a/c compressor cycles and radiator fan not running

[240tim]

hello,

i have an auto '93 850 with 130k. i am having problems with the radiator fan. it was working fine and then i replaced the oil trap. after that, it has not worked correctly.

symptoms:
if i turn on the a/c, the fan does not run at low speed. i hear the compressor seems to 'click' on for about a second and then 'click' off. it waits another second and then repeats the same cycle. it doesn't sound normal to me. this started the same time that the fan stopped working (after i put things back together).

also, if i run the engine until the temp gauge reaches normal temp and the top radiator pipe is warm, the fan does not run at high speed. if i then turn the engine off and disconnect the ect, the fan will run for a few minutes at high speed.

fan relay check:
i checked the fan relay is ok by connecting each of the center pins directly to the battery ground, one at a time. the fan runs fine at low and high speeds when i drive the relay manually this way.

ect check:
I measured the ect resistance. warm, it measures ~380 ohms (~2800 ohms cold). this seems to be in spec. i replaced it anyway and still no change.

i disconnected the ect and measuring the voltage coming from the ecu. it is a exactly 0.0000 v. it doesn't fluctuate like a floating connection. from what i have read, i believe this is supposed to be 5 v - ???

wire check:
i pulled the ecu's and checked the paths to the ect and the relay - all 4 wires look fine.

ecu:
i tried to 'wake up' the ecu (other forum advice) by disconnecting the fan relay center connector (to ecu), fan relay power connector and the ect connector, starting the engine and then connecting them while the engine was running. apparently, this can 'wake up' the ecu.

dubious procedures:
when i was initially reconnecting the battery, i had the key at position ii (this supposedly stops the air bag light from coming on). i had both positive and negative terminals disconnected and don't remember which one i connected first. i do remember slight arcing at the terminal when i made the second connection (everything was obviously live). this may or may not be relevent.

does anyone have any ideas about where to go from here?


many thanks in advance and regards,
tim

[Ozark Lee]

Welcome to MVS,

For starters it sounds like your A/C charge is low. A can of R-134A will likely fix it for a while. The blower circuit is triggered, via the ECU, by the A/C high pressure switch. If the A/C system is not getting up to operating high side pressure the fan will not turn on. The fan runs surprisingly little when the A/C is not on. It rarely comes on at all unless you are stuck in a traffic jam.

To test the fan relay itself ground the pins, one at a time, associated with the yellow and yellow/white wires on the center connector of the fan relay. Off the top of my head I can't remember which pin is associated with which fan speed but one should be low speed and the other high speed.

...Lee

[240tim]

lee,

thanks for the reply. i did test the fan+relay directly to the battery - both low and high speed work.

the a/c did not behave like this before i did the work. since the ecu drives the fan, i guess the a/c high pressure sensor probably is attached to the ecu. since the ecu is not responding to the ect _or_ the a/c high pressure sensor do you think i have fried one of the ecu's? (my '93 has the two-part ecu system).


thanks,
tim

[Ozark Lee]

My bet is that if you charge the A/C you will find that the fan works fine.

...Lee

[240tim]

i was about to add r134a to the a/c when i discovered that if i start the engine and disconnect the ect then the fan comes on continuously at high speed. also, if i then turn the a/c on the compressor runs continuously (at 40 psi, low side). this is something i did not know previously.

the temperature gauge also goes to zero and the 'check engine' light comes on when the ect is disconnected.

if i stop the engine, connect the ect and start up again, the fan does not run and the a/c compressor does its trick of cycling on and off. the pressure ranges from 20 to 40 psi when compressor is stopping and starting.

while the a/c may be low on refridgerant, i don't see how that is causing the compressor to cycle when the ect is connected but work fine when it is disconnected. this is the newest/most complex car i have ever attempted to work on. i don't understand what is going on. am i making this problem more complex than it is?


tim

[Ozark Lee]

Interesting,

The fan running continuously when the ECT is disconnected is totally normal, its effect on the A/C compressor is not normal.

Looking at the schematics it appears that you have a problem with one of your ECU modules. Do you have an electronic climate control or a manual climate control?

The power to the compressor is routed through both the ignition module and fuel module with the electronic unit but it is only routed through the ignition module with the manual climate unit.

If you are running 40 PSI on the low side with the compressor running then your charge is just fine.

...Lee

[240tim]

lee,

this is a '93 850 glt n/a. i think the climate control is electronic - the fan has an 'auto' setting and the temperature controls have degrees marked around them.

it has the two part ecu system - lh3.2 jetronic ecu and ez129k ignition ecu. i removed them to check continuity from the ect and fan to the ecu's. the fan is connected to slot 1 (ignition ecu) and the ect is connected to slot 2 (jetronic ecu). all 4 wires looked good, electrically.

i also rechecked the voltage to the ect. previously, i thought it had been 0 v, but tonight i disconnected the ect and the wires to the ecu had the expected 5 v on them.

i only noticed these two problems (cooling fan not running and a/c compressor cycling). i'm not saying something else hasn't escaped my notice.


tim

[Ozark Lee]

Tim,

I don't know it you have done this yet but snag a copy of the factory service manual from here:

https://www.matthewsvolvosite.com/downlo ... vo_850.zip

Once you unzip it see the file named system wiring diagrams.pdf and focus on the climate control schematics at the very top of the page. It sounds like you are competent with a VOM and it will help you track down which module it is that is giving you trouble.

If you need more detailed - albeit more cumbersome - schematics I have a link to a real slow site in Poland where you can download them.

...Lee

[240tim]

lee,

even if i probe around the ecu's i am not proficient enough to start diagnose the ecu's themselves.

thanks for the manual. the best doc i found was 'test with codes'. from that i deduced the layout in the attached pic.

i was hoping the ecu's would have some useful self diagnostics. they seem pretty basic. the best i found was that i can put the fi-ecu into test mode 2. this should flash 1-1-4 when the ign-ecu raises the 'fan on' signal and 1-3-4 when the a/c compressor starts. i will try tomorrow, but i suspect i will get both those codes to show when i disconnect the ect and turn the a/c on.

i should also be able to put the ign-ecu into test mode 3 and watch it drive the fan at low and high speed. but i already know it can do that so i think the ign-ecu is fine. my suspicion is with the fi-ecu, but i don't know how to prove it.

where can i find an ecu expert?


tim

[240tim]

the ign-ecu test in test mode 3 all seem to be fine. no surprises there.

i couldn't get the fi-ecu to show any codes in test mode 2. i got the ecu into the test mode (light flashes rapidly) but am a little unsure how to trigger the 'fan on' and 'a/c compressor on' conditions. the a/c and fan don't run when i have the key in position ii. is it safe to run the engine while i have the ecu in diagnostic mode? i can easily force both triggers if i can do that.


tim