I haven’t seen anywhere in this thread that brickdad has read out and reported any OBD codes. I believe, generally speaking, that persistent CPS sensor errors rarely go undetected by the OBD system.

If anything, Chris, it's the opposite.

A failed/failing CPS rarely sets a code and there's a good reason. It's also why so many of us can't instantly diagnose a bad CPS. In order to detect a missing rpm signal, the ECU and ignition control modules need to know the engine is actually turning over, otherwise how would it know it's missing. The only way it can do this is by having a pulsed rpm signal from the CPS. There is no other reliable sensor or mechanism for the controllers to know the difference between a bad CPS and simply not turning over at the moment. In a no-start situation you might think the controllers would know the starter motor is running and turning the crank, but there is no such signal to the controllers so they have no way of knowing.

Now as for the missing rpm signal OBD system DTCs, you now have to ask if it can't figure out a bad CPS then under what conditions do these codes get set.

For the 1-3-1 code "Ignition system RPM signal missing" on port 2: The rpm signal gets passed from the CPS to the ignition control module then to the ECU. This DTC is normally set if there is a wiring/connector fault between the ignition control module and the ECU or if there is an ignition control module failure. Both are rare. As there is minimal other handshaking between the controllers, I've always assumed this code only happens when the rpm signal suddenly goes missing while running, such as being an occasional intermittent bad/faulty signal where the engine manages to recover and continue running to distinguish that from the ignition being switched OFF. You need to realize these controllers aren't geniuses and were designed in another era where everything was mostly A+B=C.

For the 2-1-4 code "RPM sensor signal absent intermittently" on port 6, a similar story applies. It needs to know the engine is/has been turning over. This code identifies the cause of a hiccup, not a stall or no-start. Note that it says "missing". Signal interference adding a mystery pulse doesn't qualify. Interference would have to be so bad as to mask an expected pulse, begging the question how did the engine even get started in order to know it can set a code.

The above story agrees with all the experience I've ever had with bad CPS rpm sensors. I've never had an rpm sensor code in the four LH 2.4 systems I've maintained over the past 34 years.
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Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now

Dave, I see what you’re saying and it makes sense. Thanks. I guess I was thinking the system would detect the “intermittent” situation. Too bad one must swap in a new CPS to see if it was the problem.

Hi All, (Where did Brickdad go?)

Brickdad should start by checking for OBD codes before throwing parts.

He can check if EZK is handing the RPM/CPS signal over to LH with
special OBD port functions.

re: TP31361/1; 12.88 LH-Jetronic 2.4 Fuel system B230F 240 1989-..
LH2.4 Fault tracing (Pg 31)
Special tests using OBD port

TPS function
RPM signal from EZK
A/C compressor
Auto transmission
Cycles Injectors & Idle valve

If any Brickboard subscriber wants the Fuel or the companion Volvo Ignition fault tracing manual TP31397-1, give me an email address.

Are there newer Volvo manuals than these for the B230F ?

Bill

Good note, Bill.

The OBD Diagnostic Test Modes are fully described in the Engine OBD section in the FAQ here. These tests can be a very useful diagnostic aid. People should learn and remember to use them more often. They can help find a non-working device, confirm a suspect device as well as reducing the number of possibilities. They're quick and easy to run, can be done curbside, no tools, don't need work clothes, don't even get your hands dirty.

The DTM 2 sensor test can find a failed CPS causing a cold or warm no-start. It's not so good at finding a failing CPS causing a cutout stall or hiccup, such as a damaged one subject to interference and vibration when running. Among other things, it's good for confirming the adjustment and integrity of the TPS switch, making sure the ECU can actually see the click of a dirty/worn switch.

