idle speed question

[mdregister]

The shop manual for my 1985 Suburban "J" CODE 6.2 indicates the idle speed should be 650 +/- 40 rpm.

I have been leaving it set at approx. 680 rpm because I like the way it idles better there than 650 rpm.

With the truck in gear, the idle speed drops to approx. 630 rpm and with the A/C on and the truck in gear, it drops further still to approx. 600 rpm.

At 600 rpm, in gear, and with the A/C on, the idle is somewhat less than smooth.

I adjusted the idle speed to 650 with the truck in gear and A/C on and the idle is much better, but with the truck in neutral and no A/C my idle is approx. 730.


Is the drop in rpm I am seeing typical?? If not, do you have any thoughts in the cause.

What are the drawbacks to having my idle speed so high to compensate??


Thanks in advance,

Mike

[DmaxMaverick]

Is your high idle working? It should engage when the A/C is on. On the 85's, you have to "tap" the pedal to engage it, like on older carbureted vehicles with auto choke. If you turn on the A/C and the idle stays low even after you apply the go pedal and let off, the high idle is not responding. I have mine disabled and routed to a switch. In the summer, I can park with the A/C on and engage the high idle manually, which I have adjusted to about 850 RPM. This allows the A/C to work well when it's over 100

[mdregister]

Thanks for the reply. Based on your input, I checked out the high idle solenoid.

I knew the high idle responded when it is cool in the morning, but was not sure if it responded when the AC was cut on.

I ran a jumper from the battery to the hi idle solenoid and it fuction with a tap of the pedal and the jumper attached.

With the A/C on, the solenoid is not active, my meter shows 0 volts with the AC on and (of course) no jumper.

Is there an inline fuse somewhere? I tried to read the diagram you have posted, but I can not see them too well.

Sorry for the rudimentary questions.

[ 04-21-2005, 07:58 AM: Message edited by: mdregister ]

[ZZ]

Wouldn't there have have to be a diode to keep the Winter high idle power and the A/C high idle seperated?

My 1990 GMC doesn't energize the high idle for the A/C either.

[NH2112]

The high idle is only activated when the temp switch on the right head reads coolant temps below 95F. There's no other switch to energize the solenoid, although it would be a simple enough job to have the high idle come on automatically when the AC clutch engages. I personally don't like my RPM to increase at a stop with the AC on, but since I have a manual it's a moot point. You will want to put a diode in each leg of the circuit to avoid problems with B+ going where B- is expected. :confused:

[BobND]

If the idle speed governor in the IP is working correctly, it will compensate (more or less) for the added load of the A/C, and keep the idle speed nearly constant with the added load of the A/C.

In the same manner (unlike a gas engine) when the diesel is started cold, and the high-idle solenoid is activated, the governor will hold the high idle speed nearly constant, as the engine warms up, until the thermostatic switch in the head drops out the "high idle".

A non-computerized, carbureted gas engine will start on high idle, and gradually rev up unitl it's literally "racing", until you manually "kick it down", as there's no governor to hold the speed constant, as it warms up, and it needs a "throttle kicker" to open the thottle when the A/C is on, since there's no governor to respond to the drop in speed.

[DmaxMaverick]

On 1985 model 6.2's (all of them), the "idle stop solenoid" is included in the A/C circuit. If yours doesn't work the way I described above, it's broke. Mine worked that way when it was new, as did several other ~85 models I've owned/maintained (about 20 total, most with A/C). I bypassed mine because I wanted to increase the RPM while parked with the A/C on, and winter warm-ups. The wiring diagram shows the idle stop solenoid in the A/C circuit, getting power from the A/C relay. Because the power comes from the relay, a diode is not necessary. I didn't use one.

[NH2112]

My 85 K10 has AC and there's absolutely no electrical connection between the fast idle solenoid and the AC system.

[diesel65]

I owned my 85 K-5 since new and it never had idle-up with A/C.

[mdregister]

I have ran a jumper from my AC compressor 12v supply (green wire on compressor) to the idle solenoid. This has helped tremendously. I was able to set the idle speed in neutral back to approx. 690 rpm (within specs).

This gives me an idle of approx. 650 in gear with no A/C running.

With the A/C on and the idle switch "active" my idle is approx. 800. The AC seems to work much better at idle now.

With no AC and the idle switch "active" my idle is approx 820.


I need to put a couple of diodes in each line, as someone noted earlier. What kind do I ask for??


Thanks for the help.

Mike

[NH2112]

I used to just grab 12V 6A or 12V 10A diodes off the shelf at work, these were about the size of the eraser on a pencil.