Hi, All - I need some Volvo Guru Advice.

I bought an 850 off a friend of mine who was moving away. It won't start, but I told him I'd get it running and maybe somebody can get some use out of it.

Here's my situation:
1994 850 Turbo
1. On-board diagnostic thing does not work. When I plug the little wire in, I get a dim LED. If I push the button, the LED is bright when I am pushing it, but that's it. It never flashes.
2. By the OBDII plug, I do get a response from the computer, but no data. I guess that is normal. It says I have a problem with the TPS circuit. I am only getting 1 volt at the TPS (should be 5).

Do you guys suppose the ECU is shot? If that is true, I need to swap out with somebody to find out. I am in Kingsport, TN

3. I have no spark. I need to hook up a tach here and see if I am actually getting a cycle from the ECU at the coil or not. I gathered from the manual that the TPS problem wouldn't cause a no-spark.

Thanks for any help. I would think somebody has seen this onboard diagnostic thing failure before.

 

Do you have a good battery? If the battery was dead it likely cleared the codes in the OBD-I.

 

Could be; the battery did run down. It never blinks at all. when I let go of the button, it does nothing. Does that seem right?

 

I'm not real sure but i know that a dead battery can erase codes. Is the current battery good?

 

Yes, the battery is good.

 

So here's a little update - I hooked up an aftermarket tach to the coil, and I don't seem to have a tach signal at the coil itself during cranking. That confirms what the Volvo tach is showing.

I would guess that there is a cam sensor feeding the ECM, and the ECM is feeding the coil. Is that corrct? Anybody have a technique for testing the cam sensor?

 

I've seen lots of stuff about cam sensors and maybe 1 that spoke about testing it. You'll have to hunt that down. Maybe google would help.

 

Ok, it looks like this thing covers it:
http://www.volvoclub.org.uk/tech/ser...sicTesting.pdf

Their order is check for spark, check for voltage at the coil, check voltage after the coil, then engine speed (crank?) sensor, then the cam sensor.

 

As a followup, I seem to have fixed this accidentally. when it recurred, I was able to find the exact cause.

The main fuel injection relay (mounted above the radiator) dropped out due to deterioration of the plug. This is a style of plug that Dave Barton sells parts for. When it unplugged itself, it created a situation where the LED would glow dimly when I hooked up diagnostic port 2, and I had no spark. It would not display trouble codes.

This relay primarily powers the fuel injectors on the always-on side, but it goes several other places as well, and feeds back to the ECM.