4.0 liter '97 XK8

New Water Pump (metal impeller); New Thermostat (w/ New Metal housing); Engine Temp 180'F. at the thermostat housing

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High speed fan comes on at about 175-180 and remains on.

After engine shutoff, fan stops, then the high speed fan starts again for the 10-15 second cool down period. Then shuts down.

Low speed fan seems to work OK before the high speed fan problem begins

There are no "Check Engine lights", and engine is definitely not overheating (operating at 185 degrees F)

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I changed the coolant temp sensor with no result. (old one ohm'd out OK)

Cleaned the contacts on the A/C pressure sensor (on the AC line behind the radiator)

Pulled the Fan Relay but I do not know how to test it ??

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Suggestions?

 

What are you using to obtain those temperature readingsand / or where are you taking them?
(if rectally, please do not respond)

 

The temperature readings are obtained using a Raytek infrared thermometer laser beam reader.


The readings are taken at the thermostat housing outlet on either side thermostat, e.g., the aluminum housing, and the rubber outlet hose. The temperatures are fairly consistent with those of the larger diameter plastic manifold piping adjacent to the t-stat housing. gordo

 

Nice pyrometer, and it doesn't look slippery in the picture.

I have a fan relay module I can send you to test with, but I was hoping to get a look at the schematic first to see if there were any better suggestions. I'll post any other ideas if they arise when I get a look later.

 

Thanks for the offer Andy. Let me first see if my Indy here has one to swap out. But if not, this is the fan relay I need:

 

That's the only one there is, mate. I was curious about the temp readings, since I have seen a few XK's sucking the lower (LH) radiator hoses shut. Some people say this is normal, I can tell you it's not good. But your diagnosis is past that concern.

 

I am confident this is not an mechanically created overheating issue, but rather an electrical problem.

The coolant temp sensor appears to be OK. Consequently, either the ECM is sending the fan relay a bad signal; or the ECM signal is good, and the relay is switching on the high speed fan(s) at the wrong times. Somewhere in that mix, the AC line pressure switch can also turn on the high speed fans.

 

Yeah, I got that. You are still reproducing this symptom with climate control turned off?

 

Yes the symptoms occurred with climate control either on or off. And turning climate control off, did not seem to have any mitigating effect on the problem, e.g., delaying onset of the the fan activation.

 

You're not allowed to employ such eloquent verbage unless you are wearing pants. I'll look at a diagram and get back. Please be adorned in the appropriate vestments when next we meet.

 

The dress code in Florida doesn't include snow boots, flannel-lined trousers, woolen shirts, and other apparel wear you rust-belters are accustom to seeing. In January and February we might wear long pants and socks... but only if we brought some down here from up North. I leave my snow gear in my NYC apartment.

 

Now that is a fashion statement I can warmly relate to.

Let's start here: run the engine until high fanspeed is activated, unplug the relay module, plug it back in and see if the fan speed changes.

If not, unplug the relay and test the white wire and the white/ blue wire separatelyfor continuity to ground. Those are the outputs from the ECM. We'll go from there, it's not an easy circuit to read and I have never had a fault that wasn't fixed by plugging in a new relay.

 

have had a/c relays keep the high speed fans on.. how long ago have yopu changed the t state?

 

T-Stat was changed in June'08, <2,000 miles ago.

Climate Control:
The high speed fan error occurs independent of AC, or climate control, activation. However, I am assuming the AC pressure valve sensor is active even when climate controls are switched off. Also AC and climate control have no discernible operation errors.

Relay Continuity:
All pins have continuity to ground, except the smaller gauge white wire. This is true whether ignition is switched off or on.
[blockquote]I checked two (2) fuses in the passenger side bulkhead box:
[blockquote]#12 10amp Radiator Fans Control Module
#16 5amp Engine Control Module Cooling Fan
[/blockquote][/blockquote]

 

That was checking for ground potential at the harnesswith the relay unplugged, right?

Did you try unplug/ replug when the fans were running high? There are three relays and two diodes inside the fan relaymodule, I was speculating that a leaking diode might be causing the intermittant fault, and thatthe break and make test might reveal a change in operation which would condemn the relay module.

If the ECM outputs are sending ground signals to energize the module's relays, the module is fine andwe go back in the other direction on the circuit._genius is quite right, if the ECM is getting an errant input to activate fan operation than it is doing its job properly too.

 

Good News and Bad News

After playing around with the fan relay and cleaning the AC pressure sensor connections, the high speed fan no longer activates. The bad news is now neither fan will come on.

