Newb needs help with idle on m35 a2!!!

[georgia_deuce]

Hello,

I am a newbie and have a problem with the idle on my 1970 m35 a2. I adjusted the idle and got it to sit on 800rpm, however after my compressor turns off my rpms shoot to 1500rpm. I just replaced the brake airpack (had the problem before) as well as a blown air hose entering the pack. My thoughts were possibly a pressure release valve that is not operating properly and causing the compressor to kick off. However, I don't even know if the compressor is supposed to remain on or kick off as it does. I also can't figure out how to tighten the belt running from the crank to the compressor. I would greatly appreciate any advice you could give.

Thanks,

Georgia_Deuce

 

[TheBuggyman]

I cannot help with the idle issues but I do know two things:

The compressor will load and unload. (cycle) The pulley will always be turning.

You adjust the compressor belt tension by loosening the outer pulley's pinch bolt and screwing in the outer half of the pulley while holding the inner half stationary. This closes the inner dimension where the belt rides and in effect increases the diameter of the pulley since the sheaves are tapered.

 

[DDoyle]

How long does it take the compressor to build the air system to pressure - and what pressue does it reach? Does the engine jump to 1500 RPM and stay there, or does it drop back down?

Regards,
David Doyle

 

[georgia_deuce]

It only takes about a minute, it reaches and sustains 120psi (which is the limit of the gauge, the pressure may be higher) and it does stay at 1500rpm

 

[westfolk]

TheBuggyman has for belt tightening. I like to tighten a little then roll the pulley a little, then tighten a little and roll it a little and so on until I get the tension I want.

 

[georgia_deuce]

can anyone at least tell me whether the air compressor is supposed to pump at all times or does the pulley spin free when the set psi is reached?

 

[DDoyle]

I think that you have a problem with the setting of the air governor on the firewall. I believe it is asking the compressor to build an excessive amount of air - and thus straining the engine. I know that this defies logic - but a check will be easy. Disconnect - or very much loosen - the air line coming from the compressor, start the truck. If the idle goes to 1500 - which I think it will, try to set it back down to the normal range. Then, set out to find the find the problem, 'cause when you hook the air line back up, it will kill the engine.

My .02,
David Doyle

 

[georgia_deuce]

Thanks for the advice. However the engine is running well, and the release valve (on the firewall) is working. It is when the compressor reaches max psi that the engine races. My thoughts (please remember i am a newb) are that perhaps the compressor has a clutch mechanism that is engaging and allowing the attached pulley to free wheel. Because either the clutch mechanism is broken or the pressure release valve is not adjusted correctly. Of course this is assuming that the compressor even has a clutch, which I don't know..

 

[jasonjc]

No the compressor does not have a clutch. When it reaches the set pressure it unloads its like having the intake and exshust vales open at the same time the compressor is still pumping it just isn't compressing. Does this make sense.I think what daivd is saying is that the compressor is working so hard to get the pressure that it is pulling the eng rpm down. So when the compressor unloads the eng races. I hope this made sense.

 

[georgia_deuce]

Thank you very much Jason. Yes this makes sense, and I will adjust my air system as per your and Davids' advice first thing in the morning. This was my first day as a member, and I must say I will go nowhere else for MV advice. You guys are great.

Thanks again!
Cadman Kiker

 

[DDoyle]

No clutch on the air compressor....it runs, and pumps air, the whole time. The air governor on the firewall is essentially a relief valve - when the pressure reaches X pounds, the air is exhausted to the atmosphere. The setting on this valve determines how high the pressue climbs before the compressor's output is exhausted to the atmosphere - under no load. The fact that it cycles is nice - but we have no way to know if it is cycling properly - yet have several reasons to believe it is not. Among them:

"blown" air hose
pegs the pressure gauge
And of course the original issue - increase in RPM when the compressor unloads.

Before you adjusted the idle - would the truck run, and idle - or did you have to crack the throttle to keep it turning?

If you are running up a hill in a truck - you expect to give it more throttle to overcome the load placed on it by the hill. If you knock the truck out of gear with the same throttle setting, the RPMs will increase, because the load has been removed. Under my theory, improper setting of the air governor (or another restriction in the air system prior to the governor) is causing an excessive load on the engine - which you compensated for while setting the idle by in essence opening the throttle beyond the norm for idle. When the air governor FINALLY reaches its setting, it unloads to atmosphere, removing the load to the engine. Then, just as in the going uphill and knocking it out of gear example - the engine RPMs increase, because the load has been removed.

