Help with an 850 Volvo T5 - Electrical Gremlins Abound!

A co-worker has this '97 850 T5 he loves.  It has 112,000 miles, automatic, and has been pretty relaible...up until now.  Recently, while driving, the tach, temperature, and other  engine gauges would shut down, and the CEL would come on.  The engine would keep running, however.  After "rebooting" the car (key on/ off) the problem would go away...for some period of time.  He pulled the codes and found some Boost Solenoid error code, and had a mechanic replace the boost solenoid.  Also, the air conditioning was blowing on high unless he shut it all the way off.

After getting it back from the mechanic, the transmission only goes in W mode (locks out OD, and starts off in first) and the dash problem persists.  Notably, the speedometer IS NOT experiencing the problem.  The mechanic told him he basically needs a whole new wiring harness.  (He did refund all the money he charged the guy).  The latest codes he pulled from the CEL show the Boost Solenoid code persists, and there's another code for Automatic vehicle speed or something.  He tried swapping ECUs, no difference.

Any thoughts from The Hive?

This is the kind of thing that makes me fear, to a degree, the modern car. Just imagine the kind of headaches a 2017 s-class will create in 30 years. As for the OP, did all of these things occur at roughly the same time. Probably painfully obvious, as I'm no electrical genius, but there likely is something central going on. I had a bunch of weird stuff happen with my 01 xc70. We replaced the hvac control and all became normal. The hvac control went out when a shop jump started it. Ultimately the shop refunded all labor costs. 

Take it to someone who knows what they are doing.  "Replace the whole wiring harness" is Know-nothing Bozo speak for, " I don't know what I'm doing."

In reply to Streetwiseguy :

That's the crazy thing- the shop was some Volvo specialist, with a parking lot full of 90's Volvos he was working on.

I suggested he might start checking for faulty grounds or bad wiring or connections.  

My sister had a 99 V70, for some reason a lot of circuits were tied into each other like the ABS computer, the TCM and the TB.  There is a known issue with the TB's for the V70's of that vintage that would display some of the symptoms you describe. ABS computer issue would cause a speed sensor issue and random CEL's.  Sometimes they would display and other times they wouldn't. I'm not sure if the 97 models used that particular part though.  It's been a few years since she had that car, lovely to drive but with the electrical gremlins I wasn't sorry she decided to send it away..  I second the try another mechanic, someone who has access to the Volvo-specific diagnostic tools if possible.

OK...gonna have to expand on "TCM" and "TB".  Not familiar with those acronyms.  ;-)

Bad battery, I have seen low voltage cause all kinds of weird issues.

In reply to volvoclearinghouse :

Transmission Control Module and Throttle Body  And I second check the battery, they always seem to die when the seasons start to change in my experience

Bad or flaky ignition switch, this thing, can cause some peculiar instrument cluster issues.

Is the yellow shift arrow flashing?  I would suspect the PNP switch for the transmission issues, but that's about the most common transmission issue on those cars, I'd expect it to be one of the first things a "Volvo specialist," would check.

pimpm3 said:

Bad battery, I have seen low voltage cause all kinds of weird issues.

I just came in to say this. Check the grounds while you're at it.

I'll second a good look at the ign switch.

02Pilot said:

pimpm3 said:

Bad battery, I have seen low voltage cause all kinds of weird issues.

I just came in to say this. Check the grounds while you're at it.

 

I came here to say the same thing.  Whenever you start having odd electrical problems, always start by checking the grounds.

When I attended OEM class on electronics, the first thing the instructor would say, "Check the grounds".

Some of the Fiestas had problems.  It was found that the main ground, battery to body, was not making good connection due to paint.   On older cars it is rust or corrosion.

I have to say, all this advice is well grounded.  I'm charged about relaying it to him, and positive it will spark an interest in him performing the battery of repairs himself.  

I love troubleshooting threads and tortured puns. Thank you.