Engine/Transmission (1994 - 1998) Grid heater not working

[mpfeiffer]

Since the weather has gotten a little cooler here, I have noticed that the manifold grid heaters are not working properly on my 94 3500. When the key is cycled in the morning the wait light says on for only a few seconds (as if the truck is fully warmed) and once it starts there is no cycling of the heaters on on off (volts gauge stays at 14 volts). Im fairly certain there is a relay to control this, which I kind of remember hearing click in the past. Any suggestions?

 

[EKlem]

When the wait to start light is on, does the voltage drop? This will tell you whether at least one of the grid heaters is actually doing anything.

There is an air temp sensor in the intake which changes resistance with temperature and the computer senses this and determines how long to cycle the grids for. If the issue is the grids running for too short of a time and not cycling when it is cold out after the truck is started, then you need to look into this stuff. As a quick diagnostic, if you unplug the temp sensor, it should run for the maximum length of time.

It is also possible that your relays are bad since the heaters themselves almost never fail. These can be checked easily with a voltmeter by checking the voltage relative to ground on the switched side when the key is cycled before a cold start.

You need to determine whether the problem is that the grid heater relays are not getting the signal to cycle the grids or whether the relays have both quit.

 

[GAmes]

There is a lot of info on the heaters. Do a search, get a cold beverage or two. Lots of reading. I opted to bypass the PCM and have mine wired thru a toggle switch.

 

[mpfeiffer]

Are the relays the two mounted to the drivers side inner fender well sort of under the battery? The two look identical and are similar to snow plow solenoids. What is the locaation of the air temp sensor?

 

[EKlem]

You are correct about the location of the relays. If it turns out that they are bad, I would suggest getting some 100A relays from allied electronics. Many of us have done this cheap fix and they hold up much better than stock.

The air intake sensor is on the intake manifold just behind the intake horn. There is a screw in sensor that has a 3 or 4" pigtail with a plug. You can actually check it by unplugging that plug and checking resistance and comparing it to a table(I can't find it right now but if you need it, I am sure that someone has it).

 

[The patriot]

A pretty good read. It should actually be a "sticky":

https://www.turbodieselregister.com...484-stanco-relay-fix-failed-heater-grids.html