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Schematic and repair of the Ph2 heater transistor pack

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Old Jan 9th, 2017, 17:10   #1
Umski
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Default Schematic and repair of the Ph2 heater transistor pack

Following on from Gatos' article here:

http://www.volvoforums.org.uk/showthread.php?t=205206

I've now had it out for the second time! The first was actually due to the full speed relay going caput which was easily replaced (mentioned in the thread above too). However at the time I took the main transistor out (also referred to as a resistor which it actually is in Ph1 cars I believe and/or non-ECC?) and bought a new MJ802 transistor to replace the existing one (as also described by others in the same thread). As it turns out I didn't need it but at the time I drew up the small schematic of the inner gubbings (which aren't shown on the wiring diagram) but never replaced it. This time I was stuck on high speed (via the transistor) or full speed (via the relay) so out it came again. (Apologies for the weird order of the images - it was an afterthought to document this so some photos were taken during a second disassembly)



[Electronics coming up - this is for info in case anyone else wants to dig]

As it turns out, there are two (NPN) transistors in what's known as a Darlington pair within the black box/heatsink. There is a smaller transistor (2SD1407) which the ECC drives using a PWM signal (I think but in my haste I didn't verify this) which in turn drives the larger MJ802 (this is shown on the wiring diagram). On the PCB there is also a small capacitor (1nF - orange) and a resistor (600R or 0.6k - pink). In series with the ECC line (to the Base of the small transistor) there is a thermal fuse rated at 150C and glued to the heatsink. Schematic below (numbers correspond to wiring diagram above):





It's not so simple to test a transistor in this arrangement with the transistor test on a digital multi-meter (DMM) due to the other components, so I had to pull it apart anyway. Assuming the MJ802 was dead I clipped off the leads (blue and black from the Base and Emitter respectively, the Collector is the casing). You can test a transistor with using the diode function on the DMM which reports itself as two diodes back-to-back (best do a Google search for this if you want to check how). This seemed to suggest the MJ802 was okay. On the other hand the D1407 was suggesting a short which would explain why the fan was stuck on high speed (the D1407 was keeping the MJ802 fully on at all times)

So how to replace the small transistor - easier said than done as it is enclosed within the black casing! I ended up having to carefully de-solder the main wires (green circle) for the MJ802 and then even more carefully and quickly unsolder the thermal fuse (the heat from the soldering iron could easily blow this - in red). The tab is held on with a small M3 bolt which had corroded itself on to the heatsink so had to be drilled off to free the copper tab going to the PCB and the D1407. This connection is acting as the common Collector for both transistors and may explain why it died as there was a poor contact to the larger MJ802 which controls the current going to the fan.



In the meantime I did a check for the D1407 and it seems a bit of a tricky one to get hold of in the UK. The datasheet shows it is a TO-220Fa package (insulated tab which is odd as the Collectors are shared anyway) with a Base current of 0.5A and Collector current of 5A - this seemed enough to go on. I had a dig in a bag of old bits and found a BD135 (TO-126 package but it fits!) which is much lower rated (1.5A Collector current) but I figured this should suffice as the MJ802 is taking the brunt of the current. As a stop gap until I could order a suitable replacement, I tried the BD135 (uninsulated tab but common Collector to tab) and it worked just fine without changing the MJ802.

My equivalent replacement was a TIP121 (itself a Darlington pair internally!) which is rated at Ic=5A and came from CPC (online for about 50p). I should add that the lead arrangement for all these is the same (looking front-on B, C, E) so easy to replace. Once soldered in, the black plastic cap was slid on, the screw replaced and the large leads to the MJ802 soldered, followed gingerly by the thermal fuse (I bought some 144C spares in case). The fuse was then glued back on to the heatsink with a dab of epoxy. All bolted back together and re-fitted as Gatos describes.



The reason I decided to repeat the repair with the TIP121 was to be on the safe side - as the incoming air is quite cold at the moment, the heatsink will work well and heat shouldn't be an issue. I'm not so sure on the effects in the heat of summer - the thermal fuse matches the 150C temperature of doom for the transistors but it's a bit late by this stage.

Final conclusion is that this isn't a difficult repair to do with a soldering iron and some patience (plus correct components).

