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1984 500SEL EuroSpec w/ an AMG steering wheel : ).
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Discussion Starter #1
I have a relay just before my Jetronic computer box (located at the passanger side footwell). I was wondering what this relay is for. My car has been idling and running just fine, but my emissions are far too lean.

If I were to replace it... what part# relay should I purchase?
 

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1984 500SEL EuroSpec w/ an AMG steering wheel : ).
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Discussion Starter #2
ALSO: I have a 3 wire 02 Sensor (mustang version), but no catalytic converter. I've been reading that the EuroSpec engines didn't have emission control... I haven't read too far into this, but I am wondering if that relay going to the Jetronic system is leading to the 02 sensor. I feel that this was a later added modification to my system.
 

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What does this relay look like?
 

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1984 500SEL EuroSpec w/ an AMG steering wheel : ).
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Discussion Starter #4
It is black, and is about 1"x1"x1"... I will take a pic later and post. Hopefully we can figure this out. I don't think my jetronic computer is working.
 

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It is black, and is about 1"x1"x1"... I will take a pic later and post. Hopefully we can figure this out. I don't think my jetronic computer is working.
does it have a knob with numbers/letter on it?
 

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Hm...that's not what I was thinking of that I have on my euro, which is a knob that you can dial to the right number/setting to match the EZL in the engine bay. Not sure what yours is for!?
 

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Never seen one of those before.. I'll wager that was an aftermarket addition.

Jonathan
 

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1984 500SEL EuroSpec w/ an AMG steering wheel : ).
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Discussion Starter #9
I have two Jetronic units that I have tried. I am noticing nothing with the Jetronic unit on or off. I actually drove my car without my Jetronic box plugged in and noticed no difference at all... is this normal?
 

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1985 380SE; 1984 500SEC
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Does your car have a frequency valve, and if so, is it buzzing?

If your system is K-jet with lambda control, the lambda control (jetronic) unit richens and leans the mixture via the frequency valve. The fuel distributor depends on the action of the frequency valve to maintain the internal pressure differentials that enable fuel to flow to the injectors. The frequency valve needs to buzz regardless of whether you're in closed-loop or open-loop operation (i.e. whether the oxygen sensor is providing the jetronic unit with actionable intelligence).

A non-buzzing frequency valve is usually indicative of a defective OVP relay, as power to the jetronic unit comes through there. However, there are a couple of grounds for the jetronic unit that could be bad. On my U.S.-spec 500 SEC, someone had wired one of those grounds into the cold-start circuit, where it did nothing. Once connected to the engine-lifting plate at the thermostat, the jetronic unit came back to life, the frequency valve started buzzing, and proper mixture control became possible.

Hope some of that helps!
 

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1984 500SEL EuroSpec w/ an AMG steering wheel : ).
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Discussion Starter #11
Does your car have a frequency valve, and if so, is it buzzing?

If your system is K-jet with lambda control, the lambda control (jetronic) unit richens and leans the mixture via the frequency valve. The fuel distributor depends on the action of the frequency valve to maintain the internal pressure differentials that enable fuel to flow to the injectors. The frequency valve needs to buzz regardless of whether you're in closed-loop or open-loop operation (i.e. whether the oxygen sensor is providing the jetronic unit with actionable intelligence).

A non-buzzing frequency valve is usually indicative of a defective OVP relay, as power to the jetronic unit comes through there. However, there are a couple of grounds for the jetronic unit that could be bad. On my U.S.-spec 500 SEC, someone had wired one of those grounds into the cold-start circuit, where it did nothing. Once connected to the engine-lifting plate at the thermostat, the jetronic unit came back to life, the frequency valve started buzzing, and proper mixture control became possible.

Hope some of that helps!
This actually all makes since now. I have been running super rich and haven't been able to figure out why. I replaced the fuel distibutor, oxygen sensor, and the Jetronic computer. I still haven't had any change in how rich I am running. After further investigation last night, I realized the Jetronic computer is not even engaging. The OVP relay you speak of, is this the the relay with the red fuse on top in the fuse box area under the hood?
 

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On the OVP location - yes.

Once you get your frequency valve buzzing, you can test the lambda control system quite easily. Take the car for a drive to get the oxygen sensor hot. (And I mean HOT. They cool off surprisingly quickly.) If you have a diagnostic socket, the voltage across pins 2 and 3 shows you what the frequency valve is being told to do by the jetronic unit. If you disconnect your O2 sensor temporarily, the voltage across those pins will correspond to a default 50/50 on-off ratio. (It'll be somewhere in the region of 12.2V.) With O2 reconnected, you want to see your voltage cycling around that point. Adjust your mixture accordingly. If it's running well down into the 11's, the system is trying to richen the mixture (you're running lean); if it's in the high 12's or even over 13, you're too rich and the system is trying to lean the mixture out. If you see a steady 12.2, either the O2 sensor isn't hot enough to produce a voltage, or your jetronic unit isn't functioning correctly.

As a further test, create a massive vacuum leak (lean condition) by pulling off the vacuum line at the very back of the intake manifold that feeds the central locking/climate control/economy gauge. Watch the voltage drop as the system tries to compensate by richening the mixture. Alternatively, spray some carb cleaner in the intake and watch the voltage rise. The sound of the frequency valve will change as you run it through these tests.

Without a functioning frequency valve, your mixture would have been horribly lean as the pressure drop within the fuel distributor wouldn't be enough to promote fuel flow. Richening the base mixture would be the only way to compensate for this and get the car to run half-way decently. With lambda control back on, you may now have to make some significant adjustments in the lean direction.
 
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