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940 Mass airflow sensor-- how to test?

jambo

Member
Joined
May 15, 2005
I go 12 to 30 miles without a hitch, then usually at a stop, the rpm's rev down precipitously-- once, twice, three times, and so on-- then a little ways down the road, it cuts out entirely. Once last week it would not restart. Then I opened the filter box-- looked clean-- unhooked the MAF connector and it started right away. Reconnected, and everything is back to normal. Reading around a bit, thought it might be the sensor needed cleaning. So I did that last night, and today it ran like a champ until off the highway it starts cutting out again. Got to my destination by pulling over, pulling the sensor off its mount, remounting, then, usually, it runs. Usually everything looks clear but once there was white-grey smoke in the hose when I pulled it off.

I'm just about to push the send button for a FCP Euro rebuilt model for a buck fifty (they're close and I need it soon) and it occurs to me that it should be testable. I've got five wires: right to left; brown, blue and green, blue and red, blue and white, blue and yellow, and a voltmeter.

Can anyone tell me it's just gonna be the sensor and I shouldn't stay up half the night trying to do the diagnostics?
 
What car?

The first test on a MAF is to check functionality with it plugged in and compare to it disconnected. It should run significantly better with the AMM plugged in.
Second test is to pull the AMM and check to make sure the platinum wire is intact.

If it's a 740 have you checked the timing sender? LH2.2 has it mounted up in the distributor off the back of the cylinder head where it gets baked every time the engine cools off. LH 2.4 has a sensor off the flywheel that fails as well. That will cause immediate tach drop to zero.
240 has a similar system for LH2.2, but the distributor is in a much better location thermally and it doesn't fail nearly as often.

Bosch seems to feel that outputs qualify as a state secret and so doesn't give any data in any of the books I have seen (Volvo greenbook, Bosch Fuel Injection book).
 
It's a 94 940. Runs tons better with the sensor plugged in. All wires seem intact. I stared with a headlamp while cleaning it from both ends. Nothing is fried or loose-looking.

Timing sender? Could this be why the engine has to run for a while before problems kick in?
 
It's a 94 940. Runs tons better with the sensor plugged in. All wires seem intact. I stared with a headlamp while cleaning it from both ends. Nothing is fried or loose-looking.

Timing sender? Could this be why the engine has to run for a while before problems kick in?

Could be. The 940 should be LH2.4 or something else - and 2.4 has the timing sensor mounted down on the flywheel. When the engine starts to cut out you can glance at your tach - if it falls immediately to zero and bounces back then I'd look there or at the ignition power stage (provided the '94 still has this) - it's a small black module on the fender that has a transistor switch to drive the coil. These can start to go wonky as well.
A fuel issue will usually have a more gradual change in the tach reading.

I had a rough issue/loss of power at times that turned out to be the distributor cap, so that might be worth looking at as well. The good news is that LH AMMs seem to be pretty durable.

While you're under the hood do check the thermostat in the air box door, as they can stick on hot and are one of the big reasons for dead air mass meters.
 
Btw. It's not a turbo. Thanks saq will check distributor and thermo at the air box door.

Does anyone know which wires to plug into with a voltmeter to test how the mass air sensor is/is not behaving as the throttle is opened or closed?
 
It's not MAF.

Probably ignition module, as mentioned above. They tend to either instantly fail, or they would work just fine while cold and then start throwing the car all over the place until it dies. No-restart is a sign too. Then, when they cool off, they again pretend to be fine, and so the cycle repeat until it fails permanently.

Try a used one first...

And yup, '94 940 with a MAF, either TIC or N/A, have LH2.4 and ignition module, crank sensor on the flywheel, etc.
 
new developments-- distributor?

