At last, I have typed the complete instructions for the loss-of-coolant sensor for Volvo 740/940 cars. These will be posted in more readable and better-formatted form at the FAQ within a few weeks, but here is the first version. LONG POST FOLLOWS:

Loss of Coolant Sensor for Volvo 740/940 Cars

These plans allow you to fabricate and install a low-coolant sensor in your car so that you can detect either slow or sudden coolant leaks and take appropriate action before your engine overheats. Volvo 760 and 960 cars have these sensors already installed.

Overall Plan.

a) Volvo 740 with Circular Oval Tank.

Using a Nohken float level sensor switch, drill a hole in the bottom of the coolant tank and install the sensor. One lead is wired to the front grounding plane near the passenger-side headlight, the other back around the engine compartment, through the firewall, to the unused instrument panel lamp denoted "Exhaust Gas Overheating (Japan Only)." When coolant is low, the switch closes, allowing 12V from the panel to travel through the switch to ground, illuminating the warning lamp. You can add a small piezo buzzer to add an audible warning, if desired. The circuit is fused just behind the panel.

b) Volvo 940 with Rectangular Tank P/N 9142001/2

Volvo included a level float in this tank, but not the sensor. Buy a "Level Guard" sensor Volvo p/n 3547710 and wire as above. Make sure you read all assembly instructions first, especially to confirm you have the float inside the tank.

Bill of Materials:

a) 740 Variant: Float Level Sensor Switch: Nohken LS-11P-0A, miniature 10VA switch with polypropylene float and stem and flat flange face. This is available from Scientific Technologies Inc. at 6550 Dumbarton Circle, Fremont, CA 94555-3611, 510-608-3400.

Order STI part number 22152; cost is approximately $15 plus shipping.

Web site is http://www.sti.com

940 Variant: Volvo "Level Guard" p/n 3547710) approx $15 from Borton's Volvo, www.borton.com

b) Volvo 12v 1.2w lamp for instrument panel: Volvo uses two kinds of bulbs and bulb holders for their panels. One is a replaceable bulb with wire leads, inserted into a holder that has a plastic locking ring. The other is a bulb soldered into a holder with metal locking tabs. You will probably have to pull your panel to determine which one your car has.

740 with Yazaki: Volvo p/n 2721 1.2w replaceable bulb with leads

940: Volvo p/n 966326-1 1.2w bulb fixed into holder

c) Instrument panel ell-shaped electrical connector Volvo p/n 949542

d) Electrical Supplies, all from Radio Shack (one each unless noted):

Insulated Wire: 20 gauge stranded approx 25 feet p/n 278-1225

Snap Connector 22-18 g, p/n 64-3085

Inline Fuse Holder 5x20mm p/n 270-1238C

GGS 5x20mm Fast-Acting Fuse 3/4 amp p/n 270-1048

Quick Disconnect Connector, Female, .25 inch, p/n 64-4040

Crimp Butt Connector, 22-18 gauge (quantity=2) p/n 64-3037

Shrink tubing for insulating connections p/n 278-1611

Option: Piezo buzzer p/n 273-059 (gives audible warning)

Soldering iron with electronic solder

e) Mechanical Supplies:

Wire ties for securing wire to existing harnesses

"OxGard" conductive electrical grease (from hardware store)

Silicone dielectric insulating grease (from auto parts store)

Volvo 940 applications: None other required

Volvo 740 applications: (from hardware store)

Neoprene O-Ring O.D. 1/2 inch, I.D. 3/8 inch

Neoprene Flat Washer O.D. 7/8 inch, I.D. 3/8 inch

f) Volvo wiring diagram to understand your panel connectors. Here is where Chilton's will finally choke you, and you will be motivated to buy the OEM wiring diagram which is absolutely the best on the market. Approx. $30 from Ken Cook Inc. (Volvo agent) at http://www.kencook.com/vcna/volvo.html

Assembly Instructions:

1. Volvo 940:

a) Drain coolant recovery tank and remove from car. Shake the tank to make sure that Volvo installed the magnetic float that activates the Level Guard inside the tank. (It rattles. If you can't hear the rattle, install the Level Guard and test it with a multimeter for resistance change as the tank is filled with water.) If your tank has the float, install the "Level Guard" sensor into the snap hole on the bottom of the tank. Insert the wire leads into the tank's wiring connector holder on the bottom. Continue below at step "e)".

If you don't have a magnetic float inside, you can either proceed as though you own a 740, or buy a new tank with a float from Borton's for about $25.

Volvo 740:

a) Drain coolant recovery tank and remove from car.

b) Drill a 10mm hole on the flat surface of the underside, approximately one inch toward the center from the threaded cap

c) Remove the float and install it so that the switch operates in the "N.O." ("Normally open") position. Test the switch operation with the ohmmeter range on a multimeter: when the float is at the bottom of the switch (corresponding to loss-of-coolant) the switch should close and complete the circuit, reducing resistance from infinite to zero.

d) Remove the fixing nut on the bottom of the coolant sensor switch. Install the flat neoprene washer on the switch. Feed the switch wires through the coolant fill hole, then into the hole you drilled on the bottom of the tank. Pull the switch snug. Install the neoprene o-ring, then the fixing nut. Snug up the nut until the switch is firmly held without possibility of leakage. If your hole was oversize, use silicone RTV to seal the gap between the hole and your o-ring.

e) Cut a piece of wire sufficient to reach the grounding panel just behind the headlight. Using a short piece of shrink tubing and a .25 inch female blade quick connector, crimp the connector onto a stripped end of the wire and add some solder to ensure a long-lasting connection. Shrink the shrink-fit tubing onto it at the crimp to serve as corrosion proofing. Clean off the grounding plane connector, install the blade connector and the wire onto the ground, insulate it with some "Oxgard" conductive grease, then feed the wire up the lamp harness back to the coolant tank. (Maintenance note: this is a good opportunity to clean all your grounds and coat with OxGard to prevent future electrical problems. Don't use OxGard on any other connectors in the car aside from grounds.)

f) Volvo 740: Install a snap connector to one wire of the float switch and the ground wire. Again, solder it. Insulate any exposed wire on the connector with shrink tubing. This connector allows you to disconnect the level sensor easily.

