Fan won't return to normal speed after coming on and after temp falls.

[Waters]

Been driving my m998a1 since august last year, the fan cooling system has always functioned correctly. I'd say it comes on around 215 and goes off about 190-200 degrees after running at engine speed for about 30 seconds. (on hot days at 55mph this cycle repeats every couple of minutes).

The other day the fan clutched in and spun at engine speed like usual, then stayed on for a couple minutes. The temp gauge showed this was taking temp down to closer to 180 before the fan slowed down. Another short trip showed this was still the case. I'd wager the return of that trip the fan stayed on about 4 minutes before I slowed down at my driveway and then speed was too slow to hear if it was still engaged or not.

Most problems with fan clutch, solenoid valve, or TDM that I have read seem to indicate the fan stays clutched in the entire time the engine is on and won't slow down at all. I haven't read of a problem exactly like mine yet, but I wonder if these symptoms aren't leading towards "always fast fan".

So far I plan to check the thermostatic switch to make sure it still breaks contact (it must do so, but maybe its sticking?). Then I read that I should disconnect the cadillac valve and blow it out with compressed air. Other users suggest disconnecting the belts and supplying the fan clutch with 90 psi to see if it moves or if it is dirty and causing issues. Finally, the TDM may be acting up and I don't know any real way to check that except for to replace it (its a $100 part).

Any ideas besides to run through these diagnostics? I haven't ever cracked the hydraulic system open and I'd rather not remove the valve if I don't have to.

 

[Waters]

update; fan clutches in fine while idling if i break the circuit at the thermo switch.

i did a resistance test at the solenoid connector pins via instructions in a troubleshoot TM. it was easy to do so i checked it. GTG there.

next i let the engine warm up and then pulled the throttle lock out some to help it reach 215ish. the fan clutched in as expected and i wuickly removed the thermo switch leads and clamped my meter to them on continuity test.

the gauge read down to about 180 (usually kicks off about 190 or 195) and still no continuity through the switch, as if it wasnt making contact after the engine cooled down. i rechecked the clamp connections and then clamp to clamp to make sure the tone was still working.

i only did the test once but it seems pretty conclusive that my thermostatic switch is not reconnecting and completing the circuit, thus the fan is staying at engine speed for a looooong time.

ive got another switch on the way, but there will be no memorial day humvee trips. i may go ahead and track down a TDM just in case that ever does fail but i have no reason to believe mine has.

 

[Milcommoguy]

Switch in the coolant bypass known to be intermittent. Gets a lot of cycling over time. Some seen to have a lot of hysteresis, meaning more or less cycle time, off to on to off, coupled with how good your overall cooling system is working, outside temp. etc. Most important... First test Cadillac valve as you did. Ohms range 60 to 70 ohms +/-. Anything less than 50 Ohms could be shorted winding's and place undue current demand on TDM / system. Have found open or shorted coils, either way, toss it. If shorted... consider blown TDM and replace valve first. Try old TDM with KNOWN new valve meeting resistance test above. Won/t hurt anything and its the only way to tell whats going on with TDM. Could get lucky. For most of the time fan is disengaged (solenoid energized) so it get a lot of ON time. Coil does run warm to the touch.

Another test... The themo switch is normally close when on the low temp setting. This applies power to the system, which energizes the solenoid, pressurizing the clutch and disengages fan. (what logic) So simple test would be to disconnect switch leads and jumper together those leads removed from switch. When connected together fan should disengage with KNOWN good TDM and valve. (short of any PS pump,clogged valve, funky clutch action) It's all of these things going on that causes a Excedrin headache number two... for some Hummwv owners.

Linked to a working TDM system is the Kick Down switch. Different location on 6.2 / 6.5 / tubro engines. While fan IS engaged, hot engine, push throttle to the floor quickly (all the way down) and see if fan disengages for 20/30 sec. This switch also drives the transmission "kick down" solenoid to drop a gear.

If one looks carefully at rear of fan edge at hub, down by belts... you can see it pull away, moving forward (maybe 1/4 inch +/-) when disengaged. Watch, listen and learn this operation. Keep you finger out of there!!

A few simple circuits... working together AND correctly just to manage the fan / engine cooling. Yankee ingenuity at it's finest. Good luck.


Signed, Rudy Goldberg

 

[Waters]

thanks for the reply! yeah the solenoid was showing about 71 ohms across leads when disconnected from the TDM. i did not run the lengthy resistance tests the TM lists for TDM testing, but the failure of the thermostatic switch to close after opening (past usual closing temp) seemed pretty ****ing. im hoping that $50 part shows up early this week and i can get back on the road!

 

[ken]

You can run it with the fan on like you have. The thermostat will maintain coolant temp. It will however drink more fuel.

 

[cwajciec]

I had this exact thing happen to me on the way back from a parade today. The fan was cycling normally all day and then got “stuck” on. Then, the fan would cycle rapidly along with the familiar hiss of the Cadillac valve. Sounds like a piece of trash got stuck in the Cadillac valve and plugged it. I think it worked itself through the system and all is back to normal.