I have checked the ground point, and there is 12 volts passing through the circuit. Despite the apparent integrity of the system electrically, there does not seem to be any heat generated in the window to be useful. Could the grid itself be defective in some manner, or is it possible that there is simply not enough current being supplied to cause heating. Does anyone know if there is a relay or other device that might be limiting current to the rear window defroster?

According to the '91 940SE wiring diagram, there should be ground at the black wire and 12v at the yellow wire where they connect to the heating grid. This is with the ignition switch in the RUN or ACC position and the defogger switch pressed once to ON. Power is from fuse 13 which also powers the cigar lighter. Does the lighter work? The circuit diagram shows the timer relay is integral with the switch. If the 12v comes on at the defogger grid and goes off about 15 minutes later, that would seem to mean the timer relay is good, unless it has a corroded connection.

With no power on, try measuring the resistance of the grid from the yellow wire to the black wire. The grid on my '93 945 measures 1.3 ohms. If you get a very high resistance either the grid connections are bad or maybe a scratch across the grid wires has broken the current path. (Easy to do as the grid is fairly delicate.) Broken grid lines can be repaired. You can usually find broken grid lines by visual inspection or with an ohmmeter.

If the grid is ok, I'd check the connectors in the wiring harness which go from the fuse panel back to the rear window. The manual shows the defogger circuit to go through a connector in the left A-post for the sedan.

Good luck. Most of my rear dfogger problems have been broken grid lines.

Thnak you for your reply. I inspected the grid lines with the intent of checking them for continuity, but they seem to be embedded into the layers of glass. I could not feel them at all, nor could I check them with the meter. This seemed unusual to me-could this be related to this car being an SE model. It seems that this car, while on paper a 940, is much more like a 960 in reality (suspension, interior, relay location, etc. Or perhaps I did not look hard enough. At any rate, there appears to be current passing through the grid, so at least one of the lines is intact. If several of the lines were broken, would that not INCREASE the current through the remaining lines, causing INCREASED heating of those? Like several parallel resistors-if one goes bad (open) then the overall resistance would increase, dissipating more energy as heat? I'm not an eletrical engineer and I may be wrong on this point. Thanks again for your help. gpt

In my prior post, I said that "the overall resistance" would increase, and I believe that this is incorrect, but my thought was that the remaining intact grid lines would share the curent of the defective ones(s), and become hotter (or at least more warm). gpt

Assuming the power source is providing 12v to the grid as designed. Each grid line, in parallel with the other lines receives 12v, no more, no less. Thus the heat developed by each line is independent of the other lines. If only one line worked, it would produce the same heat per line as if they all worked.

The TOTAL heat produced would go down as lines are broken. The power formula is P = Voltage(squared) divided by Resistance. The grid lines have a certain total resistance in parallel. As you correctly state, the total resistance goes up when a line is broken. This means the power consumed (heat) goes down. Each separate line generates heat by that same formula so the individual line's power (heat) stays the same. Boy, am I getting wordy! I hope this makes sense.

I'm not familiar with the SE but you are probably right in saying it has some of the design features of the 960. Can you measure the resistance of the grid from the yellow wire to the black (ground)? It may not be the same as on my 940 but it should be in the same general area, 1.3 ohms.