Crazy temp gauge

[AZmike]

My temperature gauge has been weird ever since I got the car. Sometimes it reads correctly (maybe 60% of the time). Otherwise it's all the way up. It doesn't change between the two states gradually, it shoots up or falls down rapidly to the correct reading. I know my car isn't overheating, I have a new thermostat and hoses, full coolant with no air. It seems to read accurately more often when the car has been running for a while, but sometimes will work fine as soon as I start it.

I've been trying to pin down the problem. I have replaced the temperature sender and have checked its resistance at different temperatures--it's good. As described in the Honda repair manual I have tested the gauge by grounding the sending cable. Just like a 'good' gauge should, it shoots up to max. The wiring harness looks like it's in good shape.

When I disconnect the gauge from the sender when it's acting up the gauge goes from high to a normal reading (while it's not corrented to anything). As soon as I reconnect the sender wire the gauge shoots back up to max. The sender seems to have a good ground, with very little resistence between any of the chassis ground and battery. The sender wire is about 7 Volts above ground when not connected.

Any idea what could be causing this problem?

Thanks for reading this monster post, I didn't want to leave anything out.

[smufguy]

yep its a long post alright just kdding man. Anywayz, It just sounds like u have checked everything that is imaginable and yet u got probs.

well, It also could be a faulty gauge on the gauge cluster on ur dash. are u checking just one sensor? which one are u checking? There is one on ur radiator, two on ur thermostat since u got a efi. I dont know which one might be wron. I got a problem with that too. Did the check engine light come on? check ur code if ur check engine light comes on.

[AZmike]

I only see the engine light while the fuel pump goes for the 2 seconds starting the car. I've been messing with the temperarure gauge sender on the back of the thermostat housing. I thought all of the other temp sensors (thermostat housing and radiator) were only for the ECU and had no effecct on the gauge.

[cruznz]

Well,...i'll take a guess....
{The sender wire is about 7 Volts above ground when not connected}
....this is with key on,motor not running.....this will be stabilised voltage ......this goes thru gauge to temp sender unit....as engine temp rise's....resistance of sender unit decrease's.... allowing more power thru to heating element in gauge ...making gauge operate.

[(When I disconnect the gauge from the sender when it's acting up the gauge goes from high to a normal reading (while it's not corrented to anything).]

When you say normal...do you mean it drops back to cold on temp gauge?

check wiring from sender unit to gauge for continuity,do this while moving wiring around,and check connections on dash/circuit board, and check to see stabilised voltage does not alter as you wiggle wiring...

hope this is of some help..

[GreenMachine]

Possible, there is air in the system, have ya tried bleeding the coolant system.

[AZmike]

Thanks for the response cruznz, you sound like you'd be able to help. By 'normal' I meant about 1/4 to 1/3 of the way up. The car was still warm from the last drive. All these measurements were taken with the key to the II 'on' position, engine either off or on (made no difference, I tried may measurements both ways).

I was thinking that it might be a ground in the sending wire (the same way grounding the sending wire sends the gauge to max), but if I unplug it it falls down, which means that the cable isn't grounded somewhere (so that idea is wrong). Another idea I had was that the sender plug might be bad (still with continuity, but with extremely high resistence, causing the gauge to read really high). It would make sense that as the sender connector heated up it would expand and make a better connection explaining why the reading as good on long drives. I got some connectors at Radio Shack that fit the sender that I'll try splicing in when I get a chance. Does that sound reasonable?

Sorry, GreenMachine, there is no air in the system (as mentioned in my first post). Every time I check it by opening the bleeder I get coolant right away. The car can be off and the gauge will go from a correct reading to full scale at once, so I know it's the gauge system, not the cooling system. Thanks for the suggestion though.

[anchovies]

Originally posted by 89LXi4dr
I thought all of the other temp sensors (thermostat housing and radiator) were only for the ECU and had no effecct on the gauge.

Those two "sensors"/switches are for the fans.
Another sensor right beside the fan switch on the thermostate housing is for the ECU.
You mean the gauge shoots up all the way to H?
I agree with GreenMachine, try to bleed your cooling system one more time (unless you already done it, which is likely the case since you've changed the thermostate and hoses and still have the problem as b4).
Try another gauge cluster, if not then it should be the wires imho.

[cruznz]

Well...i think putting another plug on will not achieve much, [unless you think the plug may be earthing out the wire to the body of the sender unit], as you say the wiring is fine from the sender unit to gauge...[ i assume you did the wiggle test?]....and that you tested the sender unit and it came up to honda spec at all temp's.

I assume you got the correct sender unit..[had to ask]...

Does it only play up when you are on the move...intermittent...?

Are you able to check engine coolant temp with thermometer and resistance of sender unit when it plays up? this will confirm all your pryor diagnosis / repairs are ok
How is the condition of the cooling system and components?

Was the voltage still around 7v when it is playing up or higher?
You really need to check both sides of the gauge.....i am uncertain as to whether our gauge's have the voltage stabiliser incorporated with gauge or is part of the circuit board, i would suggest you try another gauge/or circuit board as the VS may have an intermittent fault....hope this makes some sense
:smokin:

[AZmike]

Ok, the sender is actually froma junkyard, so I know it's the right kind. Before I put it in the car I tested it's resistance at many temps and it was correct. I don't suspect the sender since I can measure the resisance when the gauge is reading wrong and it will still show a reasonable value.

I've tried jiggling the sender connector...a lot. It doesn't do anything. I don't have to be moving for the gauge to shoot up--the engine doesn't even have to be running.

I can shut the car off after with the gauge showing a warmed up reading afer a drive (1/4 to 1/3 of the way up). Then turn the key back to II and the gauge will read high. I cooling system seems ok.

I didn't think it was the gauge since even when it was reading wrong it would pas the grounding test, but the gauge is probably more likely to go bad that the wiring.

[cruznz]

"Quote"
("I can shut the car off after with the gauge showing a warmed up reading afer a drive (1/4 to 1/3 of the way up). Then turn the key back to II and the gauge will read high").

Does this occur when engine is cold too?(when you turn ignition on in the morning for first time)

Quote,
"I've tried jiggling the sender connector...a lot. It doesn't do anything. I don't have to be moving for the gauge to shoot up--the engine doesn't even have to be running."

What about jiggling the wiring ,at the same time as checking voltage for erratic changes? Check also that there is stabilised voltage at both ends of gauge
Your description of the problem,( to me anyway...) is that the gauge is getting an intermittent earth or the hotwire in the gauge is playing up once it has been on for awhile. Or possibly the sender unit is playing up when it has reached operating temp.
I would try another gauge and maybe a new sender unit to confirm.

[AZmike]

I did get a few spikes when meausrement quite a few things. I thought it was the fact that I was working with a dirty surface so if I bumped the probes at all they might move to an area that wasn't in good contact with the conductor.

I'll go ahead and try a new sender and check the wire to the gauge.

Thanks again

[socal3rdgen]

hmm. mine just recently stated working kinda. it goes to the normal temp. then when i i get to HIGH rpms. it works like a reverse tach. it goes down. then goes back to normal.

[anchovies]

socal3rdgen, yours should be the sender unit. I had the same problem last year around the same kind of weather.