Water Temp Gauge Secrets

You ask a question – “So what was the water temp?”  The answer – “Almost but just above the third mark.”  We are used to that, but it begs the question – what the hell does that mean?

Our research started when we posed the question that if you have an aftermarket VDO water temp gauge (you know, the ones with actual numbers on them), will it properly and accurately use the stock 944 sending unit?  In the past, racers have used aftermarket gauges by fixing up all descriptions of adapters to power the gauge.  We don’t think that is necessary.

Our side of the argument, normally over adult beverages, has been that the only difference between senders of the same brand (VDO) is that the threads and lengths are different to accommodate different engines.  The insides – the sender itself – are the same.  So if you buy a generic VDO gauge and wire it to the stock 944 temp sender, it should work properly and accurately.

Time for an experiment.

We made a simple harness that would plug into the sensor and then feed that signal back to the stock gauge and a “loose” VDO water temp gauge. The loose gauge has three tabs on the back – one for positive power, one for ground, and one for the sender.  We hooked it up to our 1987 924S (Sparky) and cranked him up.

Sparky has been rigged for Florida heat with both a cooler thermostat and cooler fan switch.  The fans come on between the first and middle marks (170F-180F?) on the stock gauge, and it rarely runs over that middle mark, even on the hottest days.  But then the question arises – what the hell is the middle mark?  Or any of them?

Google and Clark’s Garage told us what we needed to know about the dash gauge on the 924, 924S and Series I 944 (three gauge pod)

• The first box ranges from 104F to 154F/40C to 68C.
• The first mark is 176F/80C.
• The middle mark is 192F/89C.
• The third mark is 207F/97C.
• The bottom of the red box is 221F/105C.
• The gauge pegs at 248F/120C.

The Series II 944 has some different readings on the marks, as follows:

• The first box ranges from 104F to 154F/40C to 68C.
• The first mark is 176F/80C.
• The second mark it 212F/100C
• The third mark at the top is 239F/115C.

So now that Sparky has warmed up and the fans came on, we knew that the water temp was somewhere around 175F/80C.  I checked the gauges.

BOTH gauges were reading low, probably because of my home-made harness and the resistance created with one sender trying to feed two gauges.  The stock gauge was in the middle of the first box and the loose VDO gauge was reading about 130F.  However, the stock gauge was in the middle of the box ranging from 104F to 154F, or about 130F.  BOTH GAUGES READ THE SAME.

We then disconnected the stock gauge from the harness and warmed the car again to the point when the fans come on – should be about 175-185F.  Surprise! It just touched the 180F mark, and before I could snap the photo, the fans reduced the temp to 170 – again, normal occurrence for Sparky while idling.  

So this little test taught us two things – first, if you are going to use an aftermarket water temp gauge in your track car/race car, get a VDO gauge and hook it up to the stock sender and second, instead of talking about it for months on end, just try it and see what happens.  The whole process took about thirty minutes and was well worth it.

The only problem with this particular testing process was that the sending unit is buried under the intake boot and some other hoses – long needle nose pliers and a couple of screwdrivers were essential.  We also invented a few new words.

Also, we now know the meanings of the little marks on the stock gauge.  Bonus information is always good.

We are also making the assumption that the same theory will hold out on the oil pressure sender and the oil temp sender.  VDO senders drive VDO gauges, whatever they are.  When we get to putting these gauges into the 924S944.com track car, we’ll let you know.

Special thanks to fellow Porschephile, racer and pilot Mark Peebles for using his spurs to make this happen.