1992 740 Warning lights do not come on

jbwelker · Post by **jbwelker** » 09 Dec 2014, 19:41

Can anyone tell me why all the warning lights on the dash don't come on when i turn on the car? The oil light and seat belt light come on and the one on the far left side (don't remember which one) work but none of the others do. I have replaced all the bulbs and re soldered all the solder joints but still no go. Is it the ECU? Is there a specific ground wire that could be the problem? Any help would be appreciated.

jbwelker · Post by **jbwelker** » 10 Dec 2014, 10:27

Small wire is conected to a spade conection on the alternator. Underneath the battery post on the altenator is a smaller insulated post with nothing hooked up to it. Is ther supossed to be anything coneccted to this terminal?

93Regina · Post by **93Regina** » 10 Dec 2014, 10:45

jbwelker wrote:warning lights ...oil light and seat belt light...

Oil light has a ground at oil pressure sensor, whereas seatbelt has its own circuit

If oil light lights up, it means there is voltage to other bulbs.

Your temp gauge works?

jbwelker · Post by **jbwelker** » 10 Dec 2014, 13:44

All guages work

93Regina · Post by **93Regina** » 10 Dec 2014, 14:27

jbwelker wrote:All guages work

Check Engine light, Bulb fail light, and alternator light, for instance, are all supplied 12 volts like oil pressure light; so there must be a crack somewhere in that flat-copper printed board.

I'm reading via 1991 schematic, which shows a linear view...and may or may not show actual sequential sequence of this 12 volt bus supply. Gauges come first, then light bulbs. It shows Check Engine light as last in the "food chain" for 12 volts.

jbwelker · Post by **jbwelker** » 10 Dec 2014, 14:46

It was a bad alternator. Thanks for your help

93Regina · Post by **93Regina** » 10 Dec 2014, 19:32

jbwelker wrote:It was a bad alternator.

Last I knew, those warning lights tend to light up when alternator is not charging, or when key is switched to on position.

At least on my buddy's 744, his lit up when alternator was dead, and engine was running.

This can be seen in the schematic....6 lights should light up when alternator's idiot bulb is lit up...those six diodes shown in schematic were installed for that reason, at least up to 1991 vehicles. Most interesting....

jbwelker · Post by **jbwelker** » 10 Dec 2014, 19:42

Yea it was a new one for me. My alternator died yesterday without the warning light coming on. So i started digging around and I found out with help from this thread and another post online that said to ground out the alternator exciter wire and see if the lights came on and they did.

Volvo Manic · Post by **Volvo Manic** » 05 Dec 2015, 09:05

93Regina wrote:
jbwelker wrote:It was a bad alternator.
Last I knew, those warning lights tend to light up when alternator is not charging, or when key is switched to on position.

At least on my buddy's 744, his lit up when alternator was dead, and engine was running.

This can be seen in the schematic....6 lights should light up when alternator's idiot bulb is lit up...those six diodes shown in schematic were installed for that reason, at least up to 1991 vehicles. Most interesting....

The alternator failed on my daughters 850 and the warning lights lit up like a Christmas tree. Not just the battery symbol.
Are you saying that each diode is wired to turn on an individual indicator light if it goes bad? If s that would be a great feature as partial diode failures can sneak by other automakers warning systems without an indication.

Partial diode failure happened on my Benz Diesel. Fortunately she'll run until you turn her off, but lights would be nice, and I always drive with the lights full on, which will quickly deplete the battery with no alternator output.