1969 280sl transmission pressure switch wiring help

[edwardburak@erols.com]

1969 280sl usa version automatic

I am having issues wiring my transmission

first, I have a wire coming down from the firewall ( green/red) this wire is from the small relay beside the coil, opposite from the larger relay (#1) in the firewall picture. 

I found that this relay does not have a ground so this (green/red wire) must be its ground, so I placed it on the small screw on the terminal block.   

Is this correct? or does it go to the drivers side pressure switch along with the green/ brown wire?

My car is supposed to have a wire connecting the passenger side pressure switch and the drivers side, correct?

I am missing that wire, is it available through MB, or should I buy a spool of cloth covered wire?

I was already recommended lowbrow customs for that wire.

and finally, I am missing the metal shield that goes over the terminal block, does anyone have that part number?

thanks for your help

[edwardburak@erols.com]

b

[edwardburak@erols.com]

b

[Pawel66]

I can help just partially.

Yes, the green-red wire is indeed providing ground from both hydraulic switches that are on the sides of the transmission. They have eyelets at their ends that go under the black thumb nuts on the switches. The wire goes from the right and left hand side switch, to the plate with connections and one wire goes from there to engine bay in harness (in some cars I have seen these wires meet at the left hand side switch and then one wire put to the harness to go to engine bay).

On your pictures I see one eyelet hanging and one of the switches not connected - it is probably the eyelet for this switch.

The harness is not available to buy, but those wires are easy to re-create.

In my car (non US) this one wire that goes to engine bay, goes (provides ground) to Constant Speed Solenoid - and CSS' job is to be activated when one of these two hydraulic switches on the sides of the gear box feel the oil pressure inside as the gear (R or D) is selected. They provide ground to CSS that is powered from fuse 5 and when CSS is activated, it pushes the linkage to increase idle to compensate for the load on the engine related to gear selection and thus preventing engine from stalling.

I am not sure, however, where this wire/ground goes in your case - you say to the relay, which suggests you have Emission Control System, that I am not good at. In the US cars with Emission Control CSS is not responsible for pushing the linkage to increase idle upon gear selection, the EC system does it. CSS may still be there, but it is used for Air Condition (same function - help engine), or it is not used at all, just stays there if a car does not have AC.

There is a cover for the connector plate on the gear box, PN A 112 257 04 90, available, ca $30. There is no cover on the connector plate for the starter/alternator (just anticipating one of your subsequent questions).

[edwardburak@erols.com]

ok, they hanging wire was before I connected it to the small screw on the terminal block. 

So, if I am understanding you correctly, my placement of the green/red wire is correct (on the small screw on the terminal block), and all I need to do is create a wire connecting the two pressure switches?

thanks for your help

[Pawel66]

NOT to the 3 screws (small, medium and large) holding the wires there. It is separate. I will look at it, do not remember how it looks like exactly.

For now:
small screw is for the kick-down switch (+)
medium is for the throttle switch (+)
large is ground

In my car there is another screw there. Let me look for some pictures of that plate.

Before I found pictures, here is the connections: https://www.sl113.org/forums/index.php?topic=31675.msg229745#msg229745

https://www.sl113.org/forums/index.php?topic=28850.msg208754#msg208754

Cannot find it now. If you do not have any other connector place (except for these 3 S M and L screws) then the wire from the right hand side switch goes over the transmission to the left hand side switch and from left hand side switch to engine bay.

[edwardburak@erols.com]

OK, I am still confused, in my harness there are two wires that go from the pressure switch and terminal block up the engine bay.

Both wires go to the small relay beside the coil.

The green/ brown wire changes to black/red, but still goes to the #5 post on the connector.

the green/ red wire runs in its own separate wire loom and goes to the #$ post on the connector.

do both of these wires go to the left pressure switch?

thanks

[WRe]

Hi,
I assume you have a 280SL with a working US Emission Control System (ECS).
If so you should have a look into this document:
https://www.sl113.org/wiki/Electrical/EmissionControlSystem?action=download&upname=EmissionControlSystem.pdf.
Both wires from the pressure switches (green/red) go to the Relay Box (via the terminal connector; from there only one still green/red) and from there to fuse no. 5 (black/red/green).
...WRe

[edwardburak@erols.com]

Yeah it has all of the emission control equipment. I will look further at that document

[edwardburak@erols.com]

In that document, the pressure switches go up to the firewall and connect to a relay with 8 post connections, mine goes to a connector with 6 posts. I have read many of the previous posts and topics but they contradict themselves. I know that the green/brown wire goes on the left pressure switch.

My cloth wire should arrive soon so that I can make a wire from the right to the left pressure switch.

I am guessing that the green/ red wire is just another ground for the relay, but I am not sure.

according to my own reasoning, the green/red wire goes to the small screw as a ground.

I may just put it there and see if the idle increases as it should when I shift gears.
thanks

[Pawel66]

If by small screw you mean the first from the left, smallest of the three - no. This small screw is one of the two that control gearbox.

You will cause short.

The red\green wire does not go from switched to that screw. It provides ground from switches to whatever increases the idle.

[edwardburak@erols.com]

Ok, so the green/ red wire goes to the left pressure switch, along with the green/ brown wire that is already there?

