vfrv4

Hi there. New owner here, 1 month with a nice 1990 silver metallic GT, 77K, no cats and an authority chip. I have put 250 entertaining miles on so far.

Taken care of the Toothed belt alarm (loose tensioner), misc. leaks, new tires (direzza star spec) and misc. check outs. Now on to bigger stuff. Like non operational A/C.

Using the Dr. Bob/L. Ott/Wally write ups I determined the blue relay in the control head was bad. No power at all to freeze switch.

Pulled the head unit, took the covers off and could hear something rattling in the little blue relay. Pried the relay cover off and 1 of the arms is broke/burned off. So onto Dr. Bob's relay repair. Hooked everything back up. Still no compressor clutch. Damm!

So re-performed the following tests:
Freeze switch. pull back the 2 leads, but not remove completely from the switch. probe on 1 lead and probe to ground, get 12 volts. Probe on other lead and to ground, get 12 volts. Now I am confused, but a little happy, looks like I got the relay going right. But still no compressor clutch.

Pulled leads off of freeze switch, hooked jumper wire between..... nothing.

Pulled low pressure plug off of switch and installed jumper wire to plug....... nothing.

Pulled the red connector to the compressor. Jumped it to the passenger fender Jumper point, Compressor clutch kicked on no trouble.

Any additional advice would be helpful. Thanks.

Doug

zoltan944

The A/C is correctly filled with refrigerant right?

vfrv4

I did a static check/partial fill as the compressor is not running. Checked at 35lbs.

Even if system is low, jumping the low pressure switch should have fired the compressor to check for accurate running pressure.

Suggestions?

Alan

Be methodical...

Test the pressure switch in circuit - do you see 12v on both terminals with AC engaged? if you have 12v on only 1 terminal then you likely have insufficent freon charge. If you have 0V on both its likely a wiring issue from the freeze switch to the pressure switch, if you have 12v on both it seems like a wiring issue between the pressure switch and the clutch. Test this wire off the clutch with a voltmeter - does it go to 12v? when connected to the clutch what voltage do you see on that wire?

Alan

vfrv4

DUH!!!

Thanks Alan. Sometimes the simple things stare you right in the face. Went and checked the voltage to the low pressure switch and it is less than 1 volt. So need to hunt down the leakage or run new wiring, which I am sure is going to be no fun.

Ah, why did I not buy that cute little mga instead. Probably 7 or 8 wires on the entire car....

Thanks again for taking a tired fustrated guy and pointing him in the right direction.

dr bob

VFR4--

Get a copy of the wiring diagrams if you don't have them already.

You are good on the testing so far. The pressure switch jumper (make sure you are jumpering the switch connector and not the pressure sensor...) eliminates the possibility of low freon level preventing compressor start. If you see 12V at the pressure switch jumper but the compressor still doesn't run, look next at the 14 piin connector near the jump-start post, where the front-of-engine harness connects to the car. If the connections are corroded, obviously you'll nbeed to freshen them. That FOE harness runs from the connector towards the rear of the car over the cam belt cover, goes through a clamp that attches to the front pass side engine lift hook, under the cam belt cover and forward, and across the front of the engine. Routing is BEHIND the oil dipstick tube, towards the middle lift bracket. Right by the dipdtick tube, the harness wire for the compressorr routes out of the bundle for the connection to the captive compressor wire. That's the circuit.

vfrv4

THanks Dr. Bob,

Great help with your relay replacement write up.

At this time I am getting less than 1 volt at the pressure switch. I am getting 12 volts @ both leads on the freeze switch after relay replacment.

Got hold of a local 928 owner club guy that lives just up the street and he is going to help me trace out the no power to the pressure switch tonight.

I have a full set of WSM's that came with the car, for elec. schematics.

THanks again

WallyP

Doug,

From the antifreeze switch, the power enters the Central Electric Panel on G21. One lead runs directly to E11, and from there to the high/low pressure switch.

Checking the voltage at E11 will probably tell you instantly whether or not the problem is inside the Central Electric Panel. If you have 12 vdc at E11, the problem is almost certainly in the wire from there to the high/low pressure switch.

Inside the Central Electric Panel, one lead runs from G21 to the suppressor, Relay XXI. This is NOT a relay, but is a suppressor to control the electrical kickback from the compressor clutch coil. If the voltage is low at E11, it might be possible for this suppressor to have failed, and it might be worthwhile pulling it to see if the voltage at the pressure switch goes up.

Another lead T's off of the compressor clutch power lead inside the Central Electric Panel and runs to W24, and from there to the LH ECU. If the voltage is low at E11, you could also pull Plug W off and see if the voltage increases. We certainly hope not, because that possibly shows a problem inside the LH ECU!

Richter12x2

I'm thinking it's just the difference between other people going "Oooh" and "Ha ha" ?

Oh, I also wanted to add - MG A's are Lucas electrics, right? If so, you're probably better off where you are.

AaronSweat

Hey Doug,
Sorry I don't know much about a/c, but if you are working on the shark, let me know. My a/c also isn't working at this time.

Let me know when you want to look at that rear quarter. I just picked up a new cutting pad for the buffer.

vfrv4

I'm thinking it's just the difference between other people going "Oooh" and "Ha ha" ?

That and the HP factor. I am a speed junky.......

Wally,
Thanks for the additonal tracing info. The "New" WSM electrics are not as easy to read / decipher as the older schematics. Had a local owner with a 79 stop by and point out a couple things to trace. He kept complaing this new stuff was too hard to figure out. Popped the 14 pin connector on the pas. fender, and ran a few traces. Don't have the notes right in front of me, but he was thinking the problem is in the electric panel as well. Praying not the ECU, tho the PO had it rebuilt by sharktuner guy about 4 years ago....

Take another crack at it tonight.