Volvo 940: Using pliers, squeeze two female snap connectors so as to fit very tightly on the Level Guard lead connectors. Install one of these snap connectors onto the stripped lead of the ground wire by firmly crimping it. Place silicone dielectric grease on the connection for corrosion protection and connect the ground wire connector to one of the Level Guard leads. If you are ambitious, buy the un-insulated version of the snap connectors, solder them onto the wire, and insulate with shrink tubing.

g) Cut a long length of wire sufficient in length to travel from the coolant tank, up the wiring harness at the top of the firewall, thence through the firewall rubber wiring conduit behind the driver-side strut tower, thence into the cabin with an excess of length to reach well beyond the panel.

h) Volvo 740: Strip, crimp, and solder one end of this wire (using a crimp connector) to the other sensor switch wire. Insulate where exposed with shrink tubing and apply silicone dielectric grease to the ends of the connector to prevent corrosion.

Volvo 940: Install the other female snap connector onto the stripped lead of the panel wire by firmly crimping it onto the stripped end. Place silicone dielectric grease on the connection for corrosion protection and connect the ground wire connector to the other Level Guard lead. Again, if you are ambitious, buy the un-insulated version of the snap connector, solder it onto the wire, and insulate with shrink tubing.

i) Snake the other end of the wire up to the firewall harness and to the lower firewall rubber wiring conduit just behind the driver strut tower. Use wire ties where appropriate.

j) Cut off one of the smaller rubber wiring conduits in the lower firewall connector just behind the strut tower. Feed the wire through the conduit into the cabin.

k) Remove the battery negative connector to disable the electrical system. Remove the driver side kickpanel (two plastic body mounts at the top, pull out.) Move both the turn signal and wiper handles down, out of the way. Remove the instrument panel (740: remove plastic fascia plates covering the clock and tripmeter knobs, then the two screws holding the panel in place.) (940: unclip using the two side slits midway up the sides of the panel.) Pull the panel straight out and rest it on the steering column.

l) Working from underneath, snake the coolant sensor wire from the back of the firewall wiring conduit up through the panel opening. Keep it away from the hood release mechanism, any sharp metal edges, and the ignition module, using wire ties as needed.

m) Pull the large 12-position "C" plastic wiring connector out from the rear top center of the panel. This connector is white and roughly "ell" shaped when looked at from the side.

n) Confirm with your wiring diagram which connector you will use for the warning lamp. In the 1990 740, this is position 12 on the connector, which connects to the unused "Exhaust Gas Temperature" lamp for Japanese applications. In the 1995 940, this is position 9 on the connector which connects to the unused "High Engine Temperature" lamp.

o) Cut the coolant sensor wire, leaving enough to reach just beyond the door opening. Strip and install the in-line fuse holder using soldered crimp connectors and shrink tubing or insulating tape to cover exposed surfaces. Install the fuse. Be very careful about soldering near the interior of the car: use a metal guard or shield to keep hot solder off plastic, fabric or leather components.

p) If you want to add the piezo audible buzzer, place it in-line between the fuse and the firewall, again using crimp connectors, solder and shrink tubing.

q) Install the special ell-shaped Volvo metal wire connector p/n 949542 to the end of the fuse wire. While this crimps in place, you should add a small amount of solder to secure it.

r) Record the colors and positions of the wires at the "C" connector. Snap open the back of the connector and insert the coolant connector at the correct position. Snap it shut, making sure all the wires go back appropriately.

s) Install the Volvo instrument panel lamp at the appropriate lamp position, again per your wiring diagram. On the 1990 740, the lamp is installed at the second in from the driver's side of the panel, in the unused position. In the 1995 Volvo 940, the lamp is installed at position 7 from the passenger side of the panel. If you are confused, use a flashlight and shine it through the empty lamp receptacle to confirm the position from the front. Just make sure from your wiring diagram that you correlate the lamp position with the wiring position in the "C" connector.

t) Re-install the "C" connector. Coil extra wire and install a tie to keep it out of the way. Re-install the panel.

u) Seal the wire passage through the firewall conduit with silicone caulk.

v) Test the installation.

Volvo 740: open the coolant tank and place a pencil on the float so as to depress it to the bottom. Start the car. The lamp should go on. Demonstrate several times to wife and kids what this lamp looks like and what they need to do if it goes on (stop the engine ASAP and investigate.)

Volvo 940: Using a turkey baster or a siphon, empty the coolant reservoir. Start the car. The lamp should go on. See note above.

Regular Maintenance. Because of the peculiarities of the wiring within the instrument panel, your bulb may not illuminate when you turn the ignition to KPII before starting. Hence, periodically test the installation to make sure that the bulb, the sensor, and the circuit operate correctly by performing test v) above.