Rear defrost trouble shooting

[dmgb5]

ello, ello, ello...what all this then?

Anyone here? This forum needs some life! No recent posts for a couple weeks now.

Some of you may know me from over on SC ...so here is my first problem and post for saabworld...

The defrost function on my rear windscreen has decide to go on holiday.The car in question is a '89 T16 3dr.

What I know so far...there is no visible break or damage to the heating grid, the dash switch is good, the defrost timer/relay is good, there is 12v to positive side of the heating grid, earth side of grid checks out.

Now the confusing stuff... measuring continuity through the heating grid terminal to terminal (12v wire disconnected) I see 4 ohms. Measuring continuity of heating grid with 12v wire connected I see fluctuating reading of 500-700k ohms. I have managed to check continuity on a c900 with functioning rear defrost recently and saw 1.3-1.4 ohms through the heating grid. Can anyone confirm this to be the correct resistance levels?

If 1.3 ohms is correct than 4 ohms must be obliviously to high to allow the current to pass the through grid and heat up. How can the total resistance be so high without any visible damage to the heating grid? I have seen rear defrosters function in c900's when many of the heating lines were broken. So what is going on here?

[peva]

Breaks in the heating grid are not always visible to the naked eye. The only sure way to check it out if it's proving troublesome, is to do it in parts, ie ground to the side ground element bar on glass and then live to side live element bar on glass. 0[FONT=arial]Ω[/FONT][FONT=sans-serif] in both cases apart from the resistance in your meter leads, which can often be at least an [/FONT][FONT=arial]ohm[/FONT][FONT=sans-serif]. Check this by touching the leads together and subtract from the meter reading. Then do each element separately. An open circuit then becomes very obvious. [/FONT]

[FONT=sans-serif]PS Disconnect 12v connector on element when taking readings as resistance through car wiring circuits can give misleading information, also checking resistance on a live circuit can be very misleading unless you know exactly what you are doing.[/FONT]

[dmgb5]

Paul thanks for the reply. I understand its possible for the breaks in the grid to be very very small but I have seen the rear defrost still function with multiple breaks which were visible. For the defrost to fail completely there would have to be break in all of the heating grid lines, correct? I started off by checking my Bentley and the Service manuals on SC neither were of much help for how to test the heating grid. If I understand this right...I need to start at the negative/earth terminal and track the heating grid lines for continuity, correct?

Also I am using a digital self-ranging multimeter. The only tests conducted with a live circuit were for checking for voltage at the positive terminal and checking the relay/timer. All heating grid tests were conducted with ignition off.

[peva]

Yes I would start at the earth end but disconnect the connector at the live end first, don't just work with ignition off. Disconnect live plug at the element. Check the actual car hatch frame to the earth connection for the element, then check the earth element bar on the glass at the side. After this I would work along each element. A break has got to be there somewhere.

[dmgb5]

Okay, got it. I have already checked the earth terminal against the chassis as I thought at first I might have poor earth..not the case. I will follow your instructions and report back the results.

[dmgb5]

Paul to complicate matters my rear windscreen is tinted. I am not getting any reading(0.L) tracing the grid with the multimeter probe. Negative terminal to chassis reads between .001 Ohms and .003 Ohms. Positive terminal to chassis reads 1.5xx mOhms. Okay? Bad connection between positive terminal and heating grid? I made a small incision into the tint maybe 25mm down of the positive terminal...heating grid to positive terminal reads .001 Ohms. Heating grid incision to negative terminal reads 1.5xx mOhms. ! Comments...suggestions?

[peva]

For the readings you mention your meter is not accurate enough to measure. You are getting far more than this in the meter leads and plugs. I would suggest you concentrate on continuity or not and not the actual resistance. The readings you give obviously say there is continuity. Try to find circuits with open circuit, ie probably 0.00 on your meter. Or alternatively your meter may have a diode setting where when you touch the leads together you get a beep from the meter. You could use this for continuity.

[dmgb5]

0.L indicates infinite loop/no reading, correct? This has always been my understanding.

Yes, I have beep/chime function on my multimeter to test continuity. Should I repeat tests using this function then? Wouldn't measuring the ohms be a better indicator of whats going on?

[peva]

You don't really want to know ohms, just whether you have continuity or not. The chime feature is useful if when you are using two hands and can't watch the dial. It's not worth repeating if you have used the other method to confirm continuity. You are correct, 0.0L does indicate infinity or an open circuit. That's what you are looking for. When you have found that you will have found the break in the circuit.

PS Or you could do it from the other end. Leave the 12v connected and disconnect the ground. Then work along the circuit testing for 12v until you find where it doesn't exist. Obviously you will need the ignition on and element switched on. In the past on one of my 900s I found the glass bonded ground connection at the side of the element had become detached from the glass due to too much stress from the grounding screw.

[dmgb5]

Okay...heres the update. Disconnected the heating grid fails the continuity test setting...terminal to termianl=0.L. Still disconnected, with meter set to auto range ohms the heading grid reads 1.5-1.6 mOhms. I make an identical incision into the tint near the negative terminal. Test continuity from negative terminal to incision point in heating grid. I have continuity. Repeat test again at positive terminal. Again I have continuity. So the terminal contact points are not at fault. Test terminals again...again 1.5-1.6 mOhms. As in bl**dy mega ohms. There is now way any current could past through that!

The answer/diagnosis is becoming apparent to me now but I phone up a Saab mate and ask him if he will volunteer his c900 and rear defrost to aid in my last effort for a conclusive diagnosis. So, I build some jumper wires to bridge my defrost wires to his rear windscreen. Test continuity and voltage through jumper wires...connect them up to his functioning defroster...turn on my defrost switch and wait a couple minutes. His rear windscreen defroster heats up and functions when connected through my defrost circuit. Conclusion: defrost circuit is good and defrost grid is completely done.

This is one of those moments where I imagine Ockham wagging his finger at me, telling me ...I told you so.