BAC Wiring Harness Bypass?
 

I'm having a hell of a time getting my BAC to work. The solenoid is working and I get barely 12v at the connector and less than 1 volt (DC) with a resistor in place. The wires to the connector have been corroded and I can't seem to get a good connection now. I was wondering if I could bypass the harness to the ECU. Would I just need to run a wire from the BAC pinout at the ECU and a new ground?

From profile, I see you have 87 sport (I happen to have same car)

A crude way to tell if BAC is working is to disconnect it while idling at 750 rpm, if rpm drops, it s probably working. You can also check if rpm increases when turning wheels and/or putting on AC.

Looking at page 28 of FSM 86-88 (free from www.fc3s.org under how-to's->manuals) [section: wiring diagram]

The bac plug (connector EM-12):
B/W wire which connects to pin 3I of ECU (I think it is main 12V)
L/G wire which connects to pin 2Q of ECU

Please, double check the info above

I believe the air by-pass relay (connector F-49) and air by-pass solenoid valve (connector EM-14) are involved in BAC valve operation, so you may want to check those wires too. They connect to pins 3I-3C of ECU (check page 28 of FSM)

I think HAILERS may be able to help you better on this.

Hugues

You should have 12v on that black/white wire at the bac. Its the same 12v that feeds the other solenoids like the relief, switching, pressre, egr, port air etc so I doubt that is your problem. The other wire is a L/G or blue wire with a green stripe. It gets a pulsated ground from the ECU.

I'm not with you on the resistor across the wiring and getting 1v dc. Why not connect the plug, start the engine and either poke into the L/G wire with your positive lead and put the neg lead on a good chassis ground and see if the voltage is approx 10 volts. Then put a load on the car with headlights or a/c on and see if the voltage drops. You could do the same thing by tapping into the ECU at 2G with a wire tap.

If you have doubts about the wiring, ring it out with a meter from pin 2g to the L/G wire at the plug. Have you looked to see if one of the sockets in the plug is being pushed back? i.e. loose in the plug?

Do what Huges said about pulling the plug on and off at idle.This is a good and reasonable check at idle IF your car idles at 750 or below. Cars that have their idle jacked up at some 1000 rpm need not apply.

But the best thing to do is read and understand this article: http://www.teamfc3s.org/info/articles/idle.html He is looking for less than 5v ac with the resistor in place.

I think HUGUES is looking at the wrong pins at the ECU for a 87 model. The b/w wire should be coming from the main relay and the L/G from pin 2G on the ECU. If I'm wrong, let me know.

From 88 FSM manual, pin 2G is TPS
pin 2Q is BAC (voltage should be between 8 and 12 V).
pin 3I is main relay (12 V) - As HAILERS said, this 12 V is all over the place so you could tap it somewhere else.
I believe that should apply to an 87 sport,
but as always, I could be wrong.

To the original poster:

You should probably have a look at FSM 86-88 chapter about 'fuel and emissions systems' for systematic troubleshoothing of BAC valve and what the ECU pins are for.

If the idle speed is not set at 750 rpm but more like 1000-1100 rpms,
then the BAC valve is pretty useless IMO, and troubleshooting it may be a waste of time.

If your idle speed is not set at 750 (meaning too high), then I would adjust it to specs, then recheck the BAC (after having adjusted the TPS, which should always be done when playing with idle speed).

Regards,
hugues -

My bad. This is an 87 TII. I just checked the voltage at 2Q (2G is the TPS) and I'm getting ~13v—same at the l/g lead to the BAC. I just replaced the transistor in the ECU last night…now I'm not sure if I shorted it out again! I don't think I'm supposed to be getting 13v to the BAC at idle. I think I'm going to try bypassing the harness from the ECU at 2Q to the BAC and replace the Transistor in the ECU again this evening. As my Air Bypass valve is not kicking on at startup either I'm going to assume that's due to the failing BAC. I'm just at the point where all of my readings are so off that I'm getting confused as to where the problem might originate from.

HAILERS—the original connector, as you know, is CRAP. I had to create a new connector from female spade connectors. Without the BAC working the car idles—lumpily—at 700rpm. Gonna run outside now and pop the resistor between the leads and get a reading. I got a reading of a bout 4Mv AC. Since the ASV, ABSV and BAC all work off the same relay could one these failing have an effect on the BAC? I'm tempted to disconnect the ABSV and see.

BTW, I applied 12v to the BAC last night (afte I disconnected the connector) and the idle shot right up! I think it's safe to assume the actual solenoid works and that the problem lies between the ECU and the BAC. Ugh.

Yeah I had my head up my dark spot earlier when I wrote that about 2G. I was at work and looking at a online manual and I guess I just misread. No excuse really.

With my car, I have a wire tap on 2Q and at idle will read 9.5 to 10v with nothing else on like a/c. The 12v indicates to me the ECU isn't pulsing a ground to the bac. Like you said, try the transistor again (only a buck eighty or so) and maybe check for a short on that wire or like you said, run another wire.

I think the bac normaly works independent of that relay and the other valves.

Thanks Hailers. I'll try the new transisitor and bypass tomorrow and let you all know how it goes.

B

Well, I just replaced the transistor to make sure, but have not put in the bypass. Pulled the BAC off, ran the ohm test—passed, and it passes the 12v-click test. I checked the voltage at 2q at idle—13v. It looks like the only culprit left is the signal wire to the BAC from the ECU. I'll get the bypass on there and post and update.

B

HAILERS—Should I be able to run a wire, spliced at the ecu, through the firewall and directly to the BAC? Also, the thicker black wire in the BAC connector is the relay lead correct? Thank you so much for all of your help!

Brian

Well, now I'm stumped. I've replaced the transistor, tested the BAC and totally bypassed the wiring harness to no avail! At least I THINK I bypassed the wiring harness—I ran a wire from the ECU at 2q directly to the BAC. I don't know what else it can be.

bump

Bump. ecu. I think on the Paul Stoakes article he mentioned another fellow who traced his problem to a resistor in the ECU, but PS didn't figure what it was. ecu bad. There is a place in Florida that repair them, but a used one might be cheaper. Less than fifty bucks. No more.

I swapped the BACs and had some success. The idle is now holding at 750 and it seems that the idle is now changed by the set-screw in the BAC. Although the range is very small, I think I can work with this and really get on it when the exterior is finished and the new exhaust and diff mount are on. Still no luck with the BAC adjusting idle with a load like the A/C taxing the system.


Brian

Yep. Press on. With a good bac circuit, you'd be able to take the *spare* and with the engine running connect the plug to it, with it in your hand, and you'd see the bac vibrating like crazy i.e. pulsing. Also I've put a spare injector on that plug with the engine running and let it pulse the injector like crazy. Someday I'll do something with this arrangement. Someday you'll run across a bargain on a ECU that'll fix it.