I let the car idle in the garage until temps at the T-stat reached 230'F. I know the T-stat opened b/c the radiator outlet and hose were at the 180-190'F. range. Here is the dash gauge at 230 degrees T-stat tower temperature. But even if the T-Stat was bad, the fans should be activated at these temps.



Neither fan activated... I think my next step is to source a new fan relay.

PS. I again hecked the #12 10amp Radiator Fans Control Module fuse and the #16 5amp Engine Control Module Cooling Fan. Both OK.

What do you think?

gordo

 

I think:

It's time to put the shorts back on, you're bound to get increased cabin footwelltemps while your engine is cooking;

The relay module is probably bad, which is why I would have plugged another one on to verify so as not to waste alot of time;

That I should go back to the diagram and write you out a voltage/ ground test list for all of the relay module connector terminals. I know the ECM cooling fan fuse has nothing to do with what you are working on, and there are at least two fuses which will disable the circuit if you popped one;

That your temperature gauge is working accurately;

That your coolant temp sensor never had anything to do with the original issue, since that voltage signal talks to the ECM and is used to operate the temp gauge and fuelling strategy as well as the fan module, so you would have had a DTC if thecoolant sensor was bad;

It's hard to fix cars when you're 1500 miles away from the car;

I have a good used module to send you tomorrow if you wish, and you already know the cheapest place to buy a new factory replacement.

If you have a good fuse map, look again or test every one in the car with power on and a testlight.

 

Hey Andy. Thanks for both of your write ups.

First set of Q:

Yes the ground check was on the harness unplugged from the relay module. Any problems with the smaller white wire not giving a continity signal?

Also [When the fans were operaable] I did unplug and replug the fan relay module harness with the fans running. The fans stopped when unplugged, but restarted immediately upon reconnecting. Furthermore, that performing that procedure apparently made no significant difference thereafter.

Now onto your Second Response:

I just realized the ECM cooling fan fuse was for the fan cooling the engine computer. Duh....

But I did locate, and check, the two other fuses you mentioned. They're in the engine compartment fenderwell, left hand side: #12 30amp Cooling Fans Series/Parallel; and #14 30amp Cooling Fans Parallel. Both are OK

Agree that the temp sensor did not require replacement. And yes, I did get a DTC when it was unplugged from the connector.

Tomorrow I will hopefully source a known good fan relay here locally; If not I will email you for one. Want to deliver it FOB Daytona FL?

gordo

 

Last point first,

I'd love to drive down, but it would take me two days to peel off all the layers of clothing first. And that's not including the long johns, which may be...uh...stuck on by now.

The white and the white/ blue are the ECM outputs to ground the separate relay windings and change the fan speeds. So white/ blue was enabling its relay when you tested it, and white was not. Back to the wiring diagram for me, I'll come back if I have anything substantive to say.

I'm 99% sure that the XJ8 relay is the same, so if you verify that in your parts lookup information it might make the search for a donor easier. Obviously you should use the Skimobile for your parts chaser...which means you have the perfect vehicle to drive to Ohio for expert parts counter assistance, right?

 

First point first. Word: DAB

Regarding the relay...I found this in JTIS:

[blockquote] Outputs from the ECM control two relays contained in the radiator fans module, to operate the fans in 'Off', 'Slow' and 'Fast' mode. The fans operate in series (slow-mode) and parallel (fast-mode) under the control of the double-action radiator-mounted temperature switch as well as being controlled by the vehicle's climate control/ air- conditioning system. Hysteresis in the temperature and pressure switching values prevents 'hunting' between modes. Under hot operating conditions, the fans may continue to operate for some time after the engine has been switched off. The fans will stop automatically when the coolant temperature has been sufficiently reduced.
[/blockquote]
What's the "double-action radiator-mounted temperature switch"? is that the coolant sensor?

I was also curious about the term "hysteresis" and found this:

[blockquote]Thermal hysteresis occurs when the value of a given property of a body depends not only on the body's temperature but also on whether the temperature is rising or falling. An example a thermostat controlling a source of heat and set at some temperature T0. When the room temperature falls through T0 to some lower temperature T1, the heating power is switched on. When the room temperature rises through T0 to some higher temperature T2, the power is switched off. Thus, for temperatures lower than T1, the heat is always on; for temperatures higher than T2, the heat is always off; but for temperatures between T1 and T2, the heat may be on or off (double-valued response), depending on which of the two temperatures T1 and T2 occurred most recently in the system's history. Unlike the previous examples, this hysteresis effect is not naturally occurring; it is designed into the control system to prevent the damage to the system that would arise from switching on and off too frequently.
[/blockquote]
The internet IS the Information Revolution.... gordo