Obviously, its your truck, and you can wrench on it at your pleasure - but loosening - or disconneting the air supply line takes only seconds, and if my prediction is wrong, this would certainly eliminate the air system as the source of the problem.

Best wishes,
David Doyle

 

[georgia_deuce]

Thank you for your advise, and I fully intend to follow it. However, just for my own knowledge, how can the air system cause enough load on the engine to drop the rpms? Once again, I am conceding to your experience and advice, I just don't know what function of the system has that substainal of an effect on the engine.

 

[DDoyle]

As I said in my initial post - it defies logic - but we really don't know how much pressure the system is developing - because it pegs the gauge. Working within the design parameters, the compressor probably only pulls about 5-10 horsepower off of the engine - but we don't know that it is any longer working as designed. Is it pumping up to 125 lbs - or 500 lbs? The fact remains however that, according to your initial post, when the air system unloads, the engine RPM comes up. Inherently it seems, the air system, while in the compress mode, must be bogging the engine down. I've been wrong before, but the positive diagnosis of this is so simple - disconnect the air line at the compressor - that it can be quickly confirmed or eliminated.

The safety factor on most hoses is 4X the rated maximum pressure - I don't know what the rated max pressure of the air side of the brake system is - but we do know its at least 120 lbs. You said the truck had a burst hose - which puts us at 500 psi. Are the windshield wipers wiping demons? Or, does opening that valve fully blow the hoses off of the wiper motors (maybe not, the smaller diameter a line, the more resistant to bursting it is)?

An air compressor building excessive pressure will get hot - sometimes very hot - is there any discoloration of the paint on the head of the compressor?

Regards,
David

 

[cranetruck]

FWIW, item 6 in the image below is a safety relief valve and the pressure should not get much over 120 psi.
I have had this valve stuck open from ice and pressure never reched operational level.

 

[ida34]

Remember that anything attached to the front of the engine crankshaft by a belt is taking power away from the engine. This is why a lot of race cars have electric fans for the radiator instead of running a fan off the water pump. It is entirely possible that any belt driven accessory ( fan, water pump, air pump, alternator) can cause enough of a load to bog the engine down. The engine power is multiplied through the transmission, transfer case, and the drive axles. The air compressor seems like a small load compared to driving around a 14,000 pound truck but the compressor's load does not give the engine the mechanical advantage that the drive train has. I concur with David's assessment based on what you have presented to us in the forum. It may be something else but I would check this first.

 

[georgia_deuce]

Well, I've got news. I changed the compressor belt, adjusted the air pressure regulator, and even bled all the air out of the system while it was idling at 1500 rpm. I thought the pedal may be sticking, so ran it from the throttle on the engine with the pedal dissconected and the problem persists. Afterwards I dissassimbled the idle adjuster and everything is clean and nothing is broken. Now I am leaning towards some sort of fuel problem, any help anyone could provide with this problem would be greatly appreciated. Thanks, Cadman Kiker

 

[doghead]

Why don't you take the compressor belt off and run the truck. Then at idle let the clutch out with the trans in gear, just enough to load the engine a little bit then push the clutch back in.. See if this causes the same thing to happen. If it does then this would eliminate the air system as the problem. Then it would seem that it is a fuel system problem. btw,have you changed the fuel filters lately?

 

[Recovry4x4]

Heres a stupid question, is the return spring OK?

 

[TacticalTruck]

I think you guys are barking up the wrong tree on the compressor. Even if the compressor were locked up it would smoke the v-belt before it loaded down the engine. Also the air governor works by unloading the compressor, not venting to atmosphere, that's what the small diameter copper line running from the governor to the compressor is for. I don't think there is anything wrong with his air system. Many run at 120psi which is at the top of the gage scale.
i think he has a sticking governor in the injection pump or he has thrown it out of adjustment by adjusting the idle. How did you adjust the idle speed? I can't remember the procedure now but I do remember reading not to adjust it by the throttle stop at the arm where the linkage hooks to the pump. I would pull the engine stop cover and check for proper movement of the governer linkage under there.
Jeff

 

[georgia_deuce]

Yes, I checked the return spring. As well as the governer linkage. I am now leaning towards a fuel issue.