MCU/Duraspark Ignition

[bilbo]

My truck has an MCU and uses what appears to be Duraspark ignition and vacuum advance distributor. I'm trying to diagnose an issue where the idle speeds up after the truck warms up a bit. It behaves like a vacuum leak; I can back off the idle adjustment completely and the RPMs remain around 1000.

I've isolated the effect to a vacuum switch that connects through an orange poly hose to the vacuum port under the throttle on the carb. If I pull the hose and plug it, then apply vacuum to the switch manually, I can reliably reproduce the effect. Release the vacuum on the switch and the idle slows back down.

I'm wondering what the MCU controls in this application. I wasn't able to find any vacuum leaks, and was wondering if it's changing timing. Is the MCU capable of advancing timing with the vacuum advance distributor? Unless I'm missing something, I don't see how it could be. Is there an explanation somewhere of exactly what the MCU does, and what inputs control what outputs and when?

 

[Mark_88]

I'm not familiar with the early versions of these trucks and how everything interacted but there are others on here with similar trucks (tomw) that can probably help you.

I did have a carburetor on my truck and drove it for 14 years so I did learn a bit about them in that time. There was an Idle Control Module on the passenger side fender and also an Ignition Module with two plugs on the driver side.

The MCU I'm not exactly sure of what you are referring to but interpret that as Master Control Unit. There was, apparently, a computer like device that I never really familiarized myself with due to lack of exposure to them but others say they worked like the ICM...I have no idea...

I did find this website that has a lot of information on the early Ford F100, F150, and Bronco trucks that used very similar (or the same) systems as the early Rangers...just different engines.

Check it out...maybe you will find what you are looking for if nobody else chimes in on this...

EDIT:

put in wrong link....

http://www.garysgaragemahal.com/vacuum-diagrams.html

The above link has diagrams and there are many photos on this site that show earlier equipment...

EDIT II:

Some people on here have switched to the GM HEI ignition and found it much cleaner and easier to work with...

http://www.therangerstation.com/forums/showthread.php?t=158944

In case you can't figure it out and want to replace the current system...I never went that route but looked into it a few times over the years...mostly when I had problems but I usually figured them out and got mine working again.

 

[bilbo]

Thanks for the suggestions; MCU stands for Microprocessor Control Unit. It's in a battery-ish sized box on the passenger fender that also contains the vacuum switches and a couple electric solenoid valves for other vacuum devices. I'll read up on the links you provided. Thanks again!

 

[Mark_88]

OK...that makes sense. I looked through the link for gary's garage and was about to give up when I found this section...

It is a schematic but it might help you sort out what may be the problem...there is a bit of documentation there also that explains how it works...I'm sure some members might be able to help if they are familiar with these units...they have a member post section on the main page so if you don't find the link below helpful and nobody else responds on here you might try posting a question or reading through their member posts...

Almost always find someone who asks the same questions since these trucks were very popular at one time.

http://www.garysgaragemahal.com/feedback-carb-control-mcu.html

 

[bilbo]

I actually have that book, but for the 83 Ranger! (I bought it because it was suggested on here!) On the Ranger, the 'wire' going to the bank of vacuum switches is on a thermostatic switch. What I really want to know, is what the signals do inside (and out of) the black box that is the MCU. I'll browse around a bit and maybe try posting over there.