[Pawel66]

On your pictures I see just the 3 screws on the connector plate on the gearbox - SML (small, medium, large). I would forget them in the context of hydraulic switches wiring.

The red/green wire would go above the transmission from the right hand side switch to the left hand side switch and from the left hand side switch to the relay (in case of emission control steering the idle increase).

It is shown on Fig. 3 of the material sent by WRe, switches are objects 12 and 13 on that picture.

[edwardburak@erols.com]

I understand fig. 3 in that there is a wire that connects right and left pressure switches.

I have already made that wire using a white cloth wire.

I also see that a wire goes from the left pressure switch up to the engine bay.

based on the figure, the wire goes up to a 8 post connector.

my green/brown wire that is on the left pressure switch goes to a 6 post connector for the working current relay

does the left pressure switch need both the green/brown and the green/red wire connected to it?

As I said, in that figure, the left pressure switch only looks to be connected with a wire from the right pressure switch and the green/brown wire that goes to the working current relay.

I do not see another location for the green/red wire, unless it just needs a small ground on the transmission case

the green/red wire is not used for connecting the two pressure switches, as the other end of the wire also goes up the same working current relay

[Pawel66]

Again, I am not that good in Emission Control Systems, but what I see and read:
1. The wire that gives signal (ground) for idle control (green/red) goes from the right switch to left switch and then to 8pin box. No other wire goes form these switches anywhere.
2. On the 6 pin connector (which is the connector for the big speed relay) I see no wire going to gear box nor its switches. The only wire going somewhere there from speed relay (or its 6 pin connector) is the wire going to back up light switch. It is described in the Technical Manual as green/black (maybe your green/brown?).
3. I see no connection of working current relay with gearbox.

I think someone started wiring the engine and gave up having made some mistakes.

If I were in your shoes I would study these connections and see how they should be vs. what you see on the car. In particular I would check from which pin on relay your green/brown wire is departing and see on the picture where this wire should go. My bet is it is between speed relay and back up light switch.

Some of that is described in the Tech Manual/Engine/Emission Control. There are also numerous posts on Emission Control. I could not find for now a diagram that would clearly show components pins and colours of wires - that would be most useful.

[WRe]

Hi,
maybe I can give you an idea what happens with the oil pressure switches.
The two oil pressure switches only switch ground, one for driving forwards and one for driving backwards to increase the idling speed. This happens if the pressure is bigger than 1,6 kp/cm2.
1.) 280SL, automatic transmission, without ECS:
The line (green/red) goes from one switch to the other and from there to the constant speed solenoid which gets his 12V plus from fuse no. 5 to act.
2.) 280SL, automatic transmission, with ECS:
The line (green / red) goes from one switch to the other and from there to the relay box (8 pin), which switches the voltage on under certain conditions: Water temperature greater than 100 ° C or less than 17 ° C or ground from the oil pressure switches.
In the latter case to the speed relay (6 pins) and to the working current relay (4 pins).
...WRe

[edwardburak@erols.com]

some of the wiring may be incorrect, but my car is a 1969 model and does not have the emission control systems as found in the 70/71 model years.

I don't have either of the black plastic relays or the two-way valve.

so I do not have any 8 pin connector as found on the speed relay, since I do not have a speed relay on my car.

Some guys in other posts have said that they put the green/red wire onto the left pressure switch.

the green/ brown wire also has continuity to the green wire at the CSS.

I have found that the green/ red wire has continuity with the grounds (negative battery terminal, engine block, ground strap above the belk housing)
 

[Pawel66]

Ok, if you have CSS supported idle, then in "normally wired cars":
1. CSS gets ground from green/red wire connected to both switches that provide ground when either forward or reverse gear is selected. The wire goes from the right switch to the left switch and from the left switch to CSS. This wire should not connect to ground unless gear is selected when engine is running or unless one of the switches is shot.
2. CSS gets power constantly after switching on ignition via black/red/green wire from fuse no.3

There should be no connection between switches on gearbox and the Small Medium and Large screws connectors.

[edwardburak@erols.com]

Ok, I am learning more and more.

This is how I can best describe my CSS:

the black/red on the CSS has continuity with fuse #3
it also has continuity with the green/red by the transmission.

the black/red on the CSS and the green/red by the transmission are both all time grounds. ( they both have continuity with the engine block and major ground cables.
I do not know if this is faulty, or correct.

the green/ brown has continuity with green on CSS

maybe one or both of my pressure switches are malfunctioning and therefore bringing a constant ground and tampering with my readings.

thank you for your help

[Pawel66]

Please note you cannot measure continuity with CSS connected. Disconnect both wires from CSS, then measure continuity. CSS, as many coils has very small resistance. Probes or bulbs will always show continuity.

[ctaylor738]

You have the early US emissions control that shuts the pump off when the car is coasting and the throttle is closed.  There is a solenoid on the back of the injection pump that is activated by the relay next to the control unit.  The relay is activated by the control unit.  The pressure switches tell the control unit that the car is moving, needed because otherwise the pump would get shut down when the car is  idling at a stop. 

You should have a switch on the throttle linkage on the firewall that tells the control unit that the throttle is closed.

Most cars that I have seen have had the power to the control unit disconnected because as the system ages it produces a surge as the car comes to a stop and the control unit opens the solenoid.