Help me Understand the Vacuum Switch (Full Throttle)

[iflyfisher]

My old 83 Z24 wasn't running well when I got it. If I accelerated a touch too fast, it seemed to starve for fuel and bog down. I thought the issue was the old carb, particularly a bad accelerator pump. Rather than rebuild, I yanked it and dropped in a Weber. In doing so, I pulled both the vacuum switches. The truck accelerated great with the new Weber, so I thought the issue had definitely been fuel delivery with the old carb. However, in trying to get my fuel cut off solenoid wire working, I put the two vacuum switches back in. Once I did, the engine bogged down under hard acceleration/full throttle. I unhooked the Vacuum Switch (Full Throttle), and the engine was back to running great again. 

 

Based upon what I have been able to find, the vacuum switch sends a signal to the control unit to shut off the plugs on the exhaust side to minimize engine noise under hard acceleration. The engine acts like all the plugs are being shut off, not just those on the exhaust side. What I was thinking was fuel related must be spark related. I am guessing I don't really need that particular vacuum switch, but out of curiosity, I am trying to understand more how this system works and where the fault is. Has anyone else dealt with this before?

 

Thanks.

 

Ken

[datzenmike]

First check that both the intake and the exhaust side plugs are firing. The intake side are hard wired on and powered directly from the ignition in the on position. The exhaust side are on the first fuse on the left side of the fuse box. Pull a plug wire and put an old plug in the end, hold against something like the head that is grounded and have someone turn the engine with the starter. You should have spark on both sides, Yes???

 

The full throttle low vacuum switch simply signals the electronic ignition module in the distributor to stop firing the exhaust side coil. You can deactivate this by unplugging this wire at the distributor. Below you can see that there are three wires on the harness that connect to the distributor with a fourth separate wire with it's own plug. Disconnect this and all plug keep firing at all times. Try this and see what happens.

 

 

The second vacuum switch senses excessive intake vacuum as in suddenly letting off the gas at higher speeds. This will close the throttle but leave the idle circuit running and this gas just goes to waste out the exhaust. This vacuum switch turns the idle curt solenoid off saving this waster gas. Naturally if the clutch is depressed or the transmission is in neutral while slowing down the engine would stall so there are switches on both to circumvent the idle cut function. When the intake vacuum reaches a normal level the switch turns off and idle cut power is restored for proper idling.

[iflyfisher]

Well @datzenmike, that is interesting. This engine is only firing on the exhaust side. I didn't think that was possible. That would make sense as to why the engine was stalling on hard acceleration with the full throttle vacuum switch in place. I have the service manual, so I'll clean the fusible links and run through the steps in testing the distributor and coil. 

[iflyfisher]

I tested both primary and secondary circuits on both coils. Resistance on both coils tested within specs. I also checked the #1 fuse, and it looked good and tested good. I tested power at the coil with the ignition on, and I got readings of 12+V on each coil. Does the engine fire both intake and exhaust plugs at startup with the truck in neutral? If not, I am wondering if the coil wires are connected to the wrong coil inputs on the dizzy. Looking at the wiring diagram, the one coil looks to be connected to the #1 fuse and runs a Red wire and Black/White wire to the coil. The coil going to #2 fuse runs a Brown and White/Blue wire. This Brown wire runs through the vacuum switch, so I presume this is the coil for the exhaust plugs. The ignition coil going to the Intake side of the dizzy comes from the coil with the Brown and White/Blue wires. My other coil has a Red and Black/White wire, and it is going to the Exhaust side of the coil. Could the fix be as simple as swapping these coil wires? 

 

Ken

[datzenmike]

 

 

Both coils fire together at all times. With the exception of full throttle where the vacuum switch tells the ignition module to turn off the exhaust side using that forth wire in that picture I posted. Did you disconnect that wire in my picture and test for spark?????

 

 

 

[iflyfisher]

Yes, I did unplug that white wire. No change. The coil that isn't working is the one with the Brown and White/Blue wires. This coil runs through the #2 Fuse, and I confirmed I have 12V across the fuse. This is also the coil on the circuit with the vacuum switch. I have unplugged that vacuum switch. I'll go back and work through the ignition troubleshooting tables. I checked a few connections on the dizzy earlier today, but I didn't work through everything. If you have any other ideas, I'm open. 

 

As a reminder, the truck was running this way when I picked it up from my Dad's place a month ago.

 

The engine seems to run and idle fine on the one coil. Is it better that I run the truck on the intake or exhaust side until I figure out what is going on?

 

Thanks,

 

Ken

[iceman510]

I am in SE Michigan and I have spare parts from an 81 (coils, distributor, etc.) if you decide you need anything.  Just message me.

[iflyfisher]

@iceman510,Yeah, I need help! 😄 Thanks for reaching out. I'll certainly check in if I can identify the faulty component. 

