90 MGM Ac

[87GrandMarq]

I do have a vacuum issue somewhere which I thought was the check valve but a new one didn’t fix the issue. It’s got the typical switch to defrost while accelerating issue. I don’t see any vacuum leaks anywhere.

[gadget73]

Yep, very similar thing to the parking brake release and similar fail mode. Its not so much an arc issue, but when it first makes contact you're only getting a small contact area to handle a pretty good amount of current. Thats just not a great formula for a reliable contact. If it snapped open/closed so it had the full contact patch it probably wouldn't be a big deal. If you made it drive a relay to get the current out of the thermal switch it would pretty much never go bad. Could also have that same relay operate a vacuum solenoid to replicate the function of the vacuum section. Then again if you're doing that you could use something completely different thats not made of 40 year old brittle plastic and gives you the switch.

One of the lines connected to that is teed directly off the main vacuum supply so leaks there cause a lot of issues.

[87GrandMarq]

I found a cracked vacuum line one the upper port of the vacuum tree under the intake. I put a coupler on it and it doesn’t switch to defrost when accelerating anymore. A little win is a win. As far as the ac goes, I’m still not sure but don’t think it’s the blend door. With my laser thermometer, the discharge temps get into the upper 40s or so but will rise after sitting for a bit. It goes down while driving though. I’m leaning towards the condensor not being able to keep up.

[sly]

yeah... that sounds normal. The systems are rated at 1500 or 2000 rpm and not idle IIRC. Of course, higher RPM will pull more air with a mechanical fan, which helps a lot.

[87GrandMarq]

I have a pusher fun that I was going to install on my 87 mgm along with the mechanical fan but offset to one side. It might help at idle but I’m wondering if it would cause any issues if it moves more air than the mechanical fan.

[sly]

It won't pull more air than the mechanical once you're moving, but will help at a stop or slow traffic. It will block a little at speed though, but you'll be flowing more than it would matter anyhow. It would be a net plus I think.

[87GrandMarq]

Hmm maybe I’ll mock something up and test it out then.

[friskyfrankie]

I usually leave or put back to OEM unless I have a strong reason to do otherwise.

[87GrandMarq]

That’s how I feel about this car since it’s pretty original. I may swap to a newer style condensor to help ac performance but I don’t feel like dumping a bunch of money into this car at the moment.

[Toploader]

Thermal Blower Lockout (TBL). If the temperature of the coolant passing through it is not high enough it prevents heat from being used on the floor setting. Pretty useless feature and was done away with on the later version of ATC. Would not affect AC. Can be removed; plug the vac lines and I think jumper the connector.

I disconnected the TBLS in my '86 Tudor, glad I did

[DerekTheGreat]

Heh, I like that feature. I spent a day taking several used and one new unit apart to make some Frankenstein's monster part. Hasn't failed since and the heat kicks on when there's heat to be had on the floor setting. My original piece had become worn to the point I was losing enough vacuum on acceleration to cause the HVAC to default to defrost. Something had to be done.

[gadget73]

if you want it to work reliably, running it through a relay should make it last forever, until the plastic fails at least. The contacts nuke because it slowly engages, so when it first touches it only has a small contact area which eventually just burns away. A relay doesn't pull 20-odd amps, so that small contact area won't matter even a little bit.

I don't run one because the new part I put on exploded after 2 months and puked all of the coolant out.