RPM Cycling Situation & Vacuum Switch Location Query

[ARCHINSTL]

WARNING - This is verbose, even for me....
OK - as many know, I have been having an apparently unique problem of the engine revving to 2200, and then immediately dropping on its own to 1800 and then again going to 2200, ad infinitum. This occurred at rest, and when driving led to some big stumbles/bucking whilst in that range.
I advised yesterday that I was going to fiddle with the Deceleration Fuel Cut System and Switch "A", as those were the only systems I could find in the FSM that had any RPM reference even remotely relevant to my problem.
Today I checked , per the FSM: The EBCV, vac switches "A and "B", and the fuel cut solenoids. All checked out per the conductivity and "click" tests.
I then disconnected the vac line to the "A" switch, and lo! - the cycling stopped. Now when the vac line was connected, it cycled whether or not the switch was plugged to its electrical connector. When the vac line was disconnected, it did not cycle, whether the electrical connection was made or not.
So - I reasoned that there was a problem with the "A" switch, regardless of my earlier test. It apparently enables an air bleed into the vac line, which then introduces air into the port in the carb base under the CB. It apparently affects one of the solenoids, as well as the Evap system.
I then thought that perhaps the two switches were reversed in their position on the bracket, as one member advised once that that has happened (from OEM). Then I went to the wiring diagrams, both in the FSM and AZ's better display. It turns out that the wire colors of my switches do not match either the FSM or the AZ colors - for any year of the Wagon. The switch in the "A" position in my 1986 car has a white/black wire, and the "B" switch wire is white/red. The latter switch is toward the front of the bracket. The switches themselves do not appear to be alphabetically marked. The rear switch ("A"?) has the black plastic connector and the front one ("B"?) has the green connector.
So - I can't really tell if the vac lines are hooked up to the proper switch.
I took it out for a drive (with that "A" vac line unhooked and plugged), which included a short 70mph run on the Interstate - and it's never run/accelerated as well before! The temp gauge did seem a little higher than before (barely above the mid-line, vs. just below), but no extra funny noises were heard and no extra steam/smells were present at shut-off time. This temperature thing might be me just being my usual paranoid self, or it may be important, as the FSM states that the Deceleration System "prevents overheating and afterburning in the the exhaust system" - 'course, I don't know if the overheating means in the cooling system or the exhaust system...

So - what are the colors of the wires and connectors on your switches and which vac line goes to the switches? My vac lines are hooked up per the FSM and the label under the hood and Abused Jalopy's color drawing of last year. I just do not know if the switches are in the right place.

EDIT - EDIT > > I know, it seems odd, that if unhooking the vac line eliminates the cycling whether the line is plugged or not (permitting air to enter or not), why won't it work with the line plugged into the switch, which also may or may not permit air to enter? I can't figure this one out either - it does not make sense to me, even after ruminating about it since last night.

Thanks to all.
Tom M.
EDIT - 102608
Upon rereading the above bold and red paragraph, I realized that was incorrect, based on my ignorance at the time: The "A" switch does not introduce air through the tube to the carb; it enables vacuum from the carb to act upon the switch, opening it.

[waynehoc]

Looks like you've ruled out quite a bit. If it were me, I'd want to determine if the 'A' vacuum switch is bad. Easiest way is to sub in a known good one. Best would be with a known good spare, but if the two vacuum switches ('A' and 'B') on your wagon look to be the same, you might try interchanging one for the other, both the wires to them as well as the vac hoses to them. If the problem moves (to the CB), then you've found the root problem. Its a long shot, but worth a try, if you haven't already done this?

[xirdneh]

switches themselves do not appear to be alphabetically marked. The rear switch ("A"?) has the black plastic connector and the front one ("B"?) has the green connector.

i've only had this problem with the "B" switch (green connector)
on two cars
both cars run great when its dissconnected
i had some spare (salvaged) "A" and "B" switches that i tried
the problem did not go away
so i doubt you will find that the switch is the problem
i sure would like to know what the heck is going on
rdw

[ARCHINSTL]

I've tried this with the "B" disconnected, as you have recommended before, and that had no effect on the cycling.

Both switches "click" when vac is applied with a pump, so mechanically all is OK. Both switches test OK for continuity/no continuity per the FSM. Since "A" is tied into the Evap system and the Decel System, maybe something else is affecting one of the systems apart from "A". However, when Decel is engaged, the Evap system does not send HC into the carb via the VSV. I'm going to test further on Tuesday.

