What do the TAB, TAD, EGR and EEC-IV talk about?

Does the Megasquirt even have a provision for these functions? The computer on our cars simply sends a ground to a given solenoid to activate it (VPWR goes to each solenoid as well. It's constant with the key on). The rest is in the programming as to when the solenoids are activated and what happens when they are (i.e. having inj PW decrease and timing advance increase when EGR is functional).

Good luck.

Here's how the EGR and Thermactor Air System (smog pump) work with the stock Mustang computer...

Some basic theory to clarify how the EGR works is in order…

The EGR shuts off at Wide Open Throttle (WOT), so it has minimal effect on performance. The addition of exhaust gas drops combustion temperature, increases gas mileage and reduces the tendency of the engine to ping. It can also reduce HC emissions by reducing fuel consumption. The primary result of EGR usage is a reduction in NOx emissions.

The EGR system has a vacuum source (line from the intake manifold) that goes to the EVR, computer operated electronic vacuum regulator. The EVR is located on the back of the passenger side shock strut tower. The computer uses RPM, Load. and some other factors to tell the EVR to pass vacuum to open the EGR valve. The EGR valve and the passages in the heads and intake manifold route exhaust gas to the EGR spacer (throttle body spacer). The EGR sensor tells the computer how far the EGR valve is open. Then computer adjusts the signal sent to the EVR to hold, increase or decrease the vacuum. The computer adds spark advance to compensate for the recirculated gases and the slower rate they burn at.




Troubleshooting:
There should be no vacuum at the EGR valve when at idle. If there is, the EVR (electronic vacuum regulator) mounted on the backside of the passenger side wheelwell is suspect. Check the vacuum line plumbing to make sure the previous owner didn’t cross the vacuum lines.

Diagram courtesy of Tmoss & Stang&2birds. (the diagram says 88 GT, but the EGR part is the same for 86-93 Mustangs)


The EGR sensor is basically a variable resistor, like the volume control on a radio. One end is 5 volt VREF power from the computer (red/orange wire). One end is computer signal ground (black/white), and the middle wire (brown/lt green) is the signal output from the EGR sensor. It is designed to always have some small voltage output from it anytime the ignition switch is the Run position. That way the computer knows the sensor & the wiring is OK. No voltage on computer pin 27 (brown/lt green wire) and the computer thinks the sensor is bad or the wire is broken and sets code 31. The voltage output can range from approximately .6-.85 volt.


EGR test procedure courtesy of cjones

to check the EGR valve:
bring the engine to normal temp.
connect a vacuum pump to the EGR Valve
apply 5in vacuum to the valve.
if engine stumbled or died then EGR Valve and passage(there is a passageway through the heads and intake) are good.
if engine did NOT stumble or die then either the EGR Valve is bad and/or the passage is blocked.
if engine stumbled, connect vacuum gauge to the hose coming off of the EGR Valve
snap throttle to 2500 RPM (remember snap the throttle don't hold it there).
did the vacuum gauge show about 2-5 in vacuum?

if not, check for manifold vacuum at the EGR vacuum valve.
if you have manifold vacuum then connect vacuum gauge to the EGR valve side of the vacuum valve and snap throttle to 2500 RPM.
should read about 2-5 in vacuum


Late Model Restoration has the Ford Racing M-12071-N302 kit with the EGR valve & sensor along with the ACT & ECT sensors for $45. See http://www.latemodelrestoration.com/iwwida.pvx?;item?item_no=M12071N302 1&comp=LRS for more details


Thermactor Air System (smog pump) operation...

The Thermactor air pump (smog pump) supplies air to the heads or catalytic converters. This air helps break down the excess HC (hydrocarbons) and CO (carbon monoxide). The air supplied to the catalytic converters helps create the catalytic reaction that changes the HC & CO into CO2 and water vapor. Catalytic converters on 5.0 Mustangs are designed to use the extra air provided by the smog pump. Without the extra air, the catalytic converters will clog and fail.

The Thermactor air pump draws air from an inlet filter in the front of the pump. The smog pump puts air into the heads when the engine is cold and then into the catalytic converters when it is warm. The Thermactor control valves serve to direct the flow. The first valve, TAB (Thermactor Air Bypass) or AM1 valve) either dumps air to the atmosphere or passes it on to the second valve. The second valve, TAD (Thermactor Air Diverter valve or AM2 valve) directs it to the heads or the catalytic converters. Check valves located after the TAB & TAD solenoids prevent hot exhaust gases from damaging the control valves or pump in case of a backfire. The air serves to help consume any unburned hydrocarbons by supplying extra oxygen to the catalytic process. The computer tells the Thermactor Air System to open the dump valve at WOT (wide open throttle) minimizing engine drag. The dump valve reduces the parasitic drag caused by the smog pump to about 2-4 HP at WOT.






Computer operation & control for the Thermactor Air System
Automobile computers use current sink technology. They do not source power to any relay, solenoid or actuator like the IAC, fuel pump relay, or fuel injectors. Instead the computer provides a ground path for the positive battery voltage to get back to the battery negative terminal. That flow of power from positive to negative is what provides the energy to make the IAC, fuel pump relay, or fuel injectors work. No ground provided by the computer, then the actuators and relays don't operate.

