Th400 Transmission Modulator

[BIGYELLOW78J10]

So, at the advice of the greater body of knowledge, the board, I decided that the billowing white smoke that escapes my $300 wag might be due to the modulator valve being bad. Do I tromp on off to my least favorite parts store, pep boys. I ask for and recieve said modulator. I has to be 4 to 5 times as big as the stock one. I had been told the replacements were bigger than stock, but good grief. Anyway, I go to put it in, and it won't fit. Won't go in the hole and won't clear the driveshaft. So, that was a purolater 310. Anybody know a brand or part number that works? I would be oh so greatful.

Thanks,

Daniel

[Crazy_Jeepman]

Well first how did the decision that the Mod Valve is causing the smoke? If your Mod Valve is letting tranny fluid through then the line would be full of trans Fluid. WHITE smoke I highly doubt its trans fluid doing that, billowing white smoke is a good indication of coolant in the cylinders. Disconect the Vac line to the modulator, its the only steel line on the pass. side rear of intake that used screw on fittings, 1/8" line I think it is. disconect, and plug line. Start engine to see if the smoke clears. I have heard of the modulator valve letting trans fluid pass through, personally never saw that happen. However I would think if that were the case the trans would not shift properly. The Large modulator valve requires a elbow fitting. Jeep did come with a large modulator valve and this elbow. If you find it is the mod valve and need the elbow fitting let me know I will get one out to you.

[OJTDJPR]

Just a quick note, I had to switch mine over to the small modulator valve. Tell them you want one for a TH400 GM trans not Jeep then you will get the one that will fit yours. That is what I am running works great and takes up much less room.

The big modulator uses a differant valve that slides into the trans. The one for the big modulator is longer than the one for the small modulator. Hope I made since with that, and helped some.

Just remember if you get the elbow from crazy-jeepman, he is a great guy but dont forgeet you need the valve peice that slides in the trans for it to work.

[ November 24, 2002, 10:53 PM: Message edited by: OJTDJPR ]

[BIGYELLOW78J10]

Well, as a side note, the transmission doesn't shift right, the wag will barely move sometimes, so I have a pretty good reason to blame the modulator. But I still think it could be a coolant in the engine problem. However, a $16 modulator that couldn't hurt to replace is a good first step to me.

I had been thinking that other modulator for a TH400 might be smaller. How can I force the parts house boys to give me the right one? Anybody got a cross reference, or at least anyone know a GM product with the TH400 I could ask for?

Thanks,

Daniel

[Crazy_Jeepman]

The Modulator is what sets the shift points. It will not cause the rig to hardly move, it will just shift erratically. Just ask then parts house for the small one, if they do not know what to give you........go somewhere else.

[BIGYELLOW78J10]

Thanks pete.

The wag was not shifting right when it was driving. Later, I was trying to get it up my ramps, and it didn't want ot go without some motivation rpm wise. I assume this could be due to tranny fluid passing through the modulator. I would also assume this could cause a pressure drop that might make the tranny act weird.
Anywho, thanks a lot.

Daniel

[trickc]

CJ -I drove this wag before daniel bought it and it was shifting ok, I suspect the modulator is sucking the fluid out causing it to barely move, also it wasn't smoking but was real weak engine wise.

[OJTDJPR]

Pete is right, make if it is sucking the fluid out, it would be low on fluid making it not move very well. If it is sucking fluid out there should be some in the rubber line some where.

And yes if they dont know what you are talking about with the smaller valve go somewhere else. But as for the older GM TH400 just say from a 77 Chevy truck with the TH400. If they cant help you then they are not much of a parts store.

[BIGYELLOW78J10]

Thanks guys, I will go on another modulator search soon.

Daniel