High dwell on 911 SC ‘81 lambda (US)

[Funracer]

Originally Posted by Bob Kontak

Pretty sure the tests mysocal911 is referring to are done on the Lambda ECU connection under the passenger seat.

Doh. Thanks

[mysocal911]

Originally Posted by Bob Kontak

Pretty sure the tests mysocal911 is referring to are done on the Lambda ECU connection under the passenger seat.

Correct!

[Bob Kontak]

Originally Posted by Funracer

Doh. Thanks

You had the sense to ask. That's a win.

I can't remember how small the male/female connections are inside that big connector.

You may need to get your craft on and find a small adapter (e.g., sewing pin/small wire) that allows you to check the female side of the connection.

I have never done this but check both wiring harness side and Lambda side for ground.

What do you use to check duty cycle?

[Funracer]

Originally Posted by Bob Kontak

I have never done this but check both wiring harness side and Lambda side for ground.

What do you use to check duty cycle?

I am a novice at trying to find shorts. I have a few questions if you have a moment to help me out. These are specific to testing the OXS 14 pin connector pins 9-14.

Do I need to have the key in run or off or either battery post disconnected?
I pulled the connector under the seat and could not find pin numbers. Is there a pin diagram somewhere? Am I looking for the same voltage or resistance at each pin or can I just use a test light to test continuity?

I have two DVM I use to check DC. Also have 2 dwell meters but they are both showing a needle reading of 6% when 8 cylinder is selected which does not seem right.

Thanks

[AndrewCologne]

He refers to the big plug connected to the ECU below the passengers Seat.

[AndrewCologne]

Originally Posted by Funracer

Conclusions: From what I have read the FV is supposed to go to 65% when O2 is disconnected. This is not happening on my car unless the 15C switch is grounded.

Not sure where to go from here

No, as written, the duty cycle normally Switches to 65% when engine is below 15°c degrees.
So, Below 15°c the switch leads to ground, ... At 15°c that switch opens the circuit.

When o2 Sensor is disconnected above 15°C the ecu switches to a static 50% duty cycle, where i.E. in this state you also adjust the CO value.

I have two DVM I use to check DC. Also have 2 dwell meters but they are both showing a needle reading of 6% when 8 cylinder is selected which does not seem right.

You need a powered dwell meter means beside the green wire also coming with a black and red wire.
You should better use a DMM with duty cycle Option.

[Funracer]

Originally Posted by AndrewCologne

No, as written, the duty cycle normally Switches to 65% when engine is below 15°c degrees.
So, Below 15°c the switch leads to ground, ... At 15°c that switch opens the circuit.

When o2 Sensor is disconnected above 15°C the ecu switches to a static 50% duty cycle, where i.E. in this state you also adjust the CO value.


You need a powered dwell meter means beside the green wire also coming with a black and red wire.
You should better use a DMM with duty cycle Option.

Mine is positively not going to 65% or 50% when O2 disconnected. Nothing changes it stays at 75-85%. The only thing that will change it is if 15C switch wire is grounded to case it goes to 65%.

I will look for a powered dwell meter. They are getting a bit hard to find on eBay

Both my DVMs have HZ%.

Thanks

[Funracer]

I believe the 15C switch is working properly. It goes to 65% when grounded. It is almost never below 20C here. If it were the switch would ground and take the FV to 65% if O2 disconnected. It’s either the O2 circuit or the OXS ECU that is not doing what it should. I will be investigating further today I hope.

[AndrewCologne]

Originally Posted by Funracer

I believe the 15C switch is working properly. It goes to 65% when grounded.

Even if you live in the sunny state, remove the 15°c sensor, put it in the freezer and check with your DMM for continuity. Below 15°c it shout be closed = continuity.

And tell us the bosch part No. of your ECU on its sticker.

Mine is positively not going to 65% or 50% when O2 disconnected. Nothing changes it stays at 75-85%.

