Variability in Dash Readings Oil / Water / Oil Pressure etc

[craigcaf]

I have noticed that the dash display readings for oil temp, oil pressure, water temp, fuel etc oscillate about a lot. It's quite a frequent rhythm of oscillating from high to low readings (hunting), and for example I might see water temp oscillate from 90-105degC and oil pressure from say 30-40. The frequency of the oscillation appears to be the same for all readings.

With an old analogue car instrument panel, the first thing I would be doing is searching out the voltage stabiliser. However is there such a thing on the T-cars? The fact that all readings do this implies that it is something to do with the electrical system on the car.

Any ideas where to start?

C

[Don1]

Both of mine do much the same, I just tend to take the mean and ignore the flash high readings.

[K4TRV]

craigcaf said:

snippity.............Any ideas where to start?......snip.

C

Have you access to a laptop with the diagnostics software, available??

It will show you what the ECU sees and if any errors/faults etc have been logged?

You may also see the discreptancy of water temp the ECU sees and the display temp.

Also reflects battery voltage and whether it is swinging in sympathy with your readings??

Software is freely available, just need a laptop with Win XP and a serial adaptor cable (avail from Clev Trev's/RGs/TVR Power?).

HTHs

Trev

[craigcaf]

Yep - I have the laptop and software and will check this. The battery voltage reading swings as well on the dash maybe from say 13.1-13.5V when running - whether it is a symptom or cause, I'm not sure. I will also put a voltmeter on the battery and see it that is a real swing or not.

[SergSC]

Mine also does this, only after I come to a stop and idle for a few seconds, IME related to temperature... someone should be along soon to tells us check bulkhead connectors and/or earths?

[craigcaf]

So its clearly quite a common issue. Would be nice to find a solution that gives much steadier dash data that we could all put in place.

C

[alex_gray255]

Dash ECU software is... not the best. It has variance in the mappings between real and displayed values.

However, after speaking with Paul S about this in the past, I'm just fixing the water mapping.

The other problems will be solved by some software being developed which bypasses the Dash ECU display
all together and pushes the information straight to my SatNav system.

You can achieve a similar effect with an OBD2 compliant chip like Syvecs and appropriate display systems or direct sensor reading. Due to restrictions I won't bore people with, I'm going down a slightly different route.

There are quite a few AM systems that do something similar.

[alex_gray255]

Meant to add, the Dash software is not flashable, it is all micro-coded into the hardware (the chip), so you effectively need to design a new chip from scratch to correct all the issues and some of it would have to be done on a per-car basis.

As such, doing a new chip to fix the problems does not make sense. Easier to bypass the entire system and use something proper.

Hope that makes sense, Craig.

[paul1962]

Not all cars have the wandering readings.
One of mine is perfectly steady, the readings on the other start to wander when the car gets hot.
I'm repeatedly told it's an earth issue and I've had all earth points checked several times but so far nobody has managed to fix it.

There's also a difference between the dash display and engine ecu but as Alex has said this varies from car to car.

[alex_gray255]

The restrictions and garage it throws up sometimes has convinced me to seek solutions else where...

[twinreal]

Have you already checked bulkhead connectors and/or earths?

But seriously, maybe it has something to do with the switching on/off of the fans. My car doesn't have the various dash readings, but when hot and the fans kick in there is always a noticeably "short flicker" in the display.
It could be an inductive interference, maybe from a switching relay or even the alternator. Or maybe an earth problem

[SergSC]

Its always an earth problem, isn't it?

[craigcaf]

I think in that the fluctuations are still there even when the ignition is on and the engine not running would rule out fan switching (also the frequency of fluctuations is much more regular), and of course the alternator would not be charging then. The reason I likened it to the old analogue voltage regulators is that they could sometime show the same symptoms as the bimetallic spring heated up and cooled down and made and broke contact. But if there is a voltage regulation function for the dash panel I would presume it is electronic and probably buried in the dash ECU. I will put the laptop on to see if the main ECU values also fluctuate with a similar frequency - whether that happens or not might give some pointers.

Perhaps there is another drain on the electrical system somewhere that cycles frequently and causes the issue with the dash display?

[twinreal]

It's worth measuring the (real) battery voltage with engine off / ignition on/off and compare all with the dash values.
If this problem only occures when hot and even with engine off, then maybe could be a thermal fault (you call it so in english, too ?) of the dash ECU ? By the way, there is a fan in dash pod, too

[paul1962]

craig, yours sounds like a fault with the dash ecu especially as it happens with engine off too.
If you know someone else with a sag/t350/tam you could try swapping the ecu to check.

[craigcaf]

Yes I think that's quite possible. I'll do some simple battery voltage checks as mentioned above, but I think the cycling is coming from the dash ECU, so could well be worth a test of another one.

I also had a water sensor error message on the way home tonight - I know it's not the sensor as I replaced that at the weekend - suspect that the cycling is causing the maximum difference between the dash water temp sensor and ECU water temp sensor to trigger the fault code occasionally. The check on the laptop diagnostics will also help - I'm expecting to get a nice steady signal on water temperature from the main ECU - will report back results.

C

[SergSC]

Seems the dash is just not made of reliable stuff. Mine used to permanently read water temp 15c above ecu, it would beep at me all the time in slow traffic, then I fried the dash ecu by accidentally reconnecting the battery with the key on, then it got repaired, the repair changed the permanent temp offset into this intermittent offset when hot and idling.

its been ages since an update to this thread...
http://www.pistonheads.com/gassing/topic.asp?h=0&a...

I fitted this square peg into the round hole...