Removal of ignition retard vacuum

[scia63]

Hi Graham:

What is it that I have backwards ?

The manual does not say that the ECU cancels the timing retard above idle. Why would it not explain about the ECU involvement in the same section that deals with the 2 way valve. ( no answer needed here)

I understand that when you first open the throttle the drop in engine vac brings the vac for the retard to ZERO " Hg, but it does not stay at Zero through the entire higher RPM range as you drive,does it ?

The manual clearly states that the retard is canceled when the 2 way valve is energized when the AC is turned on or when the coolant temp is 212F or higher. I would think that the manual would also include,if any, other ways that the retard vac could/would be canceled.

Is there a section in the 117 engine manual or D-Jet manual you can refer me to, so I can come to the same understanding that you have on this minor topic ?

My inquiry and research into this topic started because I was getting very low MPG, and very low manifold vac levels.
I did put a Vacuum gauge on my car and found at idle the manifold vacuum levels readings were between 8- 10 "Hg vacuum. The timing was set at 5deg ATDC @ 750 RPM with vacuum line connected to the dizzy.

Such low readings would, therefore, effect the MPS operation and
influence the MPG.

Upon disconnecting the Vac retard and timing to 7deg BTDC with out vac, my vacuum level reading went up to 18.5" Hg and I have not seen my gas gauge taking a steep and rapid decline.

I have no vac leaks, and the engine is in fine tune.

As far as the multiple threads, I posted this topic on the forums I thought were the most relevant to the '72 450SL and the topic.

Thanks for your advice

[Tomguy]

The way I always understood the functioning is that (from psfred a while back) the vac retard only retards in 1st and 2nd gear with A/C off and below 212°F coolant temp, outside of those circumstances there is no vacuum (the "Switch" closes off the vac from the throttle body). Turning A/C on, being in 3rd gear, having the engine above 212°F, and to my knowledge, flooring it (activating the "Kickdown switch"), there is no vac applied. The purpose is to prevent pinging and lower idle unless a higher idle is needed, thereby reducing fuel consumption at idle.

Disconnecting the vacuum won't harm emissions and it won't harm the engine as long as it's not pinging under light throttle in 1st or 2nd gear on small hills. It will just make the engine idle faster. Plug the vacuum source line - not the pot on the distributor - if you do disconnect it, so unfiltered air does not go into the intake.

[scia63]

Originally Posted by 230/8

There are three engine signals that cancel the power to the retard solenoid. The throttle position sensor cancels the power when the throttle is opened past idle speed. The AC cancels the retard solenoid power when AC is switched on. The 100C engine temperature sensor cancels the retard solenoid power when engine temperature exceeds 100C. The retard solenoid only receives power to retard the spark at idle speed and only idle speed.

If you want your engine to run better, set the timing at 30 degrees advance at 3,000 rpm without the vacuum line connected. Re-connect the vacuum line and reset your idle speed to the book value. You will then have a smooth even idle and also when the AC kicks on or the temperature exceeds 100C your spark will still advance slightly to raise the idle speed and compensate for the higher AC drive loads or to lower the rising temperature. If you fail to use the retard feature then your engine will slow down and may stall at idle when the AC is turned on or it gets hot.

Hi: My '72 450SL does not have a retard solenoid, it has a 2 way valve.

Where would I find the information you posted?


Your explanation is in reverse of how my retard works.

My retard is canceled when the 2 way valve is energized.

The system you describe has constant power and cancels the retard when the power is turned off.

My retard timing is canceled, and timing advances when the AC is turned on.
So, if my ignition retard is not connected there would be no issue in the car stalling when the AC is on because the ignition advance would already be present.


Thanks

[Graham]

Originally Posted by scia63

Hi Graham:

What is it that I have backwards ?

At idle you have max vacuum - as soon as you open throttle it drops off.

The manual clearly states that the retard is canceled when the 2 way valve is energized when the AC is turned on or when the coolant temp is 212F or higher. I would think that the manual would also include,if any, other ways that the retard vac could/would be canceled.

It does - read page 14.1-050/2 second item.

I did put a Vacuum gauge on my car and found at idle the manifold vacuum levels readings were between 8- 10 "Hg vacuum. The timing was set at 5deg ATDC @ 750 RPM with vacuum line connected to the dizzy.

Upon disconnecting the Vac retard and timing to 7deg BTDC with out vac, my vacuum level reading went up to 18.5" Hg and I have not seen my gas gauge taking a steep and rapid decline.

I have no vac leaks, and the engine is in fine tune.

It sounds like you could have a leak in the retard system or you had the timing retarded when you measured the low vacuum. At retard, the vacuum IS lower.

But if YOU are HAPPY with the retard disabled why not just leave it that way??

[230/8]

Originally Posted by scia63

Hi: My '72 450SL does not have a retard solenoid, it has a 2 way valve.

Where would I find the information you posted?


Your explanation is in reverse of how my retard works.

My retard is canceled when the 2 way valve is energized.

The system you describe has constant power and cancels the retard when the power is turned off.

