Hi,
My Volvo 940 is having issues starting and running. When I can get it started, it seems as if fuel isn't getting to the engine, and it ends up turning off on its own after about a minute in idle at 1000 RPM. When I give it gas, the RPM does not increase as it should and hiccups. The engine dies immediately after putting it in reverse, which I know is set to be 800 RPM. We have tried a jump start that didn't seem to make any difference. I have checked the codes and got a 2-1-4 in Module A socket 6, pointing to a faulty RPM sensor, wiring or incorrect installation. The car has over a half tank full of gas. I found a loose vacuum hose on the engine and reattached it, but this had no effect either. Any advice is greatly appreciated.

Dear veed,

Hope you're well and stay so. When the RPM (crankshaft position) sensor fails, the loss of its signal causes the Engine Control Module (ECM) to shut-down the fuel pump.

So, the first thing to do is to replace the RPM sensor. If the in-service unit is factory original, it is long past the end of its service life. I suspect you'll find that the insulation sheath has cracked or even fallen-apart. If the wires touch, or if moisture gets to the wires, that causes signal loss. The fuel pump at once shuts-down.

The Idle Air Control Valve (IACV) may also need to be cleaned. See the site's FAQs for guidance as to how to do that.

Hope this helps.

Yours faithfully,

Spook

Hi Spook,

Thank you very much for your detailed response. Your diagnosis was spot on and we found the RPM sensor cord was sheared like you suggested. We ordered another cable and plan on cleaning the RPM censor as well as the IACV.

Best,
Veed.

Dear veed,

Hope you're well. If replacement of the failed wiring harness cleared the 3-1-1 code, do not remove the RPM sensor from the transmission.

To ease removal of the failed wiring harness connector from the sensor, spray the connector's housing with PB Blaster or another penetrating oil, e.g., Kroil. Repeated saturations on a daily basis will help to separate the connector's housing from the sensor.

Pry gently on the wiring harness connector's plastic housing, to try to work it loose. At the end of each wire on the harness, there's a metal "tube" that slides over a round "pin" in the sensor's housing. A "needle-nosed" pliers will be needed to grip the "tube" to remove it. Gripping the tube with the needle-nosed pliers, try turning the tube - gently - to break any corrosion bond between the "tube" and the sensor's round "pin". Go gently!!!

Once you've removed the "tubes", clean the sensor's round pins with some Deoxit (an aerosol corrosion-remover). Then, coat the pins with di-electric grease, before installing the new wiring harness.

You might want to wrap with black or gray butyl rubber tape, the exposed wires and the connector housing. This will keep water, etc., away from the connector. Be sure to overlap the layers of tape and compress them to form a water-proof "boot". Butyl rubber does not harden with time/exposure. It remains flexible.

See, for example: https://www.homedepot.com/p/50-ft-Butyl-Sealant-Tape-Roof-Accessory-in-Gray-99415/205945574#overlay .

Hope this helps.

Yours faithfully,

Spook

Hi,

I’m a 240 man so I still live in the world of a basic car with less frills! (:)
A 900 series was built up from basic and its still a good thing that is easy to work on with patience!

When something going on like in your descriptions the engine is already getting to much air to reach the higher RPMs above 750 to 800 limits. You say you are consistently on the 1000 rpm area.
On these cars with red blocks the IAC controls the idle rpm.
You need to listen to the start up and hear the engine “race up” past the normal idling point and go right back down to idle specifications.
If it doesn’t the IAC may be bad or at least dirty.

Like Blindboy is saying, check the throttle body plate for a full closure, as it may be exceptional dirty.
Check that a throttle switch is signaling a change of state reading when the plate just barely wiggles to open. An ohmmeter helps but I don’t know what type of switch you will have on a 900 series.
It maybe an on-off type or displays a variable resistance?
This switch tells the computer to hold idle speed with the IAC.
It also tells it to change things for powering up to pull away.

When you go into reverse the load is going to increase and this is where the RPM sensor comes into play. The computer is watching that closely, if the throttle switch is in idle position mode!
It’s a tight little circle or loop.
There is a start loop, warming up loop and operating temperature loop with a sensor that is in this scheme the whole time.
If there is a problem with that it could be telling the computer that you are all up to operating temperature. If you are not then you will be running lean or like too much air.
The thing to remember is in order for the engine to breathe air has to get in from somewhere!
Any air leak behind the AMM/accordion tube combination and up to the throttle is a suspect.

You see, there is a circle and you need to determine where the loop is broken.
It starts with listening to small details and eliminating air leaks and dirty adjustments!

It’s only after this procedure can you start thinking about a failed component someplace.

All we can do is start with a check off list that I have let you in on.
If you can fill in some maintenance “condition” history we can do a better job.
Any good shop mechanic will always listen to a customer, if he has coherent details!
Lots of times the person who lives with the car routinely senses something odd and that tells him what he can rule in or out.

Phil

Hi Phil,

Your response is greatly appreciated. The issue turned out to be the RPM censor like you suggested: the cable was stripped of its insulation and was overall in bad shape. We will also be checking the throttle body plate and cleaning the IAC. Thank you as well for your explanation of how each part contributes to the overall performance of the engine - it's most definitely helpful for a beginner like me.

Best,
Veed.

Hi,

I’m glad I was helpful and you wrote a good post with enough information to get responses.
I didn’t know you were new to the BRICKBOARD.
You have been doing your share of learning in order to write pertinent clues for us to help you. Good on ya!

Thanks for you feedback to us, as that helps us know that you are serious about keeping your car going down the road. It’s nice for all enthusiasts to see their brand car still on the highways.

Keep everything going!

Phil

when you step on the pedal it gives you air. not gas

clean the throttle bodyu & make sure the air filter is good.

clean the throttle bodyu & make sure the air filter is good.