MV idles at 3400rpm.....?

[spanishfly]

Been living with a fast idle on my MV engined Dehler when warm whilst I tackled mechanical improvements and worked on an interior.

Having done most of the other jobs, suspension / brakes etc turned to the high idle when warm (1200).

Checked for leaks with brake cleaner spray none found.

 I suspected the  ISV cigar tube which didn't seem to buzz much but cleaned and you can see it energising as the relay(s) in the black box clicks. Tried a spare that behaves the same.

One of the two wires (white) was bare under the rubber cover so replaced the socket properly. 

Next suspected the tail light black box. An electrically  knowledgeable friend replaced the power transistor which looked a bit burnt and checked through as many components as possible all now seemingly within spec.

New improved ISCU control box refitted but now idles at 3000+ rpm rising towards 4000!!!!!

There must be a more fundamental problem. Wiring short in the loom???  Will check temp2 next.  Any other
ideas help please? 

 
 

[silverbullet]

Check all rubber hoses for splits, vacuum for servo too . Might be a coincidence that you have disturbed something.

[spanishfly]

Thanks, I did try and check for vacuum leaks. Just changed the temp 2 to rule it out no different. Found another ISCU black box tried that and bingo 850rpm. 
I can see the transistor has been well fitted and no other discolouration or signs of distress on the board. Is it possible the transistor (which is the correct one) is the wrong way round? That might be a dumb question . 

Or as I don't profess to understand the workings of the black box is there another component down stream that would contribute to making the motor think it needs to run flat out......? 

I haven't had a chance to let it get warm yet so quite possibly still over revving when warm........ 

[Aidan]

the idle control unit only manages the engine when the switch on the bottom of the throttle body tells it to, so check this is switching correctly, then it takes all the inputs from the temp 1 and temp 2, afm, and hall sender and by managing the fuelling and the timing and the air bypass valve tries to keep the idle at the set level. If you  have power steering or air con then it also gets a signal from them and will run more revs.
Leaks on the intake side might cause poor running, but unlikely to run at such high revs, check contacts for the idle control unit as they do suffer from corrosion and check the vacuum pipe going to the fuel pressure regulator, if this isn't working then the fuel pressure will not be dropping at idle and can cause over fuelling, the spigot the pipe is connected to on the plenum chamber can be blocked by oil residues, a small screwdriver can be used to ream it out if it is blocked, and check the blanked off spigot on the throttle body hasn't got a split in the  rubber blanking cover as well

[spanishfly]

Will do, thanks Aidan.
 

[Mr Bean]

the idle control unit only manages the engine when the switch on the bottom of the throttle body tells it to, so check this is switching correctly, then it takes all the inputs from the temp 1 and temp 2, afm, and hall sender and by managing the fuelling and the timing and the air bypass valve tries to keep the idle at the set level. If you  have power steering or air con then it also gets a signal from them and will run more revs.
Leaks on the intake side might cause poor running, but unlikely to run at such high revs, check contacts for the idle control unit as they do suffer from corrosion and check the vacuum pipe going to the fuel pressure regulator, if this isn't working then the fuel pressure will not be dropping at idle and can cause over fuelling, the spigot the pipe is connected to on the plenum chamber can be blocked by oil residues, a small screwdriver can be used to ream it out if it is blocked, and check the blanked off spigot on the throttle body hasn't got a split in the  rubber blanking cover as well

Oh my gosh Aidan! I had only thought that the idle speed controller took input from the rev counter and perhaps one temp sensor. As you may remember I have been sucked into the dark world of WBX engine management on and off since I acquired my van in 2006. Although or because of, spreading my attention over a very wide range of both work and hobby related technology, I have been happy enough to just get the (damned thing  ) going again without having a really deep understanding of it. 

Incidentally I have recently had to replace an idle speed controller and was intrigued to discover a functional similarity with a project I have been working on for some time now. On this project I have to control a "H" bridge DC motor controller with a PWM signal via an Arduino micro. Not unlike the idle speed functionality. In the event I have now manged to eliminate the need for the Arduino in my project largely driven by my very limited software skills.

However it has become clear to me that that the functionality of the Idle speed controller could possibly be achieved by an Arduino or perhaps a Raspberry Pie. and a cheap Chinese (possibly not for long ) "H" bridge controller. Do wish I had got a grip on computer programming  but...