1985 F1590 5.0 EFI IAC issue

[sgauvry]

Allegedly, all IAC's produced since 1989 have a diode already installed in the valve.  That diode is signified by the marking "D" as shown in attached picture.  

The IAC I have is relatively new, yet there is no "D" marking on it anywhere.  That poses a dilemma for me as the harness I have has a diode built into it for the IAC.

So here are my questions:
1. As there is no "D" marking on my IAC, is it safe to say there is no built in diode on the unit?
2. Is there a way to test (with the IAC not installed) for a diode?
3. Shall I assume there IS a diode in the unit?

The advice I've received some time ago was that ALL IAC's produced since 1989 have a built in diode.  Based upon that information, I was also told to remove the wire harness diode to the IAC and to cap the ends.  I now have a replacement harness that is completely intact, but do not wish to remove the harness diode unless I am certain the IAC does in fact contain a diode.

By the way, I do not recall if the IAC is a Motorcraft, however, if there was a Motorcraft IAC available when I bought it, I am sure I would have bought it.  

Advice is welcome!

Steve

[85lebaront2]

Steve, the diode is the same concept as many EEC controlled functions, it is what is called a "snubber" and is connected essentially in reverse, so the reverse voltage spike caused by opening the circuit to the coil is dumped to either ground or back into the 12V supply. In the case of the IAC, it is back into the red #361 circuit (at least on the 1986) and the remaining load absorbs it. I do not think the lack of an internal diode will hurt, or having one as long as the polarity on the IAC connector didn't change.

I have a similar situation on my E4OD where the snubber diodes were in the circuit board on the early ones and moved to the EEC on later models, early boards can be used with later EECs, but later boards can not be used with early EECs.

I think you will be ok with it.

[sgauvry]

Hi Bill -

I looked through my parts and I have an older IAC with a diode in it (there was a time when I went through parts like nobody's business).

Suppose I were to install it. Would you recommend I cut out the diode in the harness and cap the ends of the wire?  Vinny and I conversed about this and he recommends getting rid of the diode in the harness.  He indicated it is a safer route to take.  

Steve

[85lebaront2]

I don't see why you would need to. If you do cut it, I would not take it completely out, just cut one end and insulate it. I have diodes in my EEC-V for my transmission solenoids and the ones in the transmission (it is a 1990 E4OD) and have had no issues of any kind.

[sgauvry]

Thanks, Bill!

[Gary Lewis]

I agree with Bill.  Having two diodes is not a bad thing.  Having no diodes is.  Leave it in and don't worry about it.

[sgauvry]

Thanks, Gary!

[vjsimone]

Steve,

When I had 2 Diodes installed in parallel, it did not seem to make a difference when I disconnected the one in the harness.
 
Ford created the TB-1398-A because they were experiencing a Rough Idle with 2 Diodes installed.

So, just keep that thought in your back pocket should you experience a Rough Idle.

I did as 85lebaront2 suggest, I disconnected the low voltage side of the Diode, tape up, leaving enough lead for re-connection.
 
All new Bypass Valves for our trucks have a built-in Diode…..