AO

I should know this, but this morning on the '87 the idle was fluctuating. I noticed the temp gauge on the pod was also fluctuating and they seemed to be related. When the needle would fall to the bottom, the idle would suddenly increase.

I believe this is driven by the Temp II sensor, right? Since I had the intake powdercoated, I'm thinking I might not be getting a good ground connection on the water bridge. Other thoguhts?

Hilton

Nope, the Temp II supplies signal to LH and EZK.

The gauge sensor on an '87 is the one with spades on top, on the other side of the water bridge (has brown Bosch connector on my '89).

However, the gauge one shouldn't affect idle - sounds more likely to be a ground issue to me elsewhere.. try engine block ground underneath maybe? Has to be somewhere common to ignition/temp sensors/gauge sensor.

AO

Thanks Hilton. Good to know that its different sonsors.

I think you're right in that it's a grounding issue. If the water bridge isn't making good ground with the rest of the car (becasue I didn't clean the p-coating off the mating surface of the bridge ) this could, account for why the idle and guage react together. I'll check the standard grounds, however, to be safe.

TheoJ

Hi Andrew,
there are 5 temp sensors in our 928. nice eh...?
1. Temp switch on top of the in take, which triggers the fans to run when the intake reaches a certain temperature (95C? I'm too lazy to look it up)
2. Temp switch on the radiator, which triggers the fans to run when the coolant reaches its max temp (95C or so?)
3. Temp 2 sensor (one half of the double sensor) for Ezk. That is at the pass.side of the waterpump and feeds engine temp to the Ezk to adjust timing
4. Temp2 sensor (second half of the double sensor) for the Lh. That is combined with the above, and feeds engine temp to the Lh to adjust fuel mixture.
5. Temp sensor for coolant temperature, driver side on the waterbridge. It is just linked one side to the dash and the other side to the central informer. I'd not expect this to lower/raise the rpm. The coolant temp sensor does not link directly to gnd, it does not use the engine ground.. The Temp2 does......

If you see a releationsghip to the temp gauge on the dash and rpm, I'd think of another cause, like electrical load. Can you hook up a diagnostic tester and watch the temp the Ezk/Lh sees? Or connect a voltmeter at the Temp2 sensor and engine ground. That should give you a good idea of the actual temp and relationship to actual rpm.

regards
Theo

WallyP

Theo,

You forgot the torque convertor temp sensor on the automatics - also triggers the fans.

And on the early cars, the temp sensor on the A/C refrigerant receiver/dryer to trigger the cooling fan.

And the outside temp sensor in the HVAC sensor circuit.

And the inside temp sensor in the HVAC temp circuit.

And the outside temp sensor that drives the late digital dash thermometer.

And the two temp sensors in the exhaust that control the Ignition Monitoring Relay.

And the two temp sensors in the Japanese cars that warn of overheating cats.

And the temp-sensing Wheatstone Bridge in the MAF sensor.

And the Temp I sensor in the early cars.

And the Time/Temp sensor in the early cars.

And I'll bet that I have forgotten at least one or two...

Vilhuer

Any list of resistance values for each sensor in room temperature for example? It would help diagnosis if sensor or wiring is acting up.

mark kibort

I dont think the top sensor triggers fans to run. I remember having the temp overheat, and the fans didnt turn on, because the fans were not seeing the over heat switch closure from the radiator.

Landseer

Depends on the year, Mark. 85 begins the setup where the intake air temperature sensor can trigger fans, but does so through a passenger fender resistor that makes the fan run at a slower speed. Can happen with ign off.

worf928

Run temporary ground leads from the base of both the TEMP-I and TEMP-II sensors on the bridge to the x-brace bolts and see if the fluctuations go away.

BTDT with the extra powder-coating.

blown 87

My rule of thumb, for most cars, if it goes to a gauge and/or drives something directly it is a sender.

if it reports to a computer and/or drives something indirectly it is a sensor.

If it has only one wire it is a sender, but this does not mean that a sender can have just one wire.

Now that I have in no way answered your question, carry on.

John Speake

Hi Andrew you can hook up your ST2 to check temp2 readings on both LH and EZK ECUs.

Do you think the gauges were fluctuating because battery was a bit low on charge and changes of RPM changed the readings ?

AO

Thanks for all the input guys. I have never done a count on all the temp sensors in our cars, WOW!

Unfortunately, I don't have my 928 DT or my ST2 with me here in Ohio. I'll try it in the next week or so (actually I'm in New Orleans next week so it will have to wait).

The interesting thing is that it seems to only do it when it's cold - like from sitting overnight. I've thought about running a temp ground just to see if that changes things, but haven't yet.

Yesterday, I had an afetrmarket alarm system that's in the car give me a fit, otherwise I would have probably checked the resistance valuse etc. Will try and do some of that stuff tonight.

killav

Andrew,

My 87 did this a couple of years ago (the temp gauge going to zero intermittently, not the idle problem). I cleaned the sensor/sender terminals and then crimped the connectors on the wire side slightly so that they fit tighter. The temp gauge has not wavered since.

AO

Thanks Mike. I just started the car for the first time since yesterday and it didn't do it. But I'll also look at the connectors and fix as needed.

mark kibort

But I was refering to the water temp on the bridge. my 87 and prior holbert (86) didnt budge the fans if the temp overheats, but the radiator is cool. (due to water pump failure, or t-stat issues)