blitz951

Is it normal for the DME relay to get hot after the car is driven?

I have my DME laying on the floor board and noticed that it gets hot while I am driving. I took it out of the box and found that it is real hot so I am considering drilling multiple holes in the box and even a square opening were the heat sink is situated(I think its called the heat sink). All that heat cant be good so I think this is a good idea to ventilate the box but I would like to get some thoughts from you all.

Thanks

Tadi

Most electronics of a sufficiently complicated nature get hot, so heat itself isn't worth worrying about too much. Excessive heat (like say, not being able to touch the DME box without some discomfort after a run) may be a problem. Do you have upgraded injectors by any chance? If you do, and you didn't use some sort of ballast resister to bring their current flow down to stock levels, the DME will get hot from the extra current. I believe the stock ohm rating on the injectors is 5ohm, while most aftermarket/upgraded injectors run at 2.5 ohms - an effective doubling of current that the DME has to provide. Someone correct me if I'm wrong!

Danno

Tadi's correct: V=IR from basic physics. The driver-transistor in the DME doesn't know what brand or flow-rate your injectors are, just their resistance. Using aftermarket low-impedance injectors without matching ballasts is OK with the DME as long as you stay away from high duty-cycle full-throttle runs. Otherwise, flowing twice as much current through that transistor will generate quite a lot of heat and reduce its durability.

The other DME overheating possibility is your grounding points on the block. There are several ground points in the harness and one of them is from the DME to the firewall. If your grounds on the block is bad, that current will flow through the DME and may actually fry it! A fellow Rennlist who's an EE found this out several months ago when he was getting sporadic coolant readings. Replaced his entire cooling system and still had the bad readings. Turned out it was his grounds! And he managed to fry a couple of DMEs along the way in troubleshooting this.

blitz951

I am running #55 Siemens with out the ballast resistors. I know I should have them but my set up is temprorary, as Danno knows, because I am waiting for the Link stand alone so I dont want to dump any money into my current set up. I know the resistors are cheap but I dont want to waste money. I dont think the box is overly hot but to reduce any heat I think would be good that is why I am thinking about making some holes in the box. What do you think?

Perry 951

I think you should add the ballast resistors. Why chance frying a $300 DME over a $10 resistor??

slivel

Where is the best place to wire in the ballast resistors?

Perry 951

I did mine about 4" off the DME, directly into the harness.

blitz951

Perry951,
Good to see you on the boards again. I did not think it was so cheap but I still dont know were to buy the resistor, how to wire it etc... Could you let me know?
By the way my car has a real rough idle after I drive it 10 minutes and the gas mileage is real bad could this be because I dont have resistors which would cause the DME to get hot making the car idle badly?

Perry 951

Very well could. The #55's are a disc type injector, and from my research, need a consistant voltage and load to work right. Once the drivers start getting real hot, you can have a variable output of resistance and voltage.

Danno sells the #55's and the proper resistors. I am sure he can help you.

Perry 951

Oh, and here is a link to a very good install. (Not mine)

<a href="http://forums.rennlist.com/forums/ultimatebb.php?ubb=get_topic;f=15;t=018551#000000" target="_blank">http://forums.rennlist.com/forums/ultimatebb.php?ubb=get_topic;f=15;t=018551#000000</a>

<img src="http://boards.rennlist.com/upload/ballastresistors1.jpg" alt=" - " />

blitz951

Perry951,
Thanks for the info. Were did you get your resistors? I would like to be able to go somewhere near by like Radio shack and get them today but dont know what to ask for. I could get some from Danno but it may take a while because I am 3000 miles away(east coast for now) and have some time the next 2 days so would like to get the resistors today and installed. By the way, how are you coming along with your Red 951? Get some more pics. of that car it is very beautiful.

Perry 951

I don't know if Radio Shack would have anything like that. Furthermore, I do not know the specs on them. Again, I think Danno would be the best bet.

My 951... it hits the road this Saturday. She's all ready, and all that is needed is a little more wet sanding and an alignment.

blitz951

Good job Perry. So finally you'll be running again and I think you will be very happy with all that work you did. Just keep us posted on how she runs and try and get some more pictures or better yet some videos.
I will email Danno soon. I have other things to talk to him about anyway. Take care.

RobNL

I believe the ideal resistance of the stock injector is 4.5 Ohm (spec is 3.5 to 5.5 Ohm). But the injectors are wired in parallel (2 x 2). So the resistance of each combination is 2.25 Ohm if both injectors are exactly 4.5 Ohm.

If you have aftermarket injectors with a resistance of 2.5 Ohm (1.25 Ohm in parallel), you might need two ballast resistors of 2.25 minus 1.25 is 1 Ohm.

I have Siemens 55# (2.8 Ohm) injectors. If the minimum specs of the stock injector is 3.5 Ohm then the difference, 1.75 minus 1.4, is 0.35 Ohm. Is it then really necessary to add ballast resistors? I might use 0.5 Ohm resistors to relieve the DME a little bit, but I'm not sure if this difference could fry a DME.

Perry 951

Rob -

Welcome to the list!!

Your calcualtions not-withstanding, several people have fried the injector drivers by using Siemens 55's. The Bosch 52's don't seem to have this problem, and if I recall, have a higher resistance.

The DME seems happy having the resistors wired in line, so why take a chance? I'd rather spend a few bucks to save that $300 DME.