A/C Idle to low

[lovehatec4]

Can anybody help me figure out this problem?

My 85 with auto trans idels to low with a/c on. It idles very smooth at normal idle @625 rpm in drive,like at stoplight. Today I turned on the a/c,and the idle speed went down to @540-560 rpms. It still idled smooth but the lower rpms turned on the low voltage light (11.2 volts). When I feathered the throttle a little light went out. I removed altinator and had it tested,Its fine. The battery and the I.A.C. are new... Is there a way to adjust the A/C on idle speed?

Really scratching my head on this one. Thanks in advance for any help

[c4c5specialist]

HI there,

If you have underdrive pulleys, or not enough reserve capacity in your battery, either of these things can cause your issue.

Your 75 class battery must have at least 500 CCA AND your battery terminals must be torqued to 11 ft lbs.

OR, you have too much electrical draw on the system.

Tell us what type of modifications you have and remember, when they test alternators, they may not be testing at YOUR specific RPM that this problem occurs.

Allthebest, c4c5

[lovehatec4]

Did'nt think of underdrive pulleys. You're right the test at autozone is at about 1200 rpm.. Battery is new and 550 CCA.

Does anybody know the diameter of a stock 85 altinater and crank pulley? I think it's origanal but who knows. After all, I did'nt buy it new.

Other than the injctors and the antenna, I thought this car was bone stock with 83,500 on the oddometer.

Is it normal for these models to idle lower with a/c on? I thought the ecm compinsates the idle speed, when a/c is on. Thanks for any help.

[c4c5specialist]

Hi there,

I do not know the diameter, but check your voltage at the rear of the alternator when its idling.

Check to see what electrical loads are on.

And with a NEW battery, your light shouldnt go immediately on at idle.

It should take a minute or 2.

Allthebest, c4c5

[lovehatec4]

You're correct, it does take a little time at idle with a/c on, for warning light to come on.

I will check voltage as you desribed. I will compare pulley diameter with new alt.

Thanks for the help...

[lovehatec4]

Update on problem..

I had the altinator tested at NAPA. It failed.
Took altinator apart,bad diode trio. Only two of the three leads were supplying voltage. Ordred all parts to rebuild. Less than 45 bucks.

Thanks c4c5 speicialist for your help.