My retard timing is canceled, and timing advances when the AC is turned on.
So, if my ignition retard is not connected there would be no issue in the car stalling when the AC is on because the ignition advance would already be present.


Thanks

Two-way valve and solenoid are the same thing, just different terms. I use solenoid because that is what it is. Look in the manual for job #14-050 for a description of how the system works on your SL.

[Graham]

One thing I forgot to mention.

Provided your retard tubing to the switchover (solenoid) valve does not have any leaks and is connected to the proper connection on the valve, you can achieve exactly same as bypassing it by switching A/C on. (My A/C doesn't work, but I use the switch to increase idle speed when it goes low after a hot start. )

As already mentioned, the retard system is only effective at idle.

[scia63]

Originally Posted by Graham

At idle you have max vacuum - as soon as you open throttle it drops off.



It does - read page 14.1-050/2 second item.



It sounds like you could have a leak in the retard system or you had the timing retarded when you measured the low vacuum. At retard, the vacuum IS lower.

But if YOU are HAPPY with the retard disabled why not just leave it that way??

Hi Graham, Thank you for your help.

I think I'm happy with the retard disconnected but, I am trying to find out if I'm truly happy by understanding this system and the short/ long term effects if any.

What manual are you referring to when you say "read page 14.1-050/2 second item"

I have the the MB service manual for the M116 and 117 engine.

Thanks again

[Graham]

Originally Posted by scia63

Hi Graham, Thank you for your help.

I think I'm happy with the retard disconnected but, I am trying to find out if I'm truly happy by understanding this system and the short/ long term effects if any.

What manual are you referring to when you say "read page 14.1-050/2 second item"

I have the the MB service manual for the M116 and 117 engine.

Thanks again

That's the manual - Section 14

[berfinroy]

The reference is to the "Service Manual V-8 Engines M116 (3.5) and M117 (4.5)," not the more commonplace "Service Manual Passenger Cars Starting 1968 Series 108 109 111 113-- Maintenance, Tuning, Unit Replacement."

There is also a more detailed discussion of the retard system operation in the "Registration Year 1973 Passenger Cars USA Version" supplimental service manual @ P121. All agree that the retard function is only operational at idle when the AC is off and the temperature is below 212F.

[scia63]

Originally Posted by Graham

That's the manual - Section 14

Hi Graham: Just one more time if you please.

I have MB Service Manual Engines M116-117, but it does not have

section 14. So, I can not "read page 14.1-050/2 second item" .

My manual does have:

Group Index 07.6 titled "USA-Exhaust Gas Emission Control"

This is where I found the ignition retard information I have tried to understand.

Are there 2 versions of the M116-117 manuals ?

Never had this much confusion or controversy with my EType systems, or may me it's me. Nope not me.

Frank

[Graham]

Originally Posted by scia63

Are there 2 versions of the M116-117 manuals ?

Never had this much confusion or controversy with my EType systems, or may me it's me. Nope not me.

Frank

Frank - Yes it IS you!

I have no idea if there is more than one engine manual. Berfinroy provided the reference to the engine manual I have above.

I have provided an excerpt below from the W116 manual which can be found on-line.

Maybe now you will believe me

[scia63]

Originally Posted by Graham

Frank - Yes it IS you!

I have no idea if there is more than one engine manual. Berfinroy provided the reference to the engine manual I have above.

I have provided an excerpt below from the W116 manual which can be found on-line.

Maybe now you will believe me

Graham:
If this horse gets any more deader .

I believe you. I want to believe!

I already know what you attached.

The retard is ON: 1. when Ac is OFF
2. When coolant is below 212F

3. When the throttle valve is at idle stop
position. (coasting)

4. Engine is at idle (same position as #3)

In both 1 & 2 above the retard is canceled when the 2 way valve is energized.
( that is when the AC is ON or the coolant temp is 212F or more)

What happens at higher speed than idle ?

The section you attached seems to implied, but does not clearly make a statement that the Throttle position in some way has the ability to cancel and turn on the retard timing.

The only reference that is stated in the manual section you attached for the canceling of the retard is when the 2 way valve is energized.

Now, if the the retard is canceled by the throttle position at higher than idle speed GREAT!

BUT by what mechanism ????
(Is the 2 way valve energized or is there another path to cancel the retard?)

I have not been able to find any thing in print of other ways that cancels the retard other than energizing the 2 way valve.

So, My position, with this topic, has been:

As long as the 2 way valve is not energized the retard time is ON.

Again I have not seen anything in print that states otherwise.


May be it is me.... learn me

[scia63]

I hope this info is of value to others. Thanks for all your responses

This is my follow-up to my post, and what I found out by actually testing on my 1972 450SL. My conclusions are specific to the 1972 450SL ignition timing retard employed in the M117 engine, with the 3 speed automatic transmission. (Which does not have the transmission pressure switch.)

Comments, corrections, greatly appreciated.

My Conclusions:

1.Removing/eliminating the OEM ignition retard from my engine would be beneficial to improvement of the overall operating efficiency of my engine and engines with the same ignition retard system.

2.I have no data or any conclusion to support that removing this ignition retard from my engine would cause ill effects.