Do the tests MySocial911 recommendet, especially check the reference voltage at pin 2 of the ecu with o2 sensor unplugged, should be 0.5v.
Just remove the o2 sensor and lead the red terminal of your DMM to the inner contect of the 02 sensor socket in the left engine compartment, black to ground ... you should read 0.5v with engine running (otherwise the ecu wont get any current).

Both my DVMs have HZ%

Then use it, means switch to "%" mode and you will be able to read duty cycle easely.

[Bob Kontak]

Originally Posted by Funracer

Do I need to have the key in run or off or either battery post disconnected?
I pulled the connector under the seat and could not find pin numbers. Is there a pin diagram somewhere? Am I looking for the same voltage or resistance at each pin or can I just use a test light to test continuity?

Off. You are looking for ground. Put your multimeter on the resistance setting that makes the beep.

Touch the probes together. Does it beep? If yes you are able to now see if the ECU wiring harness or the ECU pins are talking to the chassis of the car via continuity. That's a ground. If we figure out the right pins you should be good to test.

This is the 14 pin connector off of a 911 turbo, the one by the rear bumper left side. It routes power to a bunch of stuff in the engine like in your car and does not manage The Lambda system. Just want to make it perfectly clear that the 14 pin connector is not where you want to be.



Now the 10 pin connector between the shock towers delivers power to the frequency valve through the #1 pin if I remember. That may have been OP's issue with rough running and backfires when under load but he has not been online here for a couple of months.

Focus on the ground concept and I'll see if I can scrounge up a pinout diagram for the ECU connector.

[Funracer]

I have checked the 15C switch as you said. It checks good.

[mysocal911]

Originally Posted by Funracer

These are specific to testing the OXS 14 pin connector pins 9-14.

These pins, 9 thru 14, of the OXS ECU still haven't been tested for being grounded, right?

[Funracer]

Found the pins for the OXS ECU in Bentley. They are not diagramed in order so that confused me a bit but they are numbered and the numbers match the wire colors on my plug




Got continuity readings as follows. These were done by setting the DVM to Continuity and one probe on the pin number and other probe on the wire end.




Pin 2 to O2 sensor. .2 ohms
5 GRND. .2
7 to 15C switch. .3 ohms
8 to OXS relay. .4
12 ECU to AEC and 15C switch .1
14 ECU to AEC. .1
15 ECU to FV to OXS relay. 2.8 ohms
AEC is Acceleration Enrichment Control Unit

Got .5 VDC here with fuel pump jumpered and key to run. Is this the right way to test it?

[Funracer]

Originally Posted by mysocal911

These pins, 9 thru 14, of the OXS ECU still haven't been tested for being grounded, right?

Just finished. Checked them for continuity not sure if that is the same as checking for a grounded wire.

[Funracer]

Looks like 0 280 800 055 Bosch

[Bob Kontak]

Originally Posted by Funracer

Just finished. Checked them for continuity not sure if that is the same as checking for a grounded wire.

Not the same.

Check for harness terminal to chassis continuity. Check the OXY box terminals for same (although I am not certain this will show anything if the box is grounded through a terminal in the connector - which is disconnected)

[Funracer]

Originally Posted by Bob Kontak

Not the same.

Check for harness terminal to chassis connection. Check the OXY box terminals for same.

So DVM set to continuity, one probe in each numbered female plug and other probe to chassis? Key on/off?

[Bob Kontak]

Originally Posted by Funracer

So DVM set to continuity, one probe in each numbered female plug and other probe to chassis? Key on/off?

Key off. Ground has nothing to do with power. It's the wire talking to the chassis through touching it somewhere.

Well, I think that's the resistance setting but yes. No or little resistance means continuity.

Obviously a bare metal part of chassis.

[Funracer]

I also have a test light if that makes anything easier

[Bob Kontak]

Originally Posted by Funracer

I also have a test light if that makes anything easier

Test light needs power. You are looking for a mechanical contact between the terminal and chassis. You don't need power.

You can use the negative post on the battery as the "chassis" point if you wish but then you have to run extra wires to bridge the distance.