Mysterious wiring problem

R converter · Post by **R converter** » 21 May 2011, 16:03

Hello once again.

I have had an ongoing problems with the "speed sensor". I have replaced the sensor three times since August, and the arrow will not go out.

I have the 1996 850 that was converted to turbo.

In August, I had re-wired the car with a harness out of a '95.

I suspected that that was part of my problem (1995 is not compatable with 96), and swapped in a complete engine harness out of a 1997 850 T-5.

My computeres are as follows: TCU out of a 1996 850 turbo (has 33.3 in big numbers), ECU is a #628 out of a 1997 850R, and has worked fine in the car until now.

My problem now is, the car will not turn over (as if the engine is sized or the immobilizer is working OT), and the tachometer bounces up and down (cluster is out of a 1996 850 turbo). Other than the pesky arrow, the car has been running great, until I swapped harnesses.

What is causing this to happen? I thought that 1996 and 1997 were compatible with each other?

Any input is welcome.

Post by **jimmy57** » 21 May 2011, 21:53

Tach jumping occurs usually for one of two reasons: 1. the wiring to the speed sensor is routed to close to a battery cable or the engine to transmission face is not clean and or not all bolts there and tight.
Is your problem normal cranking but takes very long to start or does not start at times?
The starter creates some serious magnetic waves across the flywheel that can reach the RPM sensor if the marginal suppression of the aluminum bell housing does not take care of it.
A lot of professional volvo techs can attest to having installed a dirty used or painted reman'd transmission and having this problem immediately.

R converter · Post by **R converter** » 22 May 2011, 04:55

Thanks for the resopnse.

No, The car will not even click, never mind turn over (again like the immobilizer is working OT).

I am going to re- check my connections.

Another thing I did, I am also painting the car a different color, and there was a ton of sand under the battery tray. I blew the sand out, and some got into the transaxle wiring harness.

I will re- check my work again, and update as needed.

Post by **jimmy57** » 22 May 2011, 06:04

OK, I was confused by the tach bouncing part of the symptoms as tachometer bouncing while engine is cranking over is a telling symptom of certain problems.
A US version will not have Immobilizer but does have alarm function that prevents starter operation. Is the alarm indicator red LED blinking?

R converter · Post by **R converter** » 22 May 2011, 07:51

Thanks again jimmy57

Yep, the red LED is working perfect.

I opened the harness up, and checked the connectors from the engine bay harness to the harnesses that run along the fenders, and they are a perfect match.

I also compared the wires from my old harness to the new harness, and the new harness has four fewer wires, which adds up.

My old harness had two dead wires, and two wires to a metal solenoid that was next to the intercooler inlet.

The only real difference in the harnesses are the knock sensors. My old harness had the connectors at the sensors, and the new harness has a plug on top of the harness block, with solid piece wires to knock sensors (no plug ins).

The car still won't turn over. The tach did not jump this time, but now the CEL will not illuminate at all, in any key position.

R converter · Post by **R converter** » 22 May 2011, 10:17

Ok, here's where I stand as of now.

I turned out that the green wire to the starter had popped off.

The car now turns over, but doesn't fire.

Also, the gray relay near the intercooler inlet (I think the fan relay) buzzes and vibrates.

I am going to check the fuel rail for fuel. If there is fuel getting to the rail, then the problem must spark related.

Keep throwing ideas at me.

Update:

I went back out and spent a couple more hours on the beast.

Friday the car ran mint, so I know it is not a compression problem. I opened the schrader valve and got fuel, went back in and cranked it more, re-opened the schrader valve and got more fuel. Not a fuel problem.

I disconnected the solenoid that was humming and cranked it some more. This time the transmission arrow started rapidly blinking, and the tach began doing its thing again.

I re-traced the wires. four or five wires are different (a couple are completely different ) colors. In the middle wiring block (closest to the date stamp) are a brown/yellow and a green/yellow wire that the old harness is lacking al together. But this harness is out of a 97 850 T-5 and my ECU is out of a 97 850R, so I would assume that the harness / CPUs should automatically be compatible.
I checked the distributor for moisture, and found none. The coil wire had minor moisture though. Sorry for the long post.

R converter · Post by **R converter** » 22 May 2011, 12:57

All set.

It was a combo of the loose wire and moisture in the coil.

I disconnected the battery and sprayed carb cleaner in the coil, and wiped out the moisture / gunk.