3.My 1972 450SL M117 engines ignition timing has a constant 12 degs of ignition retard.

4.The ignition retard is canceled (advancing the timing by 12 degs.) only when the 2 way valve is energized or when the port vacuum is disconnect from the distributor vacuum unit)

The questions that I posed are:

• What are the effects of removing the ignition timing retard ?
• Is there another mechanism working above idle that cancels the ignition timing retard other than turning on the AC or having coolant temp. above 212 F ?

Testing the 2 way valve curcuit:

According to the MB M116-117 manual the 2 way valve (aka solenoid, ignition cross-over valve) allows ported vacuum from the throttle to the distributor to retard the ignition timing retard at idle speed when:

• the coolant temp is below 212F or 100C; and
• the AC is off.

The manual, however, is silent on what happens to the ignition retard above idle speed. So the obvious question is: Does the ignition retard canceled at higher than idle speeds or does the ignition retard remain?

According to the MB M116-117 manual when the 2 way valve is energized, the port vacuum to the distributor is disconnected, canceling the ignition timing retard thereby advancing the ignition timing by 12 degs.
The 2 way valve is energized when:

1.AC is on; and/or
2.the coolant temp is at or greater than 212F or 100C

I attached a test light to the 2 way valve circuit and here is what I found:

• As expected Grounding the coolant temp switch closed the circuit and the test light came on;

(This test indicated that this circuit was intact, and that the 2 way would be energized to cancel the ignition retard.)

• As expected Turning on the AC also closed the circuit and the test light came on;

(This test also indicated that this circuit was intact, and that the 2 way would be energized to cancel the ignition retard.)

• However, Increasing engine speed to 3,000 RPMs (well above idle speed) did not close the 2 way valve circuit and therefore does not energize the 2 way valve, and does not cancel the ignition retard.

Test Conclusions:

• • The 12 degs of.Ignition timing retard is present in my ’72 450SL as long as the 2 way valve is not energized. Port vacuum is, via the 2 way valve, connected from the throttle to the distributor.

• • The ignition timing retard is canceled only when the 2 way valve is energized that is: When the AC is turned ON and/or when the coolant temp is at 212 F or above. (Energizing the 2 way valve disconnects the port vacuum from the distributor advancing the ignition timing by 12 degs.)

• • Engine speed well above idle does not energize the 2 way valve and, therefore does not cancel the ignition retard.

[Graham]

Edited:

There is something wrong with your car. Or maybe not - but you did not actually check whether retard was canceled or not, did you?

Maybe check the throttle position switch. Test procedure is in the manual, section 7.4, I believe.

In looking at the wiring diagram 14.1-150/1, I could not see any electrical method for the idle switch to disable the retard. But check TPS anyway - it is important that the idle switch works for other reasons.

As soon as you open the throttle, the vacuum is quickly reduced, so in effect, there is no vacuum retard unless you are idling. You should be able to see this with a timing light and a vacuum gauge.

There would be retard ONLY when sufficient vacuum is present - at idle and with foot off pedal coasting.

Removing retard or not is your choice.

[scia63]

This is my follow-up to removing the ignition retard timing from my 1972 450SL.
The reason for this follow-up is because I could not find the information, and answers I was seeking on this subject

It seems to me that the ignition retard serves little good purpose, of very limited utility, creates unnecessary heat for the engine as a means of supposedly burning off hydrocarbons in the exhaust pipes, provides for poor fuel economy, and holds back the engine from working as efficiently as it could.

So for the above reasons, I have chosen to disconnect the vacuum ignition retard on my car’s engine.

Let me explain: I have found that the ignition retard is cancelled not only by energizing the 2 way valve as noted in the previous posts, but also whenever the throttle plate is in the open position.

The distributor vacuum retard unit receives vacuum via the 2 way valve from the port nipple in the base of the throttle body. The vacuum produced at idle is very weak at about only 3” Hg. This vacuum, however, is enough to activate the distributor vacuum unit to retard the ignition timing 12 deg. For my car: Ignition timing with vacuum line connected is 5deg ATDC. Without vacuum the timing is 7deg BTDC.

As soon as the throttle plate is opened (stepping on the gas pedal) the vacuum from the throttle port, drops to zero. The vacuum at the throttle port stays at zero as long as the throttle plate remains open. This in effect cancels the ignition retard timing.

Without this 3”Hg vacuum the distributor’s vacuum retard unit stops functioning, and the distributor mechanical advance takes control of the engine’s ignition timing in the advance direction.

So the only time the vacuum is supplied/available to the distributor to retard the ignition timing is whenever the throttle plate is in the closed position (at idle or any other time where your foot is off the gas pedal). It seems to me that when the throttle plate is closed the resultant lean mixture needs advance timing not retard timing to burn the air/fuel charge efficiently.

I am now thinking of ways to modify my 450SL distributor with a vacuum advance unit using manifold vacuum. With such modification the distributor would then be able to respond to not only the engine's RPMs, but also be responsive to the engine's load. This I believe would make for a more efficient and responsive operation of the engine.