1998 V70 Engine compartment wiring harness removal?

kjhiggins · Post by **kjhiggins** » 21 Jul 2014, 01:47

I've been having an ongoing problem with my V70 (287k miles) frying ECUs. It's been four in the last 2 1/2 weeks, even after an independent Volvo shop took a stab and replaced the B+ cable to the fuse box. I'm at the point now where I'd like to gain access to/remove the engine compartment wiring harness and I'm wondering if there are any tutorials on the subject? I've done many searches with various terms for the harness but I've come up empty.

I should mention that I found a corroded ground behind the airbox on the strut tower and replaced that along with a bad engine coolant temperature sensor today. I also did some more checking for shorts but didn't find anything apparent when I checked the ECU socket terminals. So I put in another salvaged ECU, started the car and let it idle for 1 1/2 hours before taking it around my condo complex's loop drive for 1.1 miles. I then ventured a bit farther, going up the hill to a school about 1/2 mile away and returned home with no problem. I turned it off and back on, let it idle another hour and then took it for a more extended drive, including freeway at 70 mph. As I was nearing home and driving down a gentle incline the car died again. It made it 11 miles and 3 1/2 hours this time, as opposed to 6.2 miles and 15 minutes and 2.2 miles and 45 minutes (mostly idling) the last two times I thought it had been fixed.

So I don't know if it's something that's slowly killing the ECU or a loose wire that just happens to make contact and shorts at random times after I start driving. The car shows no signs of running poorly right before it dies and there are no OBD-II codes being stored.

Thanks!

Post by **Ozark Lee** » 21 Jul 2014, 18:28

There is no connector for the engine wiring harness like there is on many domestic cars. It splits off to numerous places and goes through holes in the firewall on both the left and the right hand side of the car. Replacing the entire wiring harness is a daunting task and the better way to go about locating the problem area is to split the loom and inspect the harness for damage. It is still a PITA but it beats trying to replace and add new connectors to each wire.

...Lee

Post by **erikv11** » 21 Jul 2014, 20:34

kjhiggins, I sent you a PM.

Post by **jimmy57** » 21 Jul 2014, 21:16

The ECM grounds are under the RH headlight under the black fiberglass/plastic core support. If you use a mirror and a 10 mm boxed end wrench you can tighten them.

Tell us more.
What happens?
DO you have any idea what circuit is affected?

kjhiggins · Post by **kjhiggins** » 22 Jul 2014, 11:48

Thanks - I was able to finally see it and disconnect it for cleaning. It looks a bit corroded where the wires are crimped in the terminal but not overly bad. I'm in the process of pulling most of the wiring harness up from where it goes just above the crossmember so I can clean and inspect it. The plastic tray is filled with grime from what I can see but I've got to disconnect a few more things before I can pull it up far enough to do anything with it. I've read about opening up the tray, cleaning the wires, and then separating them and spraying them with wire lacquer or some other insulating material in case any of the insulation has worn away due to the abrasive nature of the dirt and grime.

The issue manifests itself with my car simply dying while I'm driving it - no sputtering, no indication it's about to die. After it dies, there is no spark from the coil until I put in a different ECU (all the usual suspects were checked before changing the ECU) but I don't know what circuit is affected other than the coil signal circuit from the ECU (presumably, since there is a no-spark condition.) I've tried comparing resistance readings between ECU pins on one of my bad ECUs and a good one but nothing stood out as different. I've opened up two of them but there is no obvious physical damage like a burned or overheated area.

Post by **jimmy57** » 22 Jul 2014, 12:30

Has the coil been changed?

Does it have injector pulses when it has no spark?

The power stage is driven by a 3.5 +/- volt + pulse that lasts 20 milliseconds. The pulse when seen on a scope will have a ramp on its upper flank from charging a capacitor. The pulse will start at 3 v and rise to 3.5 v during the 20 milliseconds.

kjhiggins · Post by **kjhiggins** » 23 Jul 2014, 09:57

Yes, the coil was changed and this has happened with both the old and the new coil. I didn't check for injector pulses - I don't have a scope but presumably the shop I took it to, where they checked 'everything' does and did check but I have no confirmation of that. We're not exactly on speaking terms since their total lack of concern that the car died 6.2 miles after leaving their shop so I don't think I can check with them to see what all they did during their two hours of diagnostic work.

Jbw99v70t5 · Post by **Jbw99v70t5** » 04 Feb 2020, 12:44

You might be having voltage spikes from your
alternator

Jbw99v70t5 · Post by **Jbw99v70t5** » 04 Feb 2020, 12:45

Sorry voltage regulator

Post by **abscate** » 05 Feb 2020, 04:51

Old thread alert, from 2014

Interesting problem from OP though.

Don’t check resistances on an ECU with a voltmeter. You are putting current into it when you do this, and ungrounded , this might not be healthy.