the block heater should be right brhind the oil filter housing.Check the cord first,if tis good replace the element. If your talking about the grid heaters,they arent that hard to diagnose.Try jumping the relays ,see if the come on,if not chck power to relays and relays, If they do,look at the wiring from the intake temp sedereaters,the sender may be bad too.
well the grid heaters havent worked since ive had the truck. but the block heater used to work and now it doesnt. the wiring on the truck is horrible so i have a hunch that it is probaly a wiring problem. i was just wondering how hard it would be to diagnose the old or just replace it.
Try checking the block heater cord with an DVOM for continuity and\or shorts.If it checks out OK,and your power source is known good,then replace the block heater.It just unscrews.
You'll need a test light to check the grid heater circuits.You should be able to follow the big leads from the battery,and see if they both have power at each relay terminal.You can also check to see if you have power out to grids on the other terminal when the grid heaters should be active.
Do they relays click ? Do you have a wait to start light at all ?
There are primary circuits to check too,but start with this stuff first.