The following is the procedure for testing VDO gauges and sending units. Just used this info to isolate problem to the sending unit on my 440 chryslers.
VDO GAUGE AND SENDER TESTING
VDO GAUGE TESTING: If a gauge is suspected to be faulty, the following tests can be performed on the VDO gauge and sender:
1) Turn the key on:
Pressure Gauge - pointer to .0.
Fuel Gauge - pointer to amount of fuel in tank
Temperature Gauge - pointer to temperature of engine water
2) With the key on, pull sender wire off the sender:
Fuel & Pressure Gauge - pointer will go to maximum right hand position
Temperature Gauge - pointer will go to maximum left hand position
3) With the key on, ground the sender wire to the engine chassis:
Fuel & Pressure Gauge - pointer will go to maximum left hand position
Temperature Gauge - pointer will go to maximum right hand position
Senders can be tested with an ohm meter that measures from 10 to 2,000 ohms.
Connect the positive lead from the tester to the sender terminal and the negative lead to a good ground. The following readings will occur if the sender is operating properly:
Temperature Sender: Cold - 700 ohms
Hot (250 degrees) - 22 ohms
Pressure Sender: Engine off - 10 ohms
Engine running 40 psi = 105 ohms; 60 psi = 152 ohms.
Fuel Sender*: Tank empty - 10 ohms; full - 180 ohms
*If the fuel gauge reads backwards, but accurate, the float arm has been installed from left to right into the pivot. Reinstall from right to left. Using a VDO 10-180 ohm gauge with a 240-33 ohm sender will also give a backward reading.
Fuel Sender Testing:
As explained before, there are many different ohm ranges in sending units. Therefore, with an ohm meter you can check to ascertain if unit is working properly.
VDO No. 226 001: E = 10 ohms; F = 180 ohms
An empty tank will read 10 ohms. As you add gas, the ohm reading will go up until tank is full and reads 180 ohms.
G.M.: Both 0.30 and 0.90. Empty tank will read .0. ohms. As you fill tank, reading will go up.
Stewart Warner & Ford: Empty tank will read high ohm range (S.W.-240; Ford- 73) and go DOWN as you fill the tank.
*All VDO electric gauge pointers peg full left hand position with the power off.