Meanwhile, it again ran GREAT tonight with "A" connected, but with its vac line plugged...
Tom M.

[xirdneh]

[

Meanwhile, it again ran GREAT tonight with "A" connected, but with its vac line plugged...
Tom M.[/quote]

i guess i should re-word what i said. When i said "disconnected the switch"
i should have said disconnected the vacuum hose to the switch and plugged it. I left the switch itself connected (wire connector).
rdw

[2wagons]

It seems that if the vac line was plugged and the car ran better, then there might be a vac leak associated with whatever the line plugs into. In this case, perhaps the switch itself has a leak?

[ARCHINSTL]

Well, I have not made it out yet today as promised - it's raining and no garage and I have turned into a wuss.
Anyway, to recap:
It cycles when:
1. "A" connected - vac connected
2. "A" disconnected - vac connected
It does not cycle when:
1. "A" connected - vac disconnected
2. "A" disconnected - vac disconnected
So - the key would be the line disconnected, ergo, it would seem that there is a leak inside the switch, as 2wagons suggested, thereby permitting air to enter the vac tube. However, it would then follow (?) that it would cycle if the line is unplugged. But with the tube disconnected, it will not cycle, whether the line is plugged or unplugged...
Electrically, the switch checks out, cold and warm, per the FSM. Also, it "clicks" when vac is applied with a pump, leading me to believe that the mechanical connection inside the switch is OK (it also perforce does not indicate a vac leak). So - the switch would seem to be OK by all testing methods. The solenoids on the carb also "click" when appropriate when excited by the battery, per the FSM.
The only thing I can come up with at the moment is that the oxygen sensor, while new a few months ago, is sending a wrong/no signal to the ECU, and this is affecting the "A" switch is some manner. OR - the charcoal canister is inoperative, also affecting the "A" switch, as the latter is also tied into the Evap system on occasion (I also checked the canister a while back, and it was OK). I will check and certainly advise. I already checked the EBCV per the FSM, and it is OK, both electrically and vent-wise. Of course, maybe a bad oxy sensor or canister affect it, which then affects the "A" switch, which then affects ...

It does seem odd that no one else has encountered this cycling problem, and it sure looks like someone at some time or another (prior to me) has encountered everything. Maybe I get to write the definitive monograph on this?
Thanks to all - Gaack - I wonder what people who have these cars and do not know about the Forum do? They must not have much hair left... There sure is not much mechanical help on the ToyotaNation site, nor on the TercelOnLine site. Any 3AC owner should be here, whether a Wagon or sedan person.
Tom M.

[ARCHINSTL]

Well - the saga continues ...
I checked the vac switches, the VSV, the oxy sensor, temp switch etc. and etc. All checked OK, both electrically and mechanically, as detailed before. When the vac line to "A" was hooked up, the cycling returned, as expected. So I disconnected and plugged the line (although redundant). Just came back from a run - ran great, and hit 75 on the short stretch of I-270 by my house, and now even has quite a little "power" sound (giggle).
However, I then checked the "A" and "B" switches for the heck of it. "B" was fine with continuity indicated as it should when hot (none when cold). Good old "A", however, for the stinkin' FIRST time out of at least 6 times tested, indicated continuity when it should have no continuity when hot - AAACK ! However, the car did not seem to run any differently than it did on the last two "Midnight Runs".
So, I'm gonna take it through the emissions test tomorrow and see what happens... If it passes, I may disconnect the canister line to the VSV and plug the line from the VSV to the carb to see what happens. Anyone do this before?
Tom M.

[dug320]

Great attention to detail !!!!

A couple of thoughts after reading your "report" it sounded like three possible problems
1. electrical short/intermitant conductivity
2. vacuum leak
3. vacuum plug

it sounds more to me like electrical short

[Chris]

To help in identifying the two vacuum switches:
Vac switch "A" is a normally closed switch ( with no vacuum applied to it you should get continuity between the wire and the switch body). The source of vacuum is from the Throttle Positioner port located just below the throttle plate. The switch should open with approximately 200mm Hg of vacuum. Vac Switch "B" is a normally open switch (with no vacuum applied to it you should have no continuity between the wire and the switch body). The source of vacuum is from the intake manifold through the Themostatic Vacuum Switching Valve (port "L" at coolant temps above 63 F) The switch should close with approximately 100mm Hg of vacuum.

