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Disassembly
All electrical cables can be disassembled in one complete harness. (Except for EDIC on diesels). Be sure to mark everything carefully, connectors and all connection points. It will greatly simplify assembly and testing.
The cables can be cleaned with a degreasing liquid.. Check carefully for wear on the insulation, or burned cables. On my car, the wire to the taillight was burned so the insulation was melted for several meters, and the rear part of the cables was replaced with some loose wires connected with quick splice connectors. A nice reminder not to use too large fuses.
Repair
Quick splice connectors must not be used. These will eventually corrode and give poor contact. Instead, joints must be soldered and insulated with a shrink tube.
On the outside, the original cable bundle is protected with a plastic strip that is twisted around the bundle. I use tape for this. But it is important to use an oil and temperature resistant tape. Cheap tape will loosen and fall off. I would recommend 3M Scotch-88 tape for this. It is expensive, but unsurpassed in quality. Do not use vulcanizing tape. It is impossible to get off again.
Also make sure to use the correct thickness of wires, and preferably different colors. It is difficult to find original colors, but for example in Europe Elfa Distrelec has bundles in different colors. At Elfa you also get shrink tubes, Scotch-88, switches and also connectors that fit with Toyota. However, the lock on the connector does not fit.
FASTON tabs Receptacle from TE Connectivity: multipole-connector-fastin-faston
All contact pins and switches are sprayed with contact cleaner. This removes oxides that give poor contact. Spray, and assemble and disassemble the connectors a few times.
Contacts and relays that work poorly can sometimes be saved by taking 800 sanding paper, laying double and pulling in and out between the contacts.
To loosen pins inside connectors, slide the cord toward the connector housing. Then insert a mini-screwdriver into the slot over the pin from the front, and press the small locking pin down. Then pull out the cord and pin from the back.
Test
Before the harness is mounted in the car again, it is a good idea to test through cables and instruments. You will need a multimeter to measure resistance and voltage. Connect the instrument panel to test this as well. Connect the fuse panel and use one or two car batteries to get power.
The temperature and tank sensors are all resistors and they can be simulated by connecting a resistor between cable termination and earth. This allows you to test both instrument and cable at the same time.
Use a resistance value that is about half the value of the sensor at full throttle. Then the instrument should show about halfway. Suitable values are:
Tank gives: 47 Ohm
Temperature sensor: 47 Ohm (equivalent to 80 degrees Celsius)
Oil pressure works differently and must be tested in the car.
Also connect switches and relays and use test bulbs to test lights and turn signals. The turn signal relay works best with the right wattage on the bulbs.
Instruments
Oil pressure gauge toasted
My gauge was toasted, the heating wire is severly burnt. It is impossible to get a new gauge. The gauge works with heating of a resistance wire wrapped around a metal bar that will bend when heated, and this movement is transferred to the indicator.
It should be possible to add a new wire if this is burned. Remove old wire and measure the length of it. Resistance wire is specified in ohm/meter (or foot). Spec for wire is 44 ohm for 12V, or 65 ohm for a 24 V gauge. The wire you need is then specified as 44 Ohm/ (length of wire) for 12 V.
Same can be done with water and tank gauge, but here resistance is 25 Ohm, both 12 and 24 Volts.
Currently I run with a 24 V sender and 12 V gauge. To get the total resistance of the gauge to 65 ohm I have added a serial resistor of 20 ohm between sender and gauge to prevent to high current in the system. This works, but the readings are 30% to low.
Voltage regulator for instruments
Fuel gauge and cooling water temperature gauges depend on constant voltage to display correctly. In order to obtain this, a voltage regulator is installed in the tank meter, which supplies power to both meters. If this is toasted, these instruments will not work. The regulator shall provide 7 volts, both on 12 and 24 volt cars.
In my cluster the fuel and temp gauge did not work at all. When connecting a 3W test bulb it just flashed sporadic, so I suspected the 7V voltage regulator in the fuel gauge to be bad. Took out the gauge and connected it to 12 V on the bench (no serial resistor when out of the cluster), and connected a 47 Ohm resistor to the sender input. This is better than the bulb, as with 47 Ohm input the gauge should read half tank.
The needle barely moved. I took a 600 sanding paper and sanded the contacts in the regulator. This had the effect that it started to spark between the contacts. Not good. I took 800 paper, sanded a bit more, sprayed with electric cleaner and compressed air, and voila, the needle moved. Then adjusted the screw until reading was half tank. Then a drop of paint to fix the screw.
Electronic regulator
On my car, the regulator worked poorly. It is difficult to find new instruments so I thought of replacing it with an electronic regulator. I have attached the schema for the electronics if anyone would like to try. I have not built this myself, but it is a simple thing so it should work ok.
It consist of a linear regulator, TS7808 (or similar) with steady 8 V output when input 12 – 35 V. To reduce to 7,3 V the diode D1 is in series. D1 is any standard diode like 1N4001, 1N4002, etc.
(Observe ring on diode. That shall be in the direction of the arrow.). Add a few capacitors for stability. Values are not important. Arrangement can be fixed on backside of cluster (where serial resistor is on 24 V systems). If 24 V car, remove serial resistor. It will work on both 12 and 24 V cars.
Parts can be ordered at any electronics supplier and will cost less than 2$. I use Elfa Distrelec, but that is in Europe.
ITO – 220 is the type of housing of the regulator. The backside if the regulator is connected to ground so be careful where to fix it. Fixing to the cluster would be ideal as that will then help cooling the device.
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