Speed Switch intermittent

[TheEngineer]

My car has a “speed switch”. It is part of the exhaust emission control used on 1968 and the early ’69 models. (p. 00-74) It switches off the fuel supply by actuating a solenoid installed on the fuel injection pump when the car is going downhill and your foot is off the gas pedal. A number of conditions must apply for the fuel to be switched off: The engine speed has to be over about 1100 RPM, the gear selector (on the automatic) has to be in 2, 3 or 4, and your foot has to be off the pedal, which closes a micro switch on the firewall. To test for operation of the fuel cut-off solenoid, one connects a test light between the solenoid terminal and ground. I suspected a malfunction and installed such a test lamp taped to the wipers, so that I can observe operation while driving. My speed switch operates intermittently: When it works, the car runs a little better after a long downhill run, or after deceleration on the freeway because fuel does not accumulate as much as when the solenoid does not work. I have traced the fault to the speed switch and have taken it apart: There is a relay controlled by a circuit board. The circuit board gets a signal from the minus terminal of the ignition coil: A pulsating voltage, the frequency depending on RPM or firing impulses. There is simply a throttle coil, which permits a voltage to pass when the frequency is high enough. The circuit board appears to be an amplifier that provides enough voltage to the relay coil to close the relay. But I may have that all wrong. At any rate, my switch operates some time, and some time not. I have cleaned the contacts of the relay and adjusted the stops on the relay. Help please!

[Cees Klumper]

Time for a new switch perhaps? That's what I would probably do, if they're not too expensive.

Cees ("Case") Klumper in Amsterdam
'69 white 280 SL automatic

[A Dalton]

Working off a schematic, I find:
Terminal 1 and 2 are the switch  contact section in the speed relay/switch . Pin 2 should always have fuse power , so you may want to give the fuse a spin to be assured you do not have an intermitent power feed problem first.  The second test would be to take your test lamp and hook it to pin 1 and do a test drive .. This will verify if the speed sw is faulty as you suspect or not. By testing w/lamp at fuel solinoid , the power has to go through a series circuit chain of parts , so a lamp at sol does not narrow your problem down to speed sw., as any other part in the chain can break the circuit to the fuel sol.
 The  series chain is:
 Fused power to sp/sw,to relay for trans oil pressure sw, to Nuetral safty sw., to micro sw , and finally, fuel soilnoid [ shut-off]
 May I ask how you diagnosed speed sw?

[TheEngineer]

A.Dalton: Thank you for your attention. I narrowed the malfunction down to the speed switch by doing just what you suggested: Checked for 12 volt power on pin 2 and connected another test light to pin 1 and ground. No need to drive the car, just run the engine for this test.
Cees: Yesterday the switch worked just fine all day. I'm sooo reluctant to replace the switch until I have really convinced myself that I can't fix it! (I'm so cheap)

'69 280SL,Signal Red,007537,tired engineer, West-Seattle, WA

[A Dalton]

Very Good..
 The only other suggestion I can make is to jumper 12v +  from bat to
 pin 2 to assure that you do not have an intermittent feed problem anywhere .. Watch it for a a few runs and that will assure you that it is SS if it continues to show a fail with your pin 1 monitor/test lamp....[ providing the input wire from coil to SS  also has a good connection on each end. of course..]
 If you decide to try this , I would put an in-line fuse on the B+ jumper...............

[TheEngineer]

A. Dalton: I think, I found the fault: The two wires, red & black, that provide the power to the relay coil from the circuit board were bundled together. Both have old style insulation. It almost looks and feels tike gutapercha: It's a little sticky, but it could also be old age. These two wires were stuck together and clamped together by a small brass clip which protrudes from the circuit board. It looks very neat, but I think, when the box got warm, they shorted out. Maybe not a dead short, but sufficient to deny enough power to the relay to pull in. I seperated the two wires and went for a test drive. So far, so good. Works like a charm. My test light is still there and when it comes on, I use zero fuel! Your advice about the fuse was timely: In experimenting around, I blew fuse #8 which supplies power to that circuit! Thank you! This bulletin board is fantastic.

[A Dalton]

Very good..
 If your monitor lamp shows any more power intermittents . you may want to resolder the IC board .. common to find cold solder joints/terminals on old automotive boards....

[Bob G ✝︎]

I have as prehaps you have the large speed switch. I got a used one from Ray Paul many years ago it was missing off the braket. when I tried to disasemble the switch the plastic bakealite on the switch craked and came apart. Does any one know if one from a sedan would work or are caliberated the same. I think the same switch is on the sedans 280SEL/8 280SE/8 280SE coupe & converible. I do not remember but I think new Tom Hanson quoted me $800.00
What happens when this switch is not working?
Bob Geco

[TheEngineer]

Bob: Nice to hear that others also have issues. As far as I know, the speed switch is only used on '68 and early '69 280SL & 280SE Us spec models. It's an emission thing. It cuts the fuel off completely when the car is coasting. IMHO it's not essential. Car will just run fine without the stop solenoid and europeans don't have it. Hans at H&R Fuel Injection feels that the stop solenoid is a bad thing because when it actuates, the engine quits. So you get rear ended. That would be a failure mode.

[Bob G ✝︎]

RE: Speed switch relay for my 1968 280SL. I would still like to fine a useable one formy car. So if anyone nows of a unused one sitting please let me now.
Does anyone know the part number?

thank You
Bob Geco
1968 280SL