PorKen

AKA, what the heck does diagnostic connector pin 'A' do?

This really bugged me, because I never saw any mention of it in the WSM or the bulletins I have, just the idle setting procedure, and using a gas analyser to adjust the MAF for CO. While researching LH2.2/EZF stuff, I stumbled across (this Volvo site). The Volvo and 928 LH ECUs are quite similar, so I had to test it out for myself.

MAFs vary in output, so to get the mixture in the range where the O2 loop can keep the cats happy, LH 2.4 adjusts itself (and remembers the offset), but 2.2 has to be set manually, so there's a circuit built into it that shows the operation of the loop.

The MAF screw is a variable resistor on a separate circuit out of the MAF (pin 6). It doesn't affect the MAF voltage directly.


Diagnostic connector
Round connector at front of engine w/cover.

_C_
B_A


* | LH |EZF| ?
A | 22 | --- | oxygen sensor pulses
B | 12 | 17 | high load throttle switch
C | 25 | 12 | ground (under airbox)


Idle speed adjustment
Engine warmed up, A/C off.

B->C jumpered - turns off LH idle control, EZF timing goes to ~6° advance

Adjust throttle body screw with 7mm socket for ~680RPM.

The method below doesn't work - see post #40 for the working method!

CO adjustment
With B->C jumpered, adjust MAF screw until the LED blinks on and off rapidly. Adjustments may take a moment to show.

The factory tool is a 3mm allen, on a universal joint to reach the MAF screw. Since I only had a short allen, I had to R&R the MAF, over, and over again to get to the right reading.

CW = richen
CCW = lean

LED off = lean
LED on = rich

Connect the plus/anode wire to the jumper post, and the negative/cathode to the 'A' terminal.




Late added notes
- A '12V' LED can be used w/o a resistor. (12V LEDs have an internal resistor.)
- The WOT switch can be tested using a '5V' LED between B & C. (B is 5V until the WOT switch is closed.)

PorKen

The adjustment window is quite small, roughly within 5 ohms where it will keep a steady beat. With B->C connected, you have to be right on the sweet spot, then it rewards you with a quick constant flash. Otherwise, it's either on/rich, or off/lean.

The MAF pot goes from 0 to 1k ohms. Measure at the MAF, pins 4 and 6. My rebuilt MAF was at 380 ohms. I got a steady flash at 120 ohms with my slightly mod'ed intake, including Ford 4-hole injectors. YMMV.



With B->C disconnected, the idle flash is lazy. Each on and off are roughly 2 seconds long, although the idle correction will throw it off occasionally, either way.

I had to raise the idle as I leaned it out. The idle is smoother, now, and it doesn't drop out like it did before, when switching quickly from R to D, for example. My L/100K (mileage) gauge shows that I'm using less gas now too. I reckon it has been too rich for the 02 loop to work.

According to that Volvo page, the LED setting may be a touch lean, compared to using a gas analyser. If I get time, I'll check it out on my friends machine.

John Speake

Very interesting Ken ! I think you mean the ilde pot is 3mm hex.....

Mike Frye

Ken that's cool stuff. Where's the CO test point?


Edit: Sorry, didn't see the part where you specified it's the 'A' terminal.

Rick Carter

Ken,
I thought this affected idle only?

Big Dave

Cool! I don't know what any of it means....but it's cool!

FlyingDog

Good work Ken! Now how do we use the blinking LED on open loop cars?

leperboy

Thanks, Ken. I've suspected my car is running rich but don't have a CO sniffer and didn't want to take it in just for that. Now I can check it out after a quick trip to Radio Shack.

Very nice.

Matt

PorKen

Fixed: 4mm->3mm allen.

IIRC, it's most effective at idle, but it does affect the cruise map, but less and less as the load increases, and that it doesn't affect the WOT map, much, if at all.

heinrich

wtf did you just say??

PorKen

Ask your mechanic.


...


Trivia: the diag connector is a old 911/914 relay socket!



It'd be cool to find an old round relay, take out the guts, put in a switch, LED, and a wire with a clamp for the jump post.

FlyingDog

I would think the effect would increase again at WOT because the LH switches to open loop. On Euro/ROW cars which are always open loop, the MAF adjustment effects mixture at all loads and rpms.

PorKen

Effect is a noun, affect is the verb (I looked it up).




On the dyno I was overrich at the top end. It could very well have been the rich MAF adjustment. Which means I should be making ~320 RWHP, now!


...

Tangent: with my '81, closing the bypass on the AFM changed the A/F ratio by ~½ point at the top end.


...


Mike - I'll edit the pic.

PorKen

BTW, it's quite sensitive to the air intake setup. While adjusting, have the air filter, and top installed.

Without the lower part of the box, IE just the MAF, without the bellmouth to smooth the airflow, the airflow becomes turbulent, and the mixture gets screwy.

I don't have the screens installed. That may change the adjustment, too.

John Speake

The effect of idle mixture when at WOT (open loop) decreases as the rpm rise (kind of square law).

The WOT map enrichment will not be affected by idle mixture pot settings.. Remember that the WOT map enrichment only adds an additonal enrichment to the values in the base map at WOT. Assuming that the values in the WOT map are larger than zero....