Before trying a (hopefully used) ignition module or crankshaft sensor I pulled off the distributor cap (for the first time; had the 940 two years and always had bigger problems than cap and rotor-- okay, maybe I was unaggressive . . . and trusted that the guys who replaced the tranny and water pump looked at it at some point) and found all four leads were badly worn from the rotor, which is pitted and badly worn, as well. Also there is a lot of black powder/granules that don't seem oily. Maybe carbon, maybe metal that wore away?

So I replace the cap and rotor. The car starts great, then seven miles down the road, it sputters a few times, and dies. Will not come close to starting again. After the tow, after everything cools, it just whines, no sign of a spark.

So I pull off the new cap and rotor, notice they are scored-- pretty lightly, but in the same place as the old cap was-- and figure my distributor is too wobbly for a new cap and rotor.

Clean off the old one, scrape the leads and rotor, put it back on, and it runs.

Does the evidence point to needing a new distributor only, or are the ignition module and impulse sensor still in play as problematic ?????????????

I'm going to try running it half an hour or so to see if a warm engine shows any re-ignition problems :roll:
 
runs good

Just ran if for an hour-- in the driveway only-- taking it up to 3100 rpms every once in a while.

Is this the same, as far as the impulse sensor and ignition module go-- the same as road miles?

Wonder if the only problem ever was a wobbly distributor, so cleaning the cap and rotor with a blade and sand paper every so often will keep it going.

At the moment, I'm not feeling fine about taking it out of the driveway without a AAA member close by . . .
 
I believe (I could be wrong) that the 94 model 940 B230F also came with the Rex-I ignition system along with the Motronic EZK system.....either or.

The cars with Rex ignition would have the Regina Fuel injection system.
The cars with the Motronic ignition systems would have the Bosch LH 2.4 Fuel injection system.

What does your ignition coil look like? Is it a smooth cyndrical coil about 2 1/2" diameter and 5" long? Or is it a module?


Bosch Mass Air Meter...6 wires from meter.
http://www.ebay.com/itm/Mass-Air-Fl...ash=item4af5191ad2:g:FMsAAOSwGWNUUvx6&vxp=mtr

Rex Air Flow Meter.....3 wires from meter.
http://www.ebay.com/itm/91-92-93-94...ash=item235c4d7219:g:KtAAAOSwhcJWN3no&vxp=mtr
 
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I believe (I could be wrong) that the 94 model 940 B230F also came with the Rex-I ignition system along with the Motronic EZK system.....either or.

What does your ignition coil look like? Is it a smooth cyndrical coil about 2 1/2" diameter and 5" long? Or is it a module?

This. If you have a square looming coil, you have Rex/Regina which means you don't have a MAF .What you would have is an intake air temp sensor .
 
It's not MAF.

Probably ignition module, as mentioned above. They tend to either instantly fail, or they would work just fine while cold and then start throwing the car all over the place until it dies. No-restart is a sign too. Then, when they cool off, they again pretend to be fine, and so the cycle repeat until it fails permanently.

Try a used one first...

And yup, '94 940 with a MAF, either TIC or N/A, have LH2.4 and ignition module, crank sensor on the flywheel, etc.

In spite of the original subject heading of this thread, it appears that the MAF is not the culprit for the symptoms the car shows. A fresh ignition module is next. The MAF looks exactly like the FCP euro pic for a '94 940.
 
When it's acting up what does the tach do? You should get some bounce on crank if it's just fuel, no bounce indicates an issue with ignition (usually sensor or power stage). A scope or meter should pick up the power stage input and differentiate between that and (likely the crank sensor, but could be the EZ-K unit if you're unlucky).

Note that this is Bosch injection - haven't had the misfortune of a Regina box (though the Bosch + no-turbo means that I have the curse of the awful 4HP22 tranny, though it hasn't failed yet).
 
The signal from the EZK box to the powerstage is a 5v square wave signal. Labscope needed to see it. Multimeter just "averages" out the signal. Might work. Might say 2.5v! I might test this to see what I can get, but I expect ZERO reading with an average multimeter.
 
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