I have been working on a 1984 240 Wagon (144K miles), manual trans, problem for days. I have searched and read at least 50 posts on the board but none have shed much light on the problem so I thought I'd try posting.

I'm fairly knowledgeable with Volvo repair since I've owned and worked on them for 20 years or so but only as a backyard mechanic. Anyway, here's what the symptoms are.

The car starts and idles smoothly but under initial acceleration in 1st gear, it cuts out once and almost completely dies throwing the surprised passengers forward into the dashboard until I let my foot off of the accelerator. The car returns to idle and from then on if I am easy on the throttle, it will go. After the initial cut out and then pick up, it seems to run fairly well. Check valve in the fuel pump maybe? But there's more...

When I rev the engine from inside the engine compartment, the engine bogs down and knocks also. It also suffers from major knocking when going up any incline as well. So this tells me that maybe the timing advance isn't correct or the knock sensor is malfunctioning.

So, I connected a timing light and set the timing to 12 degrees BTDC. When I increase the throttle the timing advances only a degree or two and when I open the throttle all the way up to about 3000 rpm the timing is still right around 15-20 degrees. It should be advancing to 52 degrees at open throttle but it doesn't advance past 15 plus degrees.

The following checks and or repairs were made after discovering that the timing wasn't advancing:

Swapped out control unit with known working unit from another car with same number 1346105.

Checked vacuum to control unit: okay.

Disconnected vacuum line from control unit while engine at 3000 rpm. Timing did not alter. So signs point to faulty distributor.

Swapped out the distributor for a known working one: no change.

Replaced coolant sensor, temp sensor and knock sensor: no change.

Checked throttle switch clicks immediately when throttle is opened.

So now the problem is pointing to the wiring harness. Checked continuity between each of the 3 wires that go from control unit to distributor: all good.

Checked control unit blue wire has voltage: good.

Checked ground to control unit: good.

Checked timing pulley hasn't slipped around crank.

Checked all vacuum lines and wiring harness connectors and all fuses.

At this point I'm stumped. The factory green books say that the wiring harness should be replaced if a control unit is changed. Perhaps the pins in the control unit aren't making contact with the wiring harness connector but I wiggled the connector while the car was running and no sign of a bad connection. I also verified that pins 3,5,9 have the extra sleeves that are required for the connection to the distributor wires.

Unless I am missing something, the distributor must not be getting a signal from the control unit to advance the timing, but if the wires from the control unit have continuity and the control unit and distributor all work fine in another car, then what would be the next thing to test?

The greenbooks give a troubleshooting chart which points to replacing the wiring harness, but why do this if the distributor wires have continuity?

Help... anybody have any ideas?