Clutch switch odditys

[720ADDICT]

Have a 86 720 that once in a while decides the clutch needs to be pushed in slightly to run.

It is at totally random times and the truck will start in gear.

Backed the switch out so the pedal no longer activates but am looking for answers.

I have searched and looked at the service manual for clues and have concluded that there is a relay somewhere for this circuit and the switch on the clutch pedal.
Apparently there is also a neutral switch on the transmission somewhere.
I can not find any detailed information on this feature but can say that this trucks wiring system is pretty much in good shape.

 

Which relay handles the clutch switch and how would I go about restoring that circuit?

[datzenmike]

The clutch switch is part of the fuel shut off feature. The fuel shut off is governed by a vacuum switch that detects high manifold vacuum, an indication the engine is decelerating, and interrupts power to the idle cut solenoid. This idle fuel is sucked into the engine even though not needed and just goes to waste. Obviously you don't want the engine to stall if the engine is in neutral or when the clutch is in use down shifting so there are switches that close to provide an alternate power path around the vacuum switch and to the idle cut solenoid. 

 

I would say that the vacuum switch that sends power to the solenoid to provide idling is acting up. Normally it should be closed and allow power through except when high vacuum is present. If it randomly stops working then closing the clutch switch would allow he engine to start and idle. Placing in neutral should also work. 

 

There are 2 vacuum switches, black plastic, smaller than a tennis ball, each with two wires to them and a vacuum hose, on the right (or passenger side) inner fender. One is for the fuel shut off the other turns the second set of spark plugs off under heavy load. Sorry I don't know which is which.

[720ADDICT]

This truck is an odd ball and a fraud.
I bought it for way too much and it reads 59000 miles.
The sensor light is still shining and the door chimes were still working untill I cut the cord.

The engine that was swapped into it when I bought it had well over 150k and I would guess over 250k looking at the score marks on the cylinders.

The tell tale sign on the engine was the yellow tint to the radiator and a slight gummy buildup.
It told me head gasket and I was right once I got it torn down.

I knew it was a fraud but bought it any way.

Out here in the middle of nowhere a 720 is a rare find and I had a good engine to put in it so I took a chance.

It had lots of original parts like starter and alternator and the original shocks until I put the new KYB's in.
I saved the intake manifold with hitachi and all the smog goodies but have since swapped my spare z24 stripped down with weber 32/36 into it ans other than the clutch switch it is running great.

It is quite possible that the truck really has only 59000 miles on it but they swapped in another engine and sold it to a sucker like me.

The trans is tight.

 

So I gather that the clutch switch circuit is run with those vacum switches and somehow controlled or monitored by the ecu?

My searching thru the service manuals and google mentioned that one of those relays on the passenger side may be a clutch switch relay.

Just to clarify the clutch switch system didn't function even with all the smog stuff that was in tact when I got it.

 

If I unplug the ecu will that turn off all the remaining sensors like the clutch switch?

 

Thanks for your input.

 

 

[720ADDICT]

https://dragoonarts.com/tannie-may-2019/

[datzenmike]

 

 They are vacuum switches not relays and I don't see the hoses in your picture anyway. ECU only controls the ECC (electronically controlled carb) that was on it.  Seeing as you have a Weber that's all out the window anyway.

[720ADDICT]

The switches you speak of are there just not showing in the pictures.

In the service manual it shows a relay involved with the clutch switch but I haven't found reference to the switch yet.
Its probably there and I haven't found it.

Not sure if it is related but I did notice that when the clutch switch issue happened the gas gauge  would change as well.

 

Wondering what the best method for disabling the clutch switch feature is.

 

I have read that keeping the trucks systems in tact and running the way they were designed is the best way to go and I might give it a shot on my 83 4x4 project.
I can't help but wonder if disabling the egr on my 85 720 contributed to its eventual demise at 257k with a bad valve.
 

 

 

[datzenmike]

 So what happens? It quits and won't restart or it quits but restarts, but won't idle??? 

[720ADDICT]

It stalls out when running.
I accidentally discovered that if I hold the clutch pedal down a bit it starts running again.
It happens at completely random times and other than that the truck is running like a champ.

Idles perfect and is getting a bit over 22 mpg on the highway doing between 65 and 75.
Yesterday I was on a long drive and ended up pulling over to back that switch all the way out so the pedal wouldn't trip it.

 

May be instead of figuring out how to disable it I will hook some vacuum back up to those switches you mentioned and see if I can get it to function like it was designed to.
I have never seen a 720 yet that that clutch switch actually worked.
May be somewhere in that service manual it goes into further detail about it but I haven't come across it yet.

 

I think its funny that all these years I have been disconnecting things like smog devices and now I am heading in the other direction and hooking them back up.

Might even put the egr back on and see if I can get it working.
If it actually cools the head that should be a good thing to do.