The root cause, I think, was corrosion on the copper terminal which bridges to the MJ802's collector and sits in the flow of dirty ambient air coming in from outside. I cleaned up the copper terminal and the heatsink and added some thermal compound under the MJ802 and checked for continuity using the DMM before replacing it all. I still have the new MJ802!









Hope this is helpful to someone else - it's information more than a 'how-to' so use at your own risk if you're loathed to pay £50-£60 on something which has about 3 quids worth of active components in it (the heatsink is probably the most expensive bit )

P.S. This was from a Ph2 V40 1.9D - am sure it may be applicable to other models in the Volvo range too!
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Last edited by Umski; Jan 9th, 2017 at 17:12. Reason: added P.S.
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Old Jan 6th, 2021, 20:12   #2
shimon340
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Thanks Umski!

This is superb information.

In combination with the post by Gatos "How to Replace the Heater Resistor" this is all great info on accessing, repair and the design of this for the phase 2 cars. Page 5 of the "How to Replace the Heater Resistor" also has images of the unit used in the phase 1 cars and the wiring diagram from Haynes.

From the info in Haynes the fan switch in the phase 1 cars (non climate control at least but maybe includes them too) just switches the electrical path to going through all 3 resistors (setting 1), 2 resistors (setting 2), 1 resistor (setting 3) or none (setting 4). I suppose one could replace this unit if needed with a setup that offered the same resistance per fan switch setting. Or any other series of steps too of your own choosing!

TBC if a later phase 1 car with climate control uses the same resistor block as shown in the Haynes manual and on page 5 of the "How to Replace the Heater Resistor" thread. Or if it uses the one from the phase 2 cars. I'll update my posts with any further info I get as I'm looking into this at the moment.

Gatos info that "later 1999 cars to 2004" use the unit from the phase 2 cars may well tie up with Model Year 2000 of the phase 1 cars (from August 1999) and the option of climate control - which I think was only on the later phase 1 cars as well. Indeed earlier phase 1 cars had air con as an option and non standard too!

Some posts on the design and fixes for the phase 2 cars on this forum suggests the fan switch / knob is moving a potentiometer. It's that output which would be the input you refer to as:

"There is a smaller transistor (2SD1407) which the ECC drives using a PWM signal (I think but in my haste I didn't verify this)".

Potentiometer output signal is then the input to the smaller transistor here rather than a PWM? I can see it could be either option (potentiometer output or PWM) and indeed one or the other (or both system types could be the same signal) for if the car has climate control or not too.

Are the cold-hot and vent position knobs also driving potentiometers on the phase 2 heater control units with and without climate control too? Or just for those with climate control? I suppose the phase 1 models with climate control could be similar too with potentiometers rather than mechanical connection to the cold-hot and vent position (phase 1 non climate control have mechanical connections controlled by the knobs). A few different ways to make these HVAC units I suppose too!

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Old Jan 7th, 2021, 01:13   #3
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all phase 2 knobs use potentiometers. Speed and temperature are ok to replace, but the vent direction one, is a different story. You will see when you open it up
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Old Jan 7th, 2021, 02:50   #4
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Thanks.

I guess for the heat one the potentiometer controls a valve that reduces or increases the coolant flow into the heater matrix?

I'm intrigued by your comment of the potentiometer for the vent position. Any pics you can share of one dismantled etc? What makes it trickier?

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Old Jan 7th, 2021, 11:09   #5
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Quote:
Originally Posted by shimon340 View Post
Thanks Umski!
Potentiometer output signal is then the input to the smaller transistor here rather than a PWM? I can see it could be either option (potentiometer output or PWM) and indeed one or the other (or both system types could be the same signal) for if the car has climate control or not too.
Glad to hear someone has found this useful Bit of a blast from the past but my last fix still seems to be working - the only odd thing is that the speed control sometimes flip-flops from 'hi' to 'full' when on 'auto' so I can hear the relay clicking on and off - not sure what triggers this - probably a dicky ECC unit. However, related is your point about potentiometers - yes the speed is an input to the ECC but probably one of many going into a microcontroller first I imagine recalling the PCB behind the dash (made by Motorola I think) - this would also include the reading from the ambient temperature from the thermistor and vent position I guess - a bit like an ECU does...so back to the original question, I still believe that the speed is driven by a PWM signal from the control unit since it is using a transistor to control the speed rather than just dropping voltage across a resistor pack like on the earlier phase...keep us posted on your discoveries
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