I would like to add a vacuum speed control system to my 1992 F150 with a 4.9L I6. I was able to source some parts at the junkyard from a 1991 F150 V8 that had the vacuum system and a 1993 F250 steering column so I now have:
1. Horn pad with speed control buttons
2. Vacuum servo (E9TA-9C735-EA)
3. Vacuum lines to engine, coffee can, and brake switch
4. Coffee can from behind the driver's side headlight
5. Amplifier (E9TA-9D843-DA)
6. BOO (?) switch that attaches just above the brake pad

I have a few questions about this process I am hoping you all could help me with:
1. Are there any parts that I am missing that I need to add to make this work?
2. The servo and amplifier are from a different model year and engine than my F150. Will they work with my truck, or do I need different ones?
3. Assuming I have everything I need, how should I go about testing the system to make sure it works properly?

I apologize if this has been answered already, but I've done some research here and elsewhere on this topic, and most of what I have found is about 1. troubleshooting existing vacuum speed control or 2. adding electronic speed control to 1993 and up trucks. I haven't seen anything about someone adding a vacuum system to a truck that didn't come equipped with it.

 

Adding the vacuum system will be more work than adding the electronic system (I've done both). And even if you get the vacuum system working perfectly, it won't work as well as the electronic one.

The truck already has a BOO - it's what turns on the brake lights. The same horn pad works with both systems. The servo & amp are the same for all engines, but the cable from the servo to the throttle is slightly different. Test either system while driving. The worst that could happen is wide-open throttle, which is NOT likely, and all you have to do is shift to neutral & turn the key off (but NOT far enough to lock the steering). But simply pressing the brake pedal should drop the throttle (with either system).

Study this page & the NEXT dozen (eventually, the electronic system is shown):

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Thank you for your answer, Steve. Since I already have the vacuum parts on hand, I'm going to give them a try before I throw in the towel and go for the electronic conversion.

I looked at the diagrams you suggested and see that the additional vacuum canister isn't shown for the 4.9L. Do you think there would be an issue with speed control operation if I install the extra canister? I have some vacuum tubing lying around so I can make a connection directly from the servo to the port on the engine. I also saw that the amplifier for the 92 is a 9D844, the one I have is a 9D843. Do you know if the parts are sufficiently different to prevent the operation of the system? I also assume the V8 servo cable may present issues, but I'm ignorant on this point and defer to your knowledge.

I'll plug all this in today and give it a test drive, see if it works as is or not. I also found this when searching through your diagrams: https://www.supermotors.net/clubs/fo...try/2742/54307. I'll try some of these tests if the system doesn't work once I get it all together.

That being said, there are quite a few more F-series trucks at the local junkyards with electronic systems, and if this doesn't work it will be more cost-effective to get those parts than to order them off eBay or elsewhere. When I was sourcing the vacuum parts, I noticed that the wiring for 93 and up models has a different connection for the electronic servo, the brake cylinder pressure switch, and the windshield wiper motor (my 92 has two connections). When you did your conversion, did you completely replace the wiring harness or did you splice these switches onto your existing harness?

 

A reservoir with an internal check valve is effectively the same as any other, but more volume (larger reservoir or more reservoirs) is better for system function. AFAIK, all amps are equivalent. The only issue with the cable is how it connects to the throttle body.

Of course the electronic servo's connector is different - its amp is on top of the servo, so all those circuits go there. The vacuum system has no master cylinder switch - it uses the dump valve on the pedal for that function. The wiper motor has nothing to do with the cruise, but that change occurred in '95.

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When I added vacuum cruise, I just plugged in the factory optional harness section to my '83's wiring.

 

On Sunday, I hooked everything up for the vacuum system and got the truck on the highway. Pressed the on button, hit set and... nothing.

Today, I had a chance to take some electrical measurements. Here's what I've gotten:
Voltage

151

No switches pressed: 0

On: Battery (12.25)

Off: 0

Set: 0 (holding On: 11.8)

Coast: 0 (holding On: 11.0)

Resume: 0 (holding On: 7.5)



296

Battery (12.25)



511

Not pressed: 0

Pressed: battery (12.25)


Ohms

150

0



151

On: 29.4

Off: .8

Set: 678

Coast: 119.7

Resume: 2180



511: -3.8



144/145: 57.9

144/146: 146.7

148/147: 25,900

148/149: 27,600


I'm not sure how to interpret the numbers for the control line circuit. The on/off buttons measure as they should from the charts, but resume, set, and coast all show nothing for voltage but are within parameters for ohms. The voltage when holding down the On button and then pressing the resume, set, or coast buttons also drops, but I don't know what the significance of that is.

 

It's not. Test the wiring between the switches inside the steering wheel.

 

I pulled off the horn pad and visually inspected the wiring, it doesn’t appear frayed or broken anywhere I can see. I’m not sure how I should test the wiring to see if it works, since it’s not connected to a power source. Apologies but I’m a novice when it comes to electrical stuff.

 

Test continuity/resistance/Ohms from one terminal of a wire to each other terminal of that wire. Then repeat for each other wire. A wire is a conductor, so it should (be continuous/have nearly 0 Ohms resistance) from any point on the wire to any other point on the same wire.

 

Thanks for all your help, Steve. I tested the wiring and the resistance was 0/close to zero, so the horn pad seems to be in good shape. I can only assume there must be an issue with either the in-cab computer, the servo, or the vacuum tubes, and it seems more trouble than it's worth at this point.

I'm giving the electronic route a try. I posted another thread asking about the differences in a 92 vs 93 wiring harness since I was able to get my hands on one: https://www.f150forum.com/f10/92-vs-...5/#post7235390