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Old 06-19-2013, 07:13 AM
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A/C with TBI question

Hello,
I have a '57 Chevy truck with a TBI engine. I am going to install A/C in this truck, but the TBI harness I am currently using ('89 Chevy) came from a vehicle without A/C and does not have the wire to connect the compressor to the computer (to tell the computer to raise the idle speed when the A/C is on).
Is it OK to use A/C without this wire, or should I swap harness? Will using the A/C without this wire make the engine run rough, or even die when idling?
Thanks!

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Old 06-19-2013, 09:56 AM
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i've never heard of a hotrod needing that wire.
oem uses a lot of ''stuff'' that in our world is useless
my 58 truk w/ efi has ac and no bump up wire
go with out it
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Old 06-19-2013, 02:25 PM
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Quote:
Originally Posted by wave1957 View Post
Hello,
I have a '57 Chevy truck with a TBI engine. I am going to install A/C in this truck, but the TBI harness I am currently using ('89 Chevy) came from a vehicle without A/C and does not have the wire to connect the compressor to the computer (to tell the computer to raise the idle speed when the A/C is on).
Is it OK to use A/C without this wire, or should I swap harness? Will using the A/C without this wire make the engine run rough, or even die when idling?
Thanks!
That wire is so the computer can keep the engine at a steady idle RPM whether the A/C is powered up or not. The drop in RPM will make the fuel metering unsteady as that is one of the fundmental measures that TBI uses for fuel metering.

Bogie
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Old 06-19-2013, 05:52 PM
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Oh good, just what I feared, Ogre tells me I do not need the wire, and Oldbogie tells me I need it!!!
I guess I need more opinions...
Also, is it possible to add the wire to the harness and to the connector at the computer? Is it possible to buy one of those tiny plug that go in that big connector?
By the way, my computer is the common TBI computer 747.
Thanks!
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Old 06-20-2013, 08:47 AM
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The first question is, are you using a stock PROM or a custom PROM? The reality is that the OEMs use the A/C sensor for idle control more to help meet EPA idle emissions requirements than anything else. The idle mixture control needs to be very precise because of this, and the extra load of the A/C matters. Now, if you have a custom PROM, it's likely that the idle mixture is not as lean as stock, in which case you're probably OK without the sense wire. You should also check the wiring on your specific ECU. The '7747 has the pin to accept the A/C signal, even if it's not in the harness. The issue is if your PROM is programmed to look for it.
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Old 06-20-2013, 10:49 AM
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What it comes down to is this. Will your engine tolerate a +- 200 rpm drop when at idle speed. My little 283 doesn't. I use a solenoid to hold the idle up when the air is on. The down side of the solenoid is when the air kicks on at idle, the rpm's drop until I give it a little gas. Then the solenoid will hold the new idle speed until the compressor shuts off. The solenoids wont open the throttle up on their own, they just wont let the throttle close completely when they're on. Edelbrock and most parts stores sell them for around $50-$60, or hit the junk yard for $5.
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Old 06-20-2013, 11:47 AM
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What it comes down to is this. Will your engine tolerate a +- 200 rpm drop when at idle speed. My little 283 doesn't. I use a solenoid to hold the idle up when the air is on. The down side of the solenoid is when the air kicks on at idle, the rpm's drop until I give it a little gas. Then the solenoid will hold the new idle speed until the compressor shuts off. The solenoids wont open the throttle up on their own, they just wont let the throttle close completely when they're on. Edelbrock and most parts stores sell them for around $50-$60, or hit the junk yard for $5.
The whole point of the idle air control motor on the throttle body is that you don't need the solenoid. The idle speed is kept constant by the ECU. The added A/C sense wire just lets the ECU know it needs to temporarily bump up the idle speed.
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Old 06-20-2013, 06:01 PM
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Looks like pin B8 - dark green ckt # 50 is the A/C input signal for the ECM.
Like joe said, The connector may have the wire in it , or not. But you can add a wire and change the prom and youre there.
You can get a whole nother ECM with an A/C equpt cal pac allready in it as well.Probably from a salvage yard.
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Old 06-20-2013, 07:13 PM
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I am thinking I will have a prom custom-programed anyway for that engine, so that is not an issue.
So, a part store (Napa...) should be able to sell me the little plug to instal in the big plastic connector, since the 7747 pcm has the pin, right? And then i can just add the wire to the A/C compressor. Seems like the safest way to go, because I fear the same problem as cal1320, engine dropping 200rpm in gear at idle could be an issue...
Do theses little plugs have a specific name, so the parts guy knows what I am talking about?
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Old 06-20-2013, 08:06 PM
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Pinout connector.
It may have one in it allready.
If not...go to the salvage yard and grab one from a junker.
Parts stores will not help.
Cut the connector off a junker and tae it home and look at it real good so you can figure out how to tae it apart.Pretty easy once you tae a good look at it.
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Old 06-22-2013, 09:15 AM
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Cool, thanks for all that!
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