The wire colors indicated in the wiring diagram should be the colors in the harness, not the leads coming off the switches themselves.

The vacuum switches do not control vacuum. They are used to sense vacuum at different sources under differing engine conditions. They are input sources to the ECU used to vary output signals to other systems(carb feedback, evap and deceleration fuel cut).

I have experienced a fluctuating idle but only when the engine is somewhere between partially warm and completely warm. It does not misbehave when cold or at normal operating temperatures. I have not been able to find the cause although every component checks out OK.

Now, after this lengthy discourse I am at a loss as to the cause of your problem. I wish there was more info on the operation of the ECU.

I hope this helps (although I doubt it).

Another thought...what if you hook up a couple of vacuum gauges to the A & B switch vacuum lines and monitored them as you drive and idle. You might find a correlation between vacuum levels and RPM fluctuation.

[ARCHINSTL]

dug320 and ever-resourceful Chris - Thanks for the info.
The electrical short for "A" switch seems most logical, as I have been over the vac hoses time and again, and can find no leaks anywhere - or plugs, for that matter. "A" also "clicked" when vac was applied by a pump. I did plug the lines from the EBCV to the HAC, and from the HAC to the distributor, as the HAC system is redundant at St. Louis' altitude. The unhooked "A" vac line is also plugged. I plugged the various lines with BBs and rehooked them in case the emissions tech looked under the hood...
It does seem a mystery why "A" tested fine so many times and overnight it decided to have continuity both cold and hot.
The RPM cycling/fluctuating I experienced before unhooking the "A" vac line was never at idle; only at the 2200 mark (and then down to 1800 and immediately back to 2200 and then back to...ad infinitum).
The wonderful POS FSM, as we know, ignores the ECU completely, other than to have it on some diagrams. It tells nothing about access to, or even removal of the ECU, let alone testing of the unit. This seems odd, as it is supposed to be the same manual the Toyota mechs would use. This FSM has been a real disappointment in so many areas, especially considering its cost.
The car passed the emissions test (separate post to follow on that) and runs really great, but I would feel better if "A" were working, as it is tied to so many other systems that may be necessary for longevity and running better/more economically. Toyota had it on for some reason, so I guess I'll call some salvage yards.
Tom M.

[xirdneh]

The wonderful POS FSM, as we know, ignores the ECU completely, other than to have it on some diagrams.


i was thinking about this the as i followed your adventure. why no info on the ECU? if i had not stumbled across it while removing glove compartment
i would not have know it existed. my problem was similiar to yours but with the "B" switch.

[ARCHINSTL]

xirdneh -
Yes, it is interesting. The only things I can hazard a guess as to why the ECU is omitted are that:
A. It's not really a Shop Manual for professional wrenches, but is intended for those of us amateurs who are handy and can't screw up too many things.
B. The Mother Ship considers even an ECU like ours (surely just one step removed from a vacuum tube type) to be beyond our expertise/understanding/testing.

So - is my "A" switch problem (or your "B" switch problem) really the switch itself, or is it where it is connected to the ECU, or did some grey cell of the ECU itself decide to go senile? How the heck do we know - especially if I got another "A" (even a new one), and the problem persisted?

I also wonder if all Toyota FSMs are like this, or only the Tercel's? I know my '88 Dodge Ram 50 (aka Mitsu) FSM is tremendous, as it explains everything, including " if Part "A" goes bad, this is what it can cause"... I have actually used it in tandem with the Toy manual to understand some systems, particularly the emissions maze (it is carbed, too).

[dug320]

I guess Chrysler manuals are so detailed because they need to be fixed sooo often.

Perhaps Chrysler believes their mechanics to be mentally challenged so they make them real simple with lots of pictures and diagrams.


The Toyota FSM really is a poor manual. There are sooooo many gaps and over sights. Chilton's is not much better.

I don't think they are interested in any revisions.

[takza]

I happen to have 3...Chilton's...FSM.....Bentley. There is a website that rates manuals...they say the Bentleys are right next to the FSMs in usefulness. They do have some good details.

Found the Bentley used.

Even with all three there are things not covered...ECU? Vacuum gizmo troubleshooting?