AFMs, ECUs, and more.......

[vwbusman215]

Ok, I hope this isn't too long. I have a couple of questions that if anyone can help with it would help me out and ease my mind a bit. Just for some background I have a '76 bus with a '79 2.0l and a, from what I can tell, '76 L-Jet FI setup.

The '76 bus FI was the only year to use a microswitch for full load enrichment. Previous years had used some other types of devices connected at the throttle body and such I believe. My bus has the microswitch. My ECU is for a '76 (looked up part #). However, my throttle body is obviously not a '76 TB because there is no hole in the TB to mount the microswitch to it. I do also have a '77 bus too. My engine is being rebuilt and I want everything to work properly, and short of getting a hole drilled in the TB to mount the switch from what I read in the Bentely my engine will not receive proper full load enrichment which the microswitch tells the ECU to deliver at WOT.

So I am checking out my part# for my AFM on the '76, but there is no VW part# on the dang thing. There is the standard bosch# but not a VW#. Where the number should be it is now smooth, just in that particular place. Almost like it was remanufactured and bosch smoothed off the VW part#. You can tell it just wasn't done by some joe blow with some sand paper or anything like that. It looks like it came that way from the factory. So I am not sure if I have a '76 AFM on my bus or not.

So a question arises while I am thinking about this. If '77 & later FI buses don't have any type of switch on the TB to sense WOT then how is full load enrichment provided. Bentely just says on the later year models it is handled through ECU, but how does the ECU know that its at WOT. My guess would be a sensor in the AFM maybe? I may be totally wrong... But it would be nice to know how this is done.

So I go to get the AFM, ECU, and wiring harness off my '77. Figured why not just use it since I already have it and that will eliminate the need for the microswitch because by whatever other means the ECU is able to tell the motor is at full load. I check the ECU part# to make sure and it is in fact a '77. That's good, but then I check the AFM off the '77 and its a AFM off a '76!! Darn!

So I guess I need to know how does '77 and later FI systems sense full load for enrichment purposes, and what's the difference between a '76 and '77 AFM? If the AFM is where the '77 and later models get their infor for full load then I would need a '77 AFM to use my '77 ECU & eliminate the need for the microswitch.

Also is there anyway to tell what year model a AFM is if the VW part# doesn't show. The part number on the chips inside are the same, but there are some small differences on how the pot looks on the inside. Also the covers are different. The one that doesn't have a part# has a flat plastic cover. The one that has a '76 model VW part# on it is shaped like the one shown in the Bentley on page 10.17. It actually angles down a little and curves around as part of the housing that the electrics are in instead of being just a flat lid that sits on top. (Sorry that's not a very good description)

Sorry about the length on this one. I just need to find out what I am dealing with here before I put everything back together again.

Thanks a bunch



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Casey
www.angelfire.com/tx5/76vwbus

[ray greenwood]

There were three methods of enrichment signaling. I can't remember if some were combined....but here they are. The microswitch is common. Others that did not have the microswitch had a throttle valve switch on the crossways boss underneath. Many call that a throttle position switch (it kinda is). Its just a micro-switch in the case running from a cam around the throttle shaft. I belive its two wire...maybe some were 3. But that is also the full load switch. I believe you can hook your micro switch wires into that throttle valve switch with no problem...both are just switches. The third method was a micro-switch contact inside the AfM, with "whisker contacts" that closed the circuit when the flap went wide open. They were less dependable at times. You may also find that on the AFM with enrichment, that the plug either has more pins...or the same number of pins...some of which were not used in vehicles with enrichment elsewhere. You could actually wire the lead to the microswitch into the contact from the AFM...a switch is a switch. But...That move from throttle valve to AFM would be significant in wiring change to the harness and since it is a slightly differnt switch type...there may be a resistance change that you cannot compensate for. Unless you have a full list of all the part #'s you might also want to pop the black cover off the AFM and look at the contact arms to see how many it has and where it goes. The corresponding numbers stamped on the plug next to each wire, will be the same # for full load enrichment function wether you are using the AFM model or the microswitch, or the TPS. If there is a blank milled off spot on the backside of the throttle shaft...you have the same throttle body...but it just hasn't been drilled for the bracket. It will usually have two screw holes underneath...tapped to hold a TPS. If it has no bracket...and a little 5mm freeze plug underneath where a longer throttle shaft would have been to activate a TPS...it was set up for a switch in the AFM. You can make any of the throttle bodies work for either system. Either drill holes for the bracket on top, pop out the freeze plug underneath and install the longer throttle shaft and TPS (which would mean that you have another throttle body anyway) or leave it alone and get an AFM with the switch inside. Ray

[vwbusman215]

Hey Ray,
I thought that was the case with the AFM signaling the full load enrichment if it wasn't the other two methods. My bus doesn't have a TPS but does have a microswitch, but it doesn't have the hole to bolt it to the TB that's why I thought I would just use my '77 stuff and do away with the microswitch and not have to worry about getting a hole drilled and tapped, but then I check the part# on the AFM that came out of the '77, and dang it! Its for a '76. Frustrating. Everything else in the '77 lines up with '77 except that. Maybe I should just the hole tapped.

But my AFM that was on my '76, like I said, has no VW part# on top. Do you know if bosch took the VW part#'s off if the did a remanufature job on them. I was wondering if there is a way to tell what year it might be since it has a differnt style cover. The way that this one is meant that the whole aluminum casting for the main unit had to be different as far as how the covers fit. Do you know if there is a way to tell the difference by the type of cover?

Thanks

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Casey
www.angelfire.com/tx5/76vwbus