m998 temp gauge trouble shooting

[M998-76’]

Just picked up an 89' m998 over the weekend. Ive been working through a lot of its issues with great success mostly due to the information on this forum but im finally stumped...

Temp gauge is totally dead- I checked ohms to the wires into the gauge and im not getting anything. The fan runs all the time even when first started. The TDM looks relatively new. What should I check or change out next?

 

[Milcommoguy]

Just picked up an 89' m998 over the weekend. Ive been working through a lot of its issues with great success mostly due to the information on this forum but im finally stumped...

Temp gauge is totally dead- I checked ohms to the wires into the gauge and im not getting anything. The fan runs all the time even when first started. The TDM looks relatively new. What should I check or change out next?

Which one do you want to fix first? Two separate problems. Look back thru the "find threads" for Fan runs and or Temp gauge problems. TON's of info.

Stumped too, CAMO

 

[M998-76’]

Which one do you want to fix first? Two separate problems. Look back thru the "find threads" for Fan runs and or Temp gauge problems. TON's of info.

Stumped too, CAMO

I would like to fix the temp gauge first.

 

[M998-76’]

Pic to come of a 3rd wire going into the top manifold that is just capped... could this be a possible issue to this problem?

 

[M998-76’]

First picture is the plug for the tsu the metal sleeve in #3 slot looks deeper than the others to a point i dont believe it would be making contract with the prongs on the tsu

Second picture is a wire that seems to lead to nowhere?

 

[Action]

First picture is the plug for the tsu the metal sleeve in #3 slot looks deeper than the others to a point i dont believe it would be making contract with the prongs on the tsu

Second picture is a wire that seems to lead to nowhere?

The big plug in your hand is for starting purposes. If the wait light functions properly, plug it back in and leave it.
The plug in the water corssover pipe, above the red circle, is for the fan system. If any wire in that system is unplugged, the fan will default to ON all the time. Do you think the fan is on all the time just because it is spinning? It will spin all the time, wether the clutch is engaged or not.
There is a section in TM 9-2320-280-20-1. You want the section "INSTRUMENT TESTS". It has a flowchart to follow and check things. One test will have you ground out the gauge to see if it maxes out. section 2-32, around page 2-319.
The sender for the gauge is in the block under the alternator / generator.

 

[Milcommoguy]

Want to help the new guy... but already gave bad info.
ME BAD..."Search Forums" to find problem discussed and "fixes" "Poking and prodding" likely will not result in a fix with understanding of what's going on, BUT is good to get familiar with "this and that"

Most if not ALL problems have been covered for HumV. While most system are simple...do not get all wrapped up going in circles or combining problems. I know it is not easy with a new "toy" that didn't come with an instruction manual. YEP, we guys don't need no manual, Got SS, LOL

Long distant diagnostics can be a bit challenging. Pictures and a history of problem very helpful. To assist, refer to TM's, wire numbers, etc.
The manuals are on-line or on this site, are a must have IMO. Think of them as the instructions to a model car or plane without the glue...If your old enough to remember those times. (old fart here)

Temp sender is left side, front of engine, hidden from view by alternator. Has one wire. Operation is typical of most. As nothing to do with fan operation. See this for general basic hook-up and operation / understanding to help or refer to TM's.
> https://www.stewartwarner.com/wp-content/uploads/2018/12/434956-Traditional-Electric-Gauge.pdf

Get ready for the fix-it gang here to jump in or on, CAMO

 

[M998-76’]

I ran an ohm test on the temp sensor and the wire feeding the gauge. on the 20k ohm setting I got a 10.5 which I believe is 10,000 ohms. I think im looking for under 2000 so im going to replace the temp sensor and see how that goes.


As for the fan it is not free spinning when off so I think its a mechanical issue.

 

[springer1981]

I ran an ohm test on the temp sensor and the wire feeding the gauge. on the 20k ohm setting I got a 10.5 which I believe is 10,000 ohms. I think im looking for under 2000 so im going to replace the temp sensor and see how that goes.


As for the fan it is not free spinning when off so I think its a mechanical issue.

Your fan should not spin free when off. You need to view the troubleshooting steps. I simply look at the fan clutch and plug and unplug the time delay cable while it is running and see if the fan clutch moves in and out. It only moves .2 inches so you need to watch it very closely. Basically after you first start it, it disengages so plugging and unplugging the delay should show it working, no need to let it warm up. The theory is basic, if anything in the fan circuit doesn't work right it engages the fan all the time. If everything works right it only engages the fan when the temp exceeds 200-220, something like that but it is unimportant in troubleshooting for now.