One side of the any relay/actuator/solenoid in the engine compartment will be connected to a red wire that has 12-14 volts anytime the ignition switch is in the run position. The other side will have 12-14 volts when the relay/actuator/solenoid isn't turned on. Once the computer turns on the clamp side, the voltage on the computer side of the wire will drop down to 1 volt or less.

In order to test the TAD/TAB solenoids, you need to ground the white/red wire on the TAB solenoid or the light green/black wire on the TAD solenoid.

For 94-95 cars: the colors are different. The White/Red wire (TAB control) is White/Orange (Pin 31 on the PCM). The Green/Black wire (TAD control) should be Brown (pin 34 at the PCM). Thanks to HISSIN50 for this tip.

To test the computer, you can use a test light across the TAB or TAD wiring connectors and dump the codes. When you dump the codes, the computer does a self test that toggles every relay/actuator/solenoid on and off. When this happens, the test light will flicker.

Booksix said:

Hissin50, you are correct that MS does not have anything built in for these sytems. Just thought I'd see if I could make something happen.

So as far as all this goes, my main option would be keep the EEC-IV running emissions and ignition (because the EGR system needs to advance spark) and use the MS for fuel only. Guess I won't be going with EDIS! Bummer. I heard a lot of guys are getting WAY better emissions and mileage with MS but they don't say if that is compared to a previously emissions control engine or not. Wish I knew if I could pull good emissions #'s on MS without the EGR, TAB and TAD stuff... anyone know much about this by any chance?

Click to expand...

It's an interesting quandry. It seems most guys that go to aftermarket puters have no emissions concerns (I'm not sure about fox III puters but 94-95 GT puters are generally stable till about 700 hp IIRC).

The simple answer is that you can pass emissions without any smog components but a car without that stuff will not be as clean as one with the stuff, pure and simple. Otherwise the OEM's would not waste the money on the components in the first place.

I would surmise that a lot of folks ditch the stock EEC set-up because they had some bug they couldn't work out. So anything (including a carb) would lead to better-running and cleaner emissions. Had the stock EEC and its systems been fixed, it would have been quite efficient (more-so than whatever new system was retrofitted).

You can kind of use your A.I.R. system without computer control. You could leave the smog pump and check valve in the system and just dump clean air into the H-pipe at all times. This is less efficient than how the OEM system works but it's the best you can do if you don't have computer control assisting you.


And instead of trying to marry two different Engine management systems, I'd rather tune the existing one for whatever mods you have. This would give you the best of all worlds. If you wanted to go aftermarket for the ability to tweak it yourself, you could tune the stock system yourself (especially nice if you would be comfy using an RT without a lot of time behind the learning curve).

Just MHO.
Good luck.

nice diagrams J !!!!!!!!!!!

With cats, a smog pump, and non-functioning EGR, it's possible you could get through a smog check as long as you don't fail the visual for any reason and really nail the tune.

Something like a J3 can be used to tune the EEC like a MegaSquirt, but you can keep all the emissions stuff. It's not as straightforward to tune, however.

I know you don't want to hear this, but smog was the main reason for me selling the '89GT I had in order to get the '68 in my sig. The SVO is staying stock and it barely passes smog as it is. CA sucks in this respect.

in addition to jrichner's writeup on the AIR system. it also dumps air to the atmosphere on rapid deceleration (ie. downshifting to a stop light) to help prevent backfiring.

booksix, i guess you could piggyback the computers. splice the sensor signal wires to both the OEM computer and the MS computer then let the OEM comp control all but the Fuel injectors and the MS only the fuel injectors. i don't know if that would work but would be interesting.

it's designed that way so the AIR system won't cause a backfire.

Quickly closing the throttle or using the gears to decelerate at higher engine speeds causes a very rich air/fuel mixture resulting in rich exhaust gasses. if the AIR system added fresh air into this, rapid combustion (backfiring) would result.

I thought the EEC monitored the TPS and decreased injector dwell/stopped firing the injectors in that situation - that's why the air/fuel tables have a "load" axis and run 16-18:1 ratios in the 5% load row.

maybe I'm wrong...................

i don't know enough about megaSquirt to say.

have you asked their tech support?? do you trust their tech support??

tmoss said:

I thought the EEC monitored the TPS and decreased injector dwell/stopped firing the injectors in that situation - that's why the air/fuel tables have a "load" axis and run 16-18:1 ratios in the 5% load row.

maybe I'm wrong...................

Click to expand...

that may be true as well. i don't have the charts but i DO trust your reading of them.

the computer will also shutoff vacuum to the AIRB valve to stop the flow of air into the exhaust and will dump it to the atmosphere.

Booksix said:

ok, but either way, MegaSquirt should be able to handle this in the tune, right?

Click to expand...

There are settings for acceleration/deceleration enrichment. You can increase it all you want, or shut it off. If shut off, it will only inject the fuel warranted for whatever cell of the fuel map the MAP pressure and rpm dictate. Accel enrichment works like an accelerator pump on a carb and is often used to eliminate tip-in stumbles. Decel enrichment fattens the fuel briefly when you chop the throttle...also to eliminate stumble.