DTM 3 is also covered in the FAQ which operates a number of ECU controlled devices. The FAQ neglects to mention that it also cycles the later 2 speed electric fan (low/high) as well as the fuel vapour solenoid, so obviously taken from an earlier manual. It's good for a quick check for a bad or disconnected injector or not being able to operate the IAC valve. Also good for exercising a sticky IAC to help confirm it's worn.
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Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now

Hi Dave,

I bought my 940 two years ago and I got all of the service records
and an honest appraisal of some chronic unsolved problems. One was
the car suddenly quitting in what appears to be limp mode.
I had it occur and the car needed a little gas peddle to keep from stalling,
too much gas it would stall.
Is this limp mode where every other cylinder is shut down by EZK?
I swapped the CPS serviced the connector, repaired the AMM that had the output side screen disappear into the engine: new screen and service that connector,
and so far so good. That doesn't mean it's fixed though.
The CPS I took out was OEM and looked new. The coil checks OK with an ohmmeter.

Somewhere I thought that the tranny bell housing had a ground strap but I couldn't
find it in my new 1993 wiring book. The shield for the CPS is grounded only
at the EZK ECU. I would have thought that Volvo would secure a ground also at the bell housing. Maybe Volvo was afraid of a ground loop issue and toasting the ECU?
If so plan B would be to ground the bell housing end of the CPS harness shield with a bypass capacitor to shunt RFI.
I wonder if anyone has noticed their tach read offscale <6000 RPM
momentarily just before the engine shut down? This would be hard to catch.

What would happen if there was enough noise on the CPS signal wire that the gap where the ECU senses the crank position was filled with a spike?
Another way RFI might corrupt the CPS signal.

Do you know of more recent LH and EZK manuals ?
Probably something nixed by the Ford guys.

Bill

Hi Brian,

The most likely culprit I can think of is the crank sensor

The most likely scenario for THIS intermittent shut down but able to restart immediately after was a speedbump or undulation on the road where the engine did shake a lot

Reason: The crank sensor's wiring has a hairline break (referring not to the plastic insulation but the inside electrical wire itself)

Remedy: Replace the sensor

OBD code: May be there or not be there. Intermittent fault rarely triggers OBD code

Experiment/Test: Try driving the car over any large speedbump and symptom recur. Or a better one, take out the crank sensor (the 900 series has ample hand space for this) connect to multimeter in continuity mode and shake it

Why did I suspect that: Had experienced this before


Hope that helps,
Amarin

Hi Brian,

First thing to do is check for OBD codes.

Post back.

Bill

I ve used silver heat tape over that wire & it seems to stop that problem at least till you can get another..

Sounds like a crank position sensor to me. The o.e. sensor brand is Bougicord, it is the best and will last a very long time.

Hi Duck,

I heard many people say that they need OEM Crank Position sensors.

What's different about the OEM and aftermarket CPS?

My guess is OEM & aftermarket sensors CPS are likely the same except
that aftermarket cut corners with crappy shielding for the critical
RPM signal wires to the ECU exposing the ECU to corruption from
EMI (noise) from the ignition.

When my 940 started acting up I swapped out the CPS with an after market,
and also serviced the AMM and both connectors, and so far so good.
(Servicing connectors needs a separate thread with some discussion
about using dielectric silicone or zinc paste / Penetrox)

The RPM (CPS) and AMM signals are two key inputs to the ECU's.
I speculate that if the engine management is corrupted by bad shielding
of the CPS harness, that can be addressed easily by adding snap ferrites
or wrapping the CPS harness in aluminum foil. (info gained from practical solutions for killing TVI from nearby high power ham radio stations)

Aging filter caps are more likely to make ECU's more affected by ignition noise.
I can't pull my ECU to replace the caps now as I haven't recovered from a shoulder replacement but it's on my bucket list.
Brickboarder Amarin has a nice post about ECU service, does anyone think
ECU's are a perpetual machine?

If my 940 suddenly lapses into limp mode again, My plan is to first get any
OBD codes, then disconnect the battery for ten minutes to clear the ECU
and see if things change.

Comments?

Bill

Hi BB,

Just to add to your reply..

The keyword here is "selected". The OE are selected parts from the manufacturer (I could only guess the criteria of selection). The OEM are just (unselected) parts made by same manufacturer or panel of manufacturers. The aftermarket are parts made by non-OEM manufactures thats supposed to have same specification.