[datzenmike]

Yes, you have power to the positive sides of both coils. Both coils check out. The negative side of the coils are grounded by the ignition module. So everything up to the module seems to be ok.

[iflyfisher]

@datzenmike, I think you are right. I went back through the testing section of the SM today. All the tests were OK. Well, except for the last one in testing the IC Unit Ground Circuit. When I had my wife hit START, my voltmeter would reset. I tried this several times and could never get a reading. I can't imagine the issue is an IC ground as one of the coils works fine.

 

I looked on eBay, and there is a Beck Arnley control module for $54. I logged onto PartsGeek and they have a Standard Motor Products ICU for $284, WAI Global for $67, or I can get a refurbished A1 Cardone distributor w/circuit board for $142. I presume circuit board means ICU. I've no experience with any of these brands. Are they any good? Any recommendations?

 

Thanks,

 

Ken

 

 

 

[datzenmike]

ICU? All there is is the ignition module under the rotor in the distributor. It has a magnetic pick up and that's it. The same magnetic signal is used by the module to fire two coils. It's completely possible that one side of the module isn't working.

 

Check under the rotor. Maybe one of the wires is not making good contact?

 

[iflyfisher]

@datzenmike, Sorry, yes, my service manual calls it the Ignition Control Unit rather than module. Did you mean something different when you noted 'module' above?

[datzenmike]

Nope, it's all good.

[iflyfisher]

I had cleaned each of the four contacts on the module when I was running through the tests. I also purchased a new rotor and cap a few weeks ago. I shouldn't have any issues with the rotor and cap as the condition pre-existed the new units. If this were your truck, would you just replace the module or do the whole distributor? A rebuilt distributor with module is cheaper than just getting the module. The rebuilt dizzy may be using a cheaper module. The current distributor doesn't look bad. The truck does have 135K miles on it. 

[iflyfisher]

Found a Standard Motors Products new old stock IC module for $120 on eBay. Going to start there as the distributor looks to be in good condition. I'll report back how that goes in a few days. Not quite clear how to get the other one out, but my manual or the internet should provide info. Hopefully no special tool is needed. 

[datzenmike]

The star wheel has a roll pin holding it on just pry it up with a screw driver. The module pretty much the same. To install use a suitable size piece of pipe or maybe a deep socket.

 

As long as both coils have power on the + terminal of the coils, and the coil negatives connect to the module, the extra 4th wire is disconnected.... if one side isn't sparking I guess it has to be the module.

[iflyfisher]

I removed the star ring reluctor. It didn't want to come off easily, and the bottom of it has a few marks and some of the lower corners of the tips are very slightly rolled over. Do you think this will be an issue? Also, the service manual says to 'always' install a new roll pin. I don't have a new roll pin. Are these a somewhat standard item I can get at an auto parts store or is really not that critical to get a new one? One last question. I am guessing the orientation is not critical as this ring was installed with the roll pin not on the flat spot of the shaft. The new ignition module is due to arrive today, and I am hoping to drop it in. 

 

Thanks,

 

Ken

[datzenmike]

Auto supply or machine shop will have one. I don't think it's critical to replace it. Try to get the star in the same place as when removed. Definitely the ignition timing may have been affected so check when you are done.

[iflyfisher]

Thanks @datzenmike

[iflyfisher]

Woowhoo!! I dropped in the new ignition control module this afternoon. I started the truck, and it had spark on both the intake and exhaust sides! I hooked back up the vacuum switch full throttle and took the truck out for a drive. The truck did not hesitate and would accelerate all the way up to full throttle without missing a beat! Sweet!

 

Thanks so much folks, especially @datzenmike. I admit, I now understand this particular vacuum switch much better now. Nice to have you guys out there to steer me in the right direction. 

 

Ken

[datzenmike]

Unless the star wheel was put back precisely it will have advanced or retarded your timing, so check that.

[iflyfisher]

Will do @datzenmike. Thanks again for being available and willing to help.

[iflyfisher]

I've been searching through my service manual, and the only reference to Timing I can find is that it should be 3 +/- 2 degrees. I can't find any reference to actually setting the timing. I found the timing gauge tucked away by the alternator. Wow, is it hard to find an avenue to see through to it. I hope I can get my light directed in that tight little space. The gauge is pretty well greased over with no legible numbers, but I believe I know what each peak and valley are. Maybe I can get a long-handled brush in there to clean it. In looking at other posts, there should be a timing mark on the block side of the pulley. I haven't rotated the engine to try and find it, but I'm sure it is there. Do both plugs on cylinder 1 fire at the same time, so I can clip my light to either the intake or exhaust ignition wire, or, is there a preferred wire? 

 

Thanks.

 

Ken

 

[datzenmike]

Both plugs fire at the same time so dealer's choice.

 

The top point is 0. Each valley or point is 2 1/2 degrees advance apart or you could say that every point is 5 degrees. So setting the timing in the first valley is close enough to 3 degrees.

 

[iflyfisher]

Thanks Mike