VAM Adjustment

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John R. Rowlands
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VAM Adjustment

Post by John R. Rowlands » Thu Aug 16, 2007 11:22 am

From http://www.merkurencyclopedia.com/
Don't know who wrote the article but here it is.

John

Today I rescued a large VAM(87-88 TC) that I had miss adjusted a while back trying to get better performance out of it.

Tools needed.

1- Digital volt/ohm meter.
2- 7mm socket.
3- Phillips screwdriver.
4- Flat tip screwdriver.
5- Air/fuel gauge or "Engine check light".
6- Running engine in good tune.
7- Patience.

Procedure
First we need to bring the wiper back to the original position if it had been repositioned, using the digital volt meter back probe the VAF pin(3rd from the front) and the Signal return pin(1st from the front) at the VAM connector. Loosen the Phillips screw that locks the wiper down in position by holding the flapper door. Then turn the key to run without starting the engine, adjust the wiper so the digital volt meter reads .246v with the flapper door fully closed then tighten the Phillip's screw to lock down the wiper in place recheck to make sure it still reads .246v.

Second we need to restore the flapper door spring tension, using the 7mm socket loosen the bolt that holts down the cog wheel lock wire just a turn, then if the flapper door is real loose tighten the spring tension clockwise by holding the locking wire away from the cog wheel and turning it with the flat tip screw driver at the teeth till you get some tension. This will let the engine start so the final adjustments can be done. At this point make sure there is no foreign objects inside the VAM and connect it to the turbo inlet hose and start the engine. Chance the engine idle is going to be erratic (wiper moves back and forward rapidly) this means the flapper door spring tension is too loose, hold the cog wheel lock wire away from it and start tightening the spring tension by turning the wheel clockwise till the erratic idle smoothes out, still you need to hit the flapper door with your finger and see if it settles right away, if the erratic idle continues tighten the cog wheel some more till the flapper door settles right away or when the engine is first started it goes in to a smooth idle.

About this time look at your air/fuel gauge and see if your engine is running rich (engine should be fully warmed up) continue to tighten the cog wheel till the Air/Fuel gauge starts moving up and down this means that the computer is finally able to control the air fuel mixture.

The same can be done with the engine check light (87-88 TC) as soon as the computer detect the rich condition it will turn the engine check light "ON" when the engine check light goes out again the computer is able to control air fuel mix at idle and the rich condition has been eliminated. This indicates a properly adjusted VAM at idle and most likely like mine it will work great at all throttle positions.

If you do not have a Air/Fuel gauge or engine check light it is going to be hard to tell when the VAM is properly adjusted since you will have to do adjustments and then run KOER till the computer gives you 11 or the 42 code is gone.

Also if the flapper door is too tight loosen the cog wheel counter clockwise till you get a rich condition on the Air/Fuel gauge then start tightening it clockwise till the rich condition clears out.

Hope this helps out some one out there that like me screwed up a good VAM.

Note: I tried this procedure with the original 85 SVO VAM that went bad, I was able to fix the rich condition but not the erratic idle, it would either run rich or the idle would get real bad.

So keep in mind that there are some that are not possible to be rescued.
If it ain't broke don't fix it!

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