1997 S90 power fluctuating at cruise

ML11

Hi...at least once a week the red arrow warning light comes on telling me there is a gearbox problem. It normally goes away after 1 or 2 short drives, but now it is also doing small "surges" at speed - as if the computer is trying to keep the car on a set speed. But Cruise control is not on.
Has anyone had a similar issue, and which computer is the one likely to be at fault? (In the past my 740 had issues and I sent the computer away to have all the solder tracks "reflowed" - worked like a charm!!
Thanks in advance for your time and attention.
Cheers,
Michael

ramrod

How does it drive otherwise?

How is the responsiveness?

Power output?

Idle is still smooth?

Fuel consumption?

What works have been carried out most recently?

ML11

I love driving the car...it makes me smile _ usually 🙂
Drives beautifully, responsiveness is good, as is the power. When it normally happens, if it wasn't for the warning light, I wouldn't notice a difference - until this surging started last drive.
Idle is smooth and economy ok.
Most recent works is a new radiator, thermostat, battery and alternator.
I still have the fan coming on problem as per my previous posts .
As an ex-computer guy I know electronics can go really weird - especially after 26 years!
Most of the connectors hanging on the tranny have been cleaned and sealed, so one less problem.

ramrod

Have you got a video of the problem happening?

ML11

ramrod
No....and even if I did, it wouldn't show anything, as it is a felt experience, in other words, my butt can feel the minute acceleration/deceleration as the car is hunting for a speed (like the cruise control).
Oh, and sometimes the speedo goes to zero and then comes back on without any apparent reason.

ramrod

Interesting problem

Where abouts are you from?

ML11

ramrod
Thanks ramrod....I'm on the Mid North Coast NSW.

jamesinc

Does the speedo issue happen at the same time as the surging? Could be an issue with the ABS system.
A '97 S90 has some computer diagnostic capabilities. A VAG-COM cable (it's a USB adapter for OBD-II) and a copy of Vol-FCR will give you more data to work with.

This is the warning light you are describing? Location may be different in your cluster.

ML11

jamesinc Hi there...thanks for looking....and yes - that's the arrow. I'll pay more attention if the speedo/surging are at the same time.....I have a vague recollection that it is - what are you suspecting??

nugget_940

Flakey speed sensor connector on the rear diff?

ML11

nugget_940 Thanks for looking.....
Worth a look - I recall that same sensor on my 740 gave me a bit of grief with the speedo..... so maybe you are on to something.

Major Ledfoot

Does the arrow flash? Or does it come on and stay on, then disappear?
If it's flashing, the PNP switch on your AW30-40LE or AW30-43LE may be feeling a bit ordinary.
Speaking of ordinary, if the connection to the PNP switch isn't good, you can get problems too.
Also, these transmissions have an RPM sensor in them for the shift computer.
You will need to check for codes.
In the 960/S-V90, the OBD connection is a little odd - e.g. although the cruise control does have a test point at the OBD connector, the test point for it isn't on the same bus used for the engine (and IIRC the transmission).
Diagram below is something I cooked up for attaching to the little box with LED and pushbutton I made for checking codes on my 960's cruise control.

ML11

Major Ledfoot
Hi and thanks....
I'm holding off a bit because I need to check the next time it happens to answer your first questions.
I am in the kindergarten of mechanics so I will pass on your comments to a fellow who DOES understand all about the codes 🙂)

Ghettobird

Check the codes before anything else.

It may have a failing lockup torque converter which would explain the surging feeling without cruise control on - the TCM can detect excess slip of the torque converter and can throw the transmission light which can cause other strange behaviour as the transmission is in "emergency" mode.

My modified a340e in my 97 S90 would exhibit such behaviour after modifying the torque converter for a higher stall. If the transmission tried to engage the lockup while cruising at a greater than usuall inclination, it would occasionally carry on as explained above.

Codes codes codes!

ML11

Ghettobird
Thank you...will look into it !

ML11

Ok....update time.
I mentioned all your suggestions to my mechanic who worked his way through them...
OBD code - P500 Speed Sensor A.
Have had this previously and he blasted air onto the front wheel sensors and it behaved itself - for a while.
Asked him to clean whatever connectors he stumbled upon - PNP one was clean, but the rear diff sensor was gunky.
Yes, the dash arrow is flashing, as are the lights on the S & E switches.
Once the OBD was cleared he started the car and the main warning light came on immediately - but no other. I drove around doing my usual rural shopping, and once again the up arrow came on and some hesitation in getting out of first gear (which was new). I am starting to wonder if the problem is heat-related, as it often happens when I am about to drive the 50klms home - in other words, after a lot of stop/start shopping.
I took the car back to him before going home, he blasted air on the front wheels and car behaved itself on the run home. (Note-I do live on a 3klm dirt road....hmmmmm)
So, wise ones.....that is the current state. I will keep blasting air if/when the light comes on, and maybe it's all happening because of dust??? Or is there more than 1 issue at play here??
Thanks for looking/reading/thinking !!

Major Ledfoot

[1] Did the person clean the PNP switch, or the PNP harness connector?

[2] With the AW30-40LE or -43LE, there's a speed sensor on the transmission. The ABS speed sensors at the wheels aren't connected to the transmission shift system. The RPM sensor (as it's called) has its own connector on the trans.

[3] If the car wants to start in 2nd, the trans is in limp home mode. You want to avoid driving it in that mode.

[4] Seems to me like the PNP switch itself is cactus.

[5] Try the following - with ignition on but without the car running, rapidly move the auto trans shifter from 'P' to 'L' (or '1') back and forth, about a dozen times. Turn off the ignition, then start the car. If you're lucky, the arrow will stop flashing and so will your mode switch lights; if unlucky, I'd put money on betting your PNP switch has failed.

How far from Berry's or another reliable Volvo specialist are you?

ML11

1) Don't know but will ask
2) Aaah, so dusty front sensors is the braking system....hmmmm
3) Amen
4) Seems like that's what's left...
5) Will do when I next see the pesky thing on...
6) Near Port Macquarie, NSW. But I know a good place I can call in Qld.
7) Thank you for your analysis !!