1998 V70 AWD No Start. Wiring Harness Question

Daddysard 850 · Post by **Daddysard 850** » 04 Sep 2012, 19:10

Working on a friends 1998 V70 AWD Turbo. Car had no start problems 6 months ago ECM for Motor was replaced was fine till a couple weeks ago then no start with Temp sensor and Crank position sensor codes. Also would crank and after a few revolutions Fan would start and run till it timed out even with ignition off. Another ECM was installed, ran for 30 some minute shut down for night and would not start in AM. Same codes and Fan running, that is when I became involved. Connectors were corroded with Green hard scum for Throttle position , Cam position sensor module , Fan relay and Block Temp sensor connectors were corroded and plastic insulators internally were dissolved. Crank position sensor side connector was brittle and broken.
Connectors were cleaned and connections made secure, Cam sensor replaced along with speed sensor. Wiring checked to ECM
connector and sensors. All looked good so another ECM installed with a injector test light installed. Attempted to start no start and Fan ran as before! Test light flashed twice then not again during extended crank, No Spark and fan ran till it went thru its cooling cycle.
Results till Yesterday.. Took apart ECM. Did a smell test. Found a broken or burnt wire at A18 in ECM it was (on further inspection ) burnt through, it was inside the nylon pass through portion. I did a continuity test of all the pins to the board and traced the open to a discolored pin in the nylon. Then check the other ECM's and found same pin open and burnt same way on all ECM's.
Now I have a couple questions that would be a big help in progressing. Since 3 sensors Cam,Temp and Throttle position all use this A18 as a "Signal Ground" Suspect problem is in the Harness as I think that the Ohm meter reading from A18 to frame ground with battery unhooked and ECM removed should be an open line but it reads as a short to ground. Other pins to modules using the Test procedures in the Volvo manual test close to normal. My assumption is that a high current is burning the connector from A18 to signal ground causing an open circuit. The time between burning out the pin seemed to be shorter as modules were replaced??
There is a 3 way connection in the harness where the grounds become one wire to the ECM Has anyone any idea where this is ??
I wish to isolate each line and test. If you read all this THANX and any help appreciated. Daddysard

p.s. Making a fused line from A18 input to Ground on ECM to test current.
would like to test each sensor individually.

Post by **Ozark Lee** » 04 Sep 2012, 20:03

Quite a detailed and thorough first post.

Schematically I can find no chassis ground bonding to signal ground. You correctly identified the components involved, the Throttle Position Sensor, the ECT Sensor, and the Cam Position Sensor. The schematics also show an optional signal ground to the outside air temperature sensor.

Did you disconnect the sensors themselves? Chassis ground may arise due to one of the sensors.

The schematics show the junction point at 23/8 - inside of the ECU box below the edge connectors.

...Lee

Daddysard 850 · Post by **Daddysard 850** » 04 Sep 2012, 20:32

Thanx for the reply! Yes all sensors were unplugged at measurement. Will check for optional ground to outside air temp sensor. My Diagrams do not show this optional ground but will look. Flying kinda blind as do not have a breakout box or all the factory manuals. The location of the connection really helps as taking out all the harness is not what I was looking forward to doing.

Thanx again Daddysard

Post by **BEJinFbk** » 05 Sep 2012, 00:03

I usually don't even think about giving up on a good Volvo,
but corrosion in the connections can be like a cancer.
And problems at the ECM puts it right at the brainstem...
I'd seriously consider a harness transplant unless you're
confident that the green rot has all been excised.

Until then,if you haven't already got them, most of the pinouts
can be identified in the wiring diagrams available here at MVS:
https://www.matthewsvolvosite.com/volvo-wiring-diagrams

Good Luck - I don't envy you with this one.

Daddysard 850 · Post by **Daddysard 850** » 07 Sep 2012, 19:59

Thanx BEJ:
I did find the better set of wiring diagrams at this site .Thanx for the Tip tho. Took awhile to figure out the Volvo lexicon!
Did find the temp sensor and Was NOT pblm. I am working part time on this so progress reports are not on a regular basis. I did take apart ECU "Box" and found where the four signal grounds come into one to A18. Did not take apart as I have a bunch of commitments this weekend so stopped, before I got involved with trying to isolate the offending line (Of the now 4). EX Computer CE from the 60's,so been here done this. "New er" Harness has already been discussed. Thanx again. John S.

Post by **BEJinFbk** » 07 Sep 2012, 20:36

Sounds like huge fun.

Again, no envy here.

But if any car is worth
it, it's one of these.

Best of Luck!

Daddysard 850 · Post by **Daddysard 850** » 10 Sep 2012, 21:57

Thanx BEJ:
Well some progress has been made. Repaired the ECU, ( Added a fuse into the line from A18 input pin to signal ground (same as the grounds to back plane)). Fuse was in case!! Found a "Test" 1998 same ECU numbers and with a lot of trepidation installed the"Repaired/Test " ECU into the running V70. (Drum Roll) It started and ran fine!! OK looks like will have a couple repaired/tested ECU's for sale soon.
Got some time to open and test out 4 signal grounds with all sensors disconnected and found the one to the block temp sensor was reading funny and weird resistance depending how the wire was twisted and fooled with! Stripped the covering back and found Black/Green not bright copper wire. Kinda like the old Black death wire from big dry cells that went bad. (If you are old enough to remember those BIG dry cells in "Portable Radios" .. Have re strung a new A18 wire and connected it to the Temp sensor. Ran out of time today.. Prolly will strip the other connection, the Yellow wire, (A31?(prints are at shop) to check its health although it tests OK. Looking for a test start Wednesday. If goes well then will check and replace any other connections that do not pass the Eyeball test. John S

Daddysard 850 · Post by **Daddysard 850** » 27 Sep 2012, 18:46

Sorry about delay in finishing this post. "Life is what happens when you had something else planned" The Volvo ran out of shop under its own power today! With new Oil cooler hoses and new Radiator (To stop all the !#$%$#@#$ stuff from corroding the new ground wires installed.) A new Cam sensor assy was installed as the connector would just break apart in your hand!
It was found that there was a combination of ground wires from the Engine block temp sensor and the throttle position sensor
that had "Funny" Ohm readings to something and suspected these were causing a current large enough to break (melt) the A21 ground connector in the ECU. I installed new Ground wires to the ECU connector that fed 4 grounds to A21 from both "Funny" brown ground wires. The ECU was repaired without the Test Fused (5 or 10 amp) jumper. I wired a connector from the input of A21 inside the ECU case to the other side of the board to a junction point of a larger ground plane point. This ECU was tested and ran well. I believe that the no-start was caused by loss of signal to the affected sensors and that the fan ran, as the circuit was floating at some value that was causing a high temp signal, turning on the Hi Fan.
As My instructors at Wards school of Electronics in Hartford, Connecticut drilled into us: "Do a Mechanical inspection FIRST"

Thanx for reading! Daddysard 850 John S

Post by **BEJinFbk** » 27 Sep 2012, 21:59

Kinda like the "A good mechanical connection is the basis of a good solder joint".
Or - "Step One - Always check the power supply. Step Two - Refer to Step One."

Glad to hear you've gotten the better it, sir.
Always a great feeling - Enjoy your Volvo!

Daddysard 850 · Post by **Daddysard 850** » 29 Sep 2012, 18:13

BEJ: I saw the triple ground connection on the wiring diagrams which kinda pointed to all the symptoms very early on BUT!!! Now have 2 spareECU's! Leaves just turning here! Thanx John S