Coolant temperature gauge issues

[Blair Bugher]

Have two (2) 6BTA5.9M2 marine engines with dealer installed VDO 0- 250 deg. F two- wire electric analog helm gauges. I know these type gauges are not high accuracy, but these are way off. Heat gun reading the sender base (and reading the accessible coolant areas will show 178-184 degrees and the gauge reads 205 (same both engines). My factory wiring diagram for these engines (12-v) shows positive battery voltage to starter solenoid, circuit breaker, then via wiring plug/harness red wire to the ignition switch, with output to all helm gauges via purple wire, then to the sender via tan wire (which all matches what I have). Other side of temp sender (black wire) reads less than 1 ohm to ground/engine block. My question is: Shouldn’t the purple wire from ignition switch to all gauges have positive battery voltage(12.6-13.2v) with the ignition switch “on”? While my red wire into the ignition switch shows 13.2v, my purple wires register about 11.4 v (all with ignition switch “on”, connected to dock shore power, battery charger “on”, and engines not running). Is this correct? (Note that the voltage on the tan wire going to the sender drops to about 8v with the engine running, with the tan wire not connected to the sender. This drops to about 4v with the tan wire connected to the sender.)

Topic

[Rob Schepis]

As to the temperature readings being off, is this:

1.  A new install?
2. Existing install w/ new issue?
3. Always been this way?

[Corey Schmidt]

The TAN wire should lead directly to the signal terminal on the back of the coolant temp gauge.

Both the RED and PURPLE wires should carry close to the same voltage after you turn the key to IGN. If not, you probably have a bad connection somewhere between the key and the run back to the engine or possible on the engine wiring itself… you’ll need to hunt it down with a volt meter to see where the voltage is dropping out.

Are you also sure that youre using the correct station sending unit? If you only have one panel per engine, it needs to be a SINGLE station sender and not a DUAL station sender with a different ohm table.

[Author]

Replies