In my hi-fi hobby I used to own budget Marantz products which sounds nice with average criteria of internal component selection. Then I upgraded to Naim Audio which practices in-house component testing and matching before selection. It sounded much better with music flows out like water. Equipment prices are already prohibitive with this selection method. I've yet to own an Audio Research or Krell with their esoteric component selection with out of this world pricing. So I'd imagine for OE parts something like this is being practiced.

In case of aging ECU filter caps the deterioration is more likely gradual than sudden. A car with aging ECU should be able to start no problem but when driving is involved thats when the symptoms appear. The filter caps unable to supply the filtering needed when ECU activity becomes more demanding (all computer microchips inside starts to demand more power) ie driving. Early symptoms could appear as intermittent misfire, escalating to complete shutdown.

You'll suspect its the ECU fault when you have taken good care with all parts replacement. You'll know its the ECU fault when its replaced with a good known part.

Hope that helps,
Amarin

To add to your good reply, Bill

In my mind the symptoms are indeed consistent with CPS/rpm sensor failure. An eventual complete no start would seem a likely next stage.

Talk about which CPS sensor brands are best intrigues me at the moment. In the past I've always tried to stick with Genuine Volvo or Bougicord. But I'm now setting my sights wider. Art B. has experimented with cheap CPS sensors from China and found them surprisingly satisfactory, at least initially, but longevity remains an unknown

CPS, aka rpm sensors don't wear out in the true sense, rather they fail. The sensors actually aren't all that complicated to manufacture, a simple pickup wire coil around a metal rod in a plastic sensor head and a small gauge wire pair to deliver a millivolt pulse to the ignition controller, which also passes that signal on to the ECU. A foil shield is used in the cable to minimize/eliminate signal interference along the way, grounded at one end only for best shielding.

What the controllers need to see is enough of a voltage pulse to detect a revolution. What they don't want to see is significant signal interference to make it think there's another revolution happening. If the shielding should suddenly fail and if the engine should then die (rather just a stumble) then you might expect the engine wouldn't easily restart until something changes, which is one symptom we often see. Another common symptom is not being able to do a cold start, but being able to do a warm start. Another variation is having the engine die when rounding a corner at speed and flexing the CPS cable. Then there seems to be everything in between.

In my mind it doesn't add up that a bad CPS is mostly a damaged shield allowing interference. Damaged cable insulation is of course associated with and the common cause of a damaged shield and a less than perfect signal getting from the sensor to the controllers, but just how easy is it for interference to ruin a pulse or create a false pulse?

What I'm wondering, actually theorizing, is that in addition to an intermittent/ broken wire, that a CPS failure might be due to moisture getting in through damaged insulation and even behind the foil shield, eventually working its way down to the sensor. It may then be that it's corrosion of the sensor coil and pickup rod that can be more a problem than interference. It would take a few careful autopsies to confirm my suspicions.

Damaged insulation, often starting out as a brittle area from age and heat, will occur first at the flex points, quite often where the cable goes into the sensor. It's often difficult to spot a small split down there unless you remove the cable and flex it for inspection. Broken or missing plastic standoff clips on the water pump return line and dipstick allow the cable to flex there. If the standoffs are still in place then the main flex point is at the upper standoff. Damage there can often be spotted visually by unclipping the cable.

With all that in mind and as well the wire strands breaking from being able to flex too much at a damaged area, I believe the key to quality and longevity is the cable construction, most important being the thickness, quality and longevity of the cable insulation (casing).

So having theorized this, I bought a Facet CPS made in Italy with cable construction that looks slightly better than the Volvo/Bougicord originals. Facet is OEM for a number of automakers. It was cheaper than the Bougicord I wasn't able to find in stock along with reasonable shipping. It also includes two new standoff clips that you don't get with Bougicord, only with the Genuine Volvo (the clips themselves are now NLA from Volvo). I've now put this in one of my 940s.

At the other end of the scale, I also bought a cheap one from China at almost 1/10th the Volvo price. Of the dozens available on Aliexpress, I chose one that appeared it might have better construction than the very cheapest ones. I actually purchased it as a spare to pack, but my current plan is to try using it in my other 940 to see how long it lasts. I'll likely not be able to fully report back on this for few more years.
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Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now