trunk or engine compartment? Are there any hidden in-line fuses or connectors between the ignition switch and AC relay that I should check?

Wil - Thanks for your input too! I think you're right and I had a hunch that power should only be to pole


Thanks again guys - this is why this website is so great!

Gabe

[Patton]

85 ground 86 power in this case and is pretty much a standard. Check fuses in lugage compartment#1 (check your fusebox listing in the cover)

above is an 1987 cover example.

[2.7RACER]

Here is the schematic from the factory manual.
This should match what you are seeing in the Bentley.


As you can see there are 2 relays. As I understand it you are working on the "Relay AC" located in the middle of the schematic. Not the cooling fan motor AC relay.
The control power for this relay originates at the ignition switch, then going to the top of fuse 11, not through fuse 11.
From the top of fuse 11 the power goes to pin 85 of the relay.
Pin 86 is grounded at the starter. Why this is I have no idea.
Putting power to pin 86 should have resulted in an arc to ground.
More likely pin 85 energized the relay.
Essentially this is telling you, as you suspected, the power is not getting to the relay from the ignition switch.
Testing the top of fuse 11 for power with the switch on will verify if the switch or wire from the switch to fuse 11 is the problem.
Also the power to the top of fuse 11 is jumpered from an adjacent unidentified fuse in my schematic. Check the jumper wire.
Of course for the AC to come on the temp switch must be set to to activate the compressor clutch.
More likely a wire than the ignition switch.
Good luck,

[Patton]

Anyone else?.....what is Porsche's convention on 85 and 86 ( which is ground? They are bi-directional as far as functioning is concerned...but Porsche wires up only one way...right?). I think Porsche uses 85 as ground.

Although, as from the factory manual as posted by Doug and here at Pelican parts http://www.pelicanparts.com/911/911_Parts/Electrical/911_electrical_82SC_AC.jpg there's certainly a change in the later models.

2.7racer +1.

[rick-l]

Hard to read through your troubleshooting but you do realize that terminal 86 of the AC relay provides ground to the relay coil through the windings in the starter.

It is this way so when you power the starter there is 12 volts on each side of the coil or 0 across it. Turns off the AC when you run the starter.

[scottbooth]

Originally Posted by 2.7RACER

From the top of fuse 11 the power goes to pin 85 of the relay.
Pin 86 is grounded at the starter. Why this is I have no idea.
Putting power to pin 86 should have resulted in an arc to ground.
More likely pin 85 energized the relay.

All of this is correct ... just a clarification on the pin 86 connection.

Many of the control circuits on my 78 (which doesn't have AC) ground through (not at) the starter. This has the effect of powering down the accessories when the engine is cranking.

If you haven't already, check the voltage between #30 and #86. You should see 12 volts there all the time. I agree with Doug, jumpering 30 to 86 should have given you a big spark and/or caused fuse #20 (as marked in the diagram) to blow.

Good luck
Scott

[2.7RACER]

rick-l,
Thanks for the analysis re: ground thru the starter. Makes sense.
Interestingly the round red fuel pump relays used prior to '84 have a diode across the control coil.
These red relays require pin 